[Richard[_11_]]

I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

[[email protected]]

On Sep 1, 2:44*pm, Richard wrote:
I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

Emission.

[Richard[_11_]]

On 02/09/2010 07:52, wrote:
On Sep 1, 2:44 pm, wrote:
I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

Emission.

Also, good pictures I would imagine.

I suppose folks intersted might want them selling as a pair?

[Don Pearce[_3_]]

On Wed, 01 Sep 2010 19:44:29 +0100, Richard wrote:

I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

I believe that having the type number clearly visible on the envelope
is the top of the list, so be careful if you decide to clean them.

d

[Richard[_11_]]

On 02/09/2010 18:41, Don Pearce wrote:
On Wed, 01 Sep 2010 19:44:29 +0100, wrote:

I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

I believe that having the type number clearly visible on the envelope
is the top of the list, so be careful if you decide to clean them.

d

Oh, yes, must not rub that out. :c)

Actually, I don't think the figures will rub out, I thhink they are
etched. Others will kmow.

I guess they will end up on Epay. I need the money. :c)

[Richard[_11_]]

On 02/09/2010 18:54, Richard wrote:
On 02/09/2010 18:41, Don Pearce wrote:
On Wed, 01 Sep 2010 19:44:29 +0100, wrote:

I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

I believe that having the type number clearly visible on the envelope
is the top of the list, so be careful if you decide to clean them.

d

Oh, yes, must not rub that out. :c)

Actually, I don't think the figures will rub out, I thhink they are
etched. Others will kmow.

It will rub out because I've got a pair of TT22's and the figures have
rubbed out a bit. Of course TT22's are like TT21, but 12.6v heaters.
Not used much I would not have thought for audio projects.

[Don Pearce[_3_]]

On Thu, 02 Sep 2010 19:15:29 +0100, Richard wrote:

On 02/09/2010 18:54, Richard wrote:
On 02/09/2010 18:41, Don Pearce wrote:
On Wed, 01 Sep 2010 19:44:29 +0100, wrote:

I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

I believe that having the type number clearly visible on the envelope
is the top of the list, so be careful if you decide to clean them.

d

Oh, yes, must not rub that out. :c)

Actually, I don't think the figures will rub out, I thhink they are
etched. Others will kmow.

It will rub out because I've got a pair of TT22's and the figures have
rubbed out a bit. Of course TT22's are like TT21, but 12.6v heaters.
Not used much I would not have thought for audio projects.

The classic use for KT88s is for guitar amps. If you do decide to
sell, be sure to include such groups in your announcement. They will
be very interested in emission, as has already been said. That is the
key to how much life they have left.

Also include close-up photos of the gettering area as you can usually
see if they are going soft in this region. A clear metallic shine is
what they want to see.

d

[[email protected]]

On Sep 1, 2:44*pm, Richard wrote:
I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

Is this NOS or a re-issue? Original British-made valves/tubes/lamps
are highly desirable.

[Richard[_12_]]

On 03/09/2010 06:52, wrote:
On Sep 1, 2:44 pm, wrote:
I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

Is this NOS or a re-issue? Original British-made valves/tubes/lamps
are highly desirable.

They are used. They have come out of a British commercial ham AM
transmitter, which employed a pair of GEC KT88's in the modulator (813
in final).

I'm in UK.

No idea of usage, but will test.

These GEC KT88's are a bit too good really for the (dismantled) Tx. If I
rebuild the Tx, I'd use an inferior set of KT88's, and you probably will
not notice listening to the audio over short-wave.

Will test these tubes today.

[Richard[_12_]]

On 02/09/2010 23:14, François Yves Le Gal wrote:
On Wed, 01 Sep 2010 19:44:29 +0100, wrote:

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Full test :
Shorts (none whatsoever), gas, grid emission, leakage (at least 500K and
preferably more than 2M between all elements).
Transconductance and current (nominal is 11.5 mA/V and 140 mA @ 250V platye
and -15V grid), another set of measurements @500V, a must with big bottles.
End of life tests, same as above but with reduced heater voltage (6.3V
nominal, 5.5V or so for EOL), transconductance and current should be at
least 95% of nominal.

Plus a full description. Add some nice hi rez pics showing the tubes as well
as close-ups of the getters.


These tubes have one getter, at the top.

How metallic-looking should the getter flash be? Does the flashing start
out metallic then get less metallic-looking?

Can you test for "softness"?

[PeterD]

On Fri, 03 Sep 2010 08:49:13 +0100, Richard wrote:

On 02/09/2010 23:14, François Yves Le Gal wrote:
On Wed, 01 Sep 2010 19:44:29 +0100, wrote:

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Full test :
Shorts (none whatsoever), gas, grid emission, leakage (at least 500K and
preferably more than 2M between all elements).
Transconductance and current (nominal is 11.5 mA/V and 140 mA @ 250V platye
and -15V grid), another set of measurements @500V, a must with big bottles.
End of life tests, same as above but with reduced heater voltage (6.3V
nominal, 5.5V or so for EOL), transconductance and current should be at
least 95% of nominal.

Plus a full description. Add some nice hi rez pics showing the tubes as well
as close-ups of the getters.


These tubes have one getter, at the top.

How metallic-looking should the getter flash be?

Very metallic. No whiteness at all (indicates leakage).

Does the flashing start
out metallic then get less metallic-looking?

It can, but that indicates a tube (valve) that is failing and is
likely gassy.


Can you test for "softness"?

Better testers can.

[[email protected]]

On Sep 3, 8:36*am, PeterD wrote:
On Fri, 03 Sep 2010 08:49:13 +0100, Richard wrote:
On 02/09/2010 23:14, François Yves Le Gal wrote:
On Wed, 01 Sep 2010 19:44:29 +0100, *wrote:

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Full test :
Shorts (none whatsoever), gas, grid emission, leakage (at least 500K and
preferably more than 2M between all elements).
Transconductance and current (nominal is 11.5 mA/V and 140 mA @ 250V platye
and -15V grid), another set of measurements @500V, a must with big bottles.
End of life tests, same as above but with reduced heater voltage (6.3V
nominal, 5.5V or so for EOL), transconductance and current should be at
least 95% of nominal.

Plus a full description. Add some nice hi rez pics showing the tubes as well
as close-ups of the getters.

These tubes have one getter, at the top.

How metallic-looking should the getter flash be?

Very metallic. No whiteness at all (indicates leakage).

Does the flashing start
out metallic then get less metallic-looking?

It can, but that indicates a tube (valve) that is failing and is
likely gassy.



Can you test for "softness"?

Better testers can.- Hide quoted text -

- Show quoted text -

What is the definition of "softness",may I inquire?

[Richard[_12_]]

On 02/09/2010 23:14, François Yves Le Gal wrote:
On Wed, 01 Sep 2010 19:44:29 +0100, wrote:

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Full test :
Shorts (none whatsoever), gas, grid emission, leakage (at least 500K and
preferably more than 2M between all elements).
Transconductance and current (nominal is 11.5 mA/V and 140 mA @ 250V platye
and -15V grid), another set of measurements @500V, a must with big bottles.
End of life tests, same as above but with reduced heater voltage (6.3V
nominal, 5.5V or so for EOL), transconductance and current should be at
least 95% of nominal.

Plus a full description. Add some nice hi rez pics showing the tubes as well
as close-ups of the getters.

I think there is a problem with these tubes.

I think both are gassy.

My friend tested these tubes on a VO VCM MK1, a very good tube tester.

This is what was done:

Settings:

Selector Switch Number: 026 540 310
Vf: 6
Neg Grid Volts: 30
Anode Volts: 250
Screen Volts: 250
Ia mA: 57
mA/V: 10

First round of checks for was for inter-electrode leaks. Seemed OK.

After leakage, the second thing done was to set to "Test" and read the
plate current. It should have stopped at 57 mA. Plate current went
above, to about 100mA and possibly higher.

Unless the tester controls were set wrong, these results would indicate
the tubes are gassy.

I may get another to test, just to get a second opinion, before junking
these tubes.

Of course, these tubes may not have been used for 25 years or whatever.
But probably that is of no significance.

[[email protected]]

On Sep 4, 2:46*am, Richard wrote:
On 02/09/2010 23:14, François Yves Le Gal wrote:





On Wed, 01 Sep 2010 19:44:29 +0100, *wrote:

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Full test :
Shorts (none whatsoever), gas, grid emission, leakage (at least 500K and
preferably more than 2M between all elements).
Transconductance and current (nominal is 11.5 mA/V and 140 mA @ 250V platye
and -15V grid), another set of measurements @500V, a must with big bottles.
End of life tests, same as above but with reduced heater voltage (6.3V
nominal, 5.5V or so for EOL), transconductance and current should be at
least 95% of nominal.

Plus a full description. Add some nice hi rez pics showing the tubes as well
as close-ups of the getters.

I think there is a problem with these tubes.

I think both are gassy.

My friend tested these tubes on a VO VCM MK1, a very good tube tester.

This is what was done:

Settings:

Selector Switch Number: 026 540 310
Vf: 6
Neg Grid Volts: 30
Anode Volts: 250
Screen Volts: 250
Ia mA: 57
mA/V: 10

First round of checks for was for inter-electrode leaks. Seemed OK.

After leakage, the second thing done was to set to "Test" and read the
plate current. It should have stopped at 57 mA. Plate current went
above, to about 100mA and possibly higher.

Unless the tester controls were set wrong, these results would indicate
the tubes are gassy.

I may get another to test, just to get a second opinion, before junking
these tubes.

Of course, these tubes may not have been used for 25 years or whatever.
But probably that is of no significance.- Hide quoted text -

- Show quoted text -

Does not seem to make any sense to me. If both of them are gassy,then
why is there any plate current at all? Oh,almost forgot. Power tubes
may check a bit strange on some tube testers-seen it myself,in the
past. And the best test,of course, is to plug them into a working amp
and see and hear what happens. And pray...

[[email protected]]

On Sep 5, 3:28*am, wrote:
On Sep 4, 2:46*am, Richard wrote:





On 02/09/2010 23:14, François Yves Le Gal wrote:

On Wed, 01 Sep 2010 19:44:29 +0100, *wrote:

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Full test :
Shorts (none whatsoever), gas, grid emission, leakage (at least 500K and
preferably more than 2M between all elements).
Transconductance and current (nominal is 11.5 mA/V and 140 mA @ 250V platye
and -15V grid), another set of measurements @500V, a must with big bottles.
End of life tests, same as above but with reduced heater voltage (6.3V
nominal, 5.5V or so for EOL), transconductance and current should be at
least 95% of nominal.

Plus a full description. Add some nice hi rez pics showing the tubes as well
as close-ups of the getters.

I think there is a problem with these tubes.

I think both are gassy.

My friend tested these tubes on a VO VCM MK1, a very good tube tester.

This is what was done:

Settings:

Selector Switch Number: 026 540 310
Vf: 6
Neg Grid Volts: 30
Anode Volts: 250
Screen Volts: 250
Ia mA: 57
mA/V: 10

First round of checks for was for inter-electrode leaks. Seemed OK.

After leakage, the second thing done was to set to "Test" and read the
plate current. It should have stopped at 57 mA. Plate current went
above, to about 100mA and possibly higher.

Unless the tester controls were set wrong, these results would indicate
the tubes are gassy.

I may get another to test, just to get a second opinion, before junking
these tubes.

Of course, these tubes may not have been used for 25 years or whatever.
But probably that is of no significance.- Hide quoted text -

- Show quoted text -

Does not seem to make any sense to me. If both of them are gassy,then
why is there any plate current at all? Oh,almost forgot. Power tubes
may check a bit strange on some tube testers-seen it myself,in the
past. And the best test,of course, is to plug them into a working amp
and see and hear what happens. *And pray...- Hide quoted text -

- Show quoted text -

As an afterthought: if Vg1 is set incorrectly too much Ia (mA)current
will flow . This is what may just be happening here. Maximum Ia is
quite large,140-145mA (from Genalex data sheet,1959).

[Patrick Turner]

On Sep 3, 3:37*am, Richard wrote:
On 02/09/2010 07:52, wrote:

On Sep 1, 2:44 pm, *wrote:
I'll probably sell my two KT88's.

http://en.wikipedia.org/wiki/KT88

I am taking the tubes to a friend who has an AVO VCM MK1 valve tester to
test their condition.

What will an audiophile like to know so he can have a good idea what he
or she would be buying? I want to describe the items so a buyer knows
the state of the tubes.

Thanks.

Emission.

Also, good pictures I would imagine.

I suppose folks intersted might want them selling as a pair?

I reply to all those contributing to this thread so far.

Anyone who is selling ancient KT88 or TT21 ( which are KT88
internally, but with anode top cap connection ), could be selling
something worn out and entirely worthless or something worth whatever
the going rate for genuinely NOS tubes.
Unfortunately, the whole NOS tube market is largely a huge SCAM and
buyers are regularly riped off when they get say KT88 in original
boxes which of course were carefully retained by the previous owner/
user of the damn tubes.

Most people have utterly no clue about what the tube tester tests
mean. Most certainly 99% of AUDIOPHILES are grossly ignorant about all
aspects of audio **engineering**, ie, all the difficult things you
have to know about to get a circuit to work properly with low
distortion and reliability, and which depend on a whole lot of
mathematical relationships.

I don't have a brand name tube such as an AVO etc but I have been
given several, and all were in such a terrible state of repair due to
burnt out resistors, ****ed switches, stuffed transformers et all that
I just canobolised them for the few good working parts. Unfortunately,
tube testers are easy to wreck with incorrect use, and having
incorrect switch settings, especially when testing power tubes.

I have built my own simple testers for octal output tubes and another
for preamp tubes.

A KT88 or any other octal OP tube is never going to ever be used with
Ea, Ia, and Eg2 and Eg1 as stated in the tube data sheets from the
1950s and 60s. I've NEVER seen an amp with Ea at 250V.

A KT88 should be able to be set up in a test circuit with Ea = Eg2 =
+500Vdc, and Eg1 adjusted so that the product of Ea x ( Ia + Ig2 ) =
40W, and once this idle condition is established the tube should
remain stable without any internal arcing or red glowing sections of
the anode and no bright glowing of screen wires which usually can be
carefully viewed through holes in the anode structure.

To measure Ia and Ig2, have 10 ohm wire wound reistors in series from B
+ supplies. The control grid G1 should be biased from a variable
negative supply with a series 100k resistance. After initially setting
up the tube for test one MUST immediately begin measuring the Ia, and
Ig2 whch should have a ratio of approximately 10:1, and then measure
Vdc across the 100k grid bias R. The grid bias voltage to begin with
should be between -45Vdc and -60Vdc. Tubes which are excessively gassy
will immediately have a considerable Vdc across the 100k grid bias R,
and the voltage will increase as the test is continued, so one has to
measure repeatedly to see how the tube copes over time. Really hard
vacuum brand new tubes will have a grid which is slightly more
negative than the bias supply. But aged tubes will all have a grid
which is POSITIVE with respect to the bias supply.
One can have grids up to about +1.0Vdc with respect to bias supply
before you need to be concerned - but only if the condition stays
stable over an hour. The "positive grid condition" means that there is
some Idc flow into the grid, and that also means the ac grid input
impedance is not as high as it would be if the grid were drawing
extremely low current which is the case when the tube is in excellent
condition. Usually with a high amount of positive grid condition, the
gettering will appear to have degradation which is a dulled or
oxidised apperance, ie, partially worn out.

Some tubes have cathodes which have a reduced ability to emit
electrons. To get such a tube to bias at Pda = 40W, the grid bias may
have to be set with an unusually low value of negative grid bias.

However, let us assume the static test without any ac signal applied
has proceeded without any fuses blowing and there has ben no smoke
produced.

Anyone building a tester should have variable B+ up to +500Vdc for
BOTH anode and screen grids. Once varied for the desired voltage, they
should remain stable, ie, have good regulation.

There should be a suitable SE output transformer in the anode supply
and set up as it would be used in a normal SE triode or SE tetrode
amplifier and capable of at least 15Watts of undistorted power at 30Hz
to 5kHz, with up to 100mAdc of Ia flow.

The primary inductance needs to be known so that its loading effect
under AC test conditions with and without a resistive load can be
known.

A suitable large air gapped choke may also be used in the anode
circuit and the load for the anode may be a resistance which is placed
across the ends of the choke. I use this type of arrangement with well
rated resistances and a switch to vary the ac load between 1k and 10k.

There must be a suitable low distortion ac signal source established
to drive the KT88 grid. This can usually be done using a 6SN7 or 6CG7
with the two triodes cascaded and RC coupled and made up to be able to
produce up to 70Vrms between at least 30Hz and 5kHz at less than 1%
THD.
( If youse cannot figure out how to to all this, try going fishing,
writing poetry, etc, but lemme tellya, it takes TIME + a boring lot of
discipline to really understand your vacuum tubes, or those tubes you
might wanna buy. When I'm offered tubes I only wanny pay for them
AFTER I HAVE TESTED THEM, and all this takes TIME, and the seller
needs to realise there is some effort involved to verify "tube
goodness.")

If anyone knows anything about power tubes, they will know that it is
FOOLISH to set then up so they will have Pda = rated maximum at the
idle condition. For KT88, Pda max = 42W, and in most PP amps the tubes
may well have Pda at idle = 25W.

In a guitar amp or in many hi-fi amps Ea = Eg2 and both may well be a
typical +470Vdc, so that Ia might be about 45mA.
If one has a 10 ohm resistance between cathode and 0V, we can measure
the cathode current which = Ia + Ig2, and this is often about 50mA
where Ea = say 470Vdc.

We should want to test the KT88 to see how it performs with signals
and measure its gain and distortion with various loads and levels.
KT88 and other beam power tubes or power pentodes can appear to have
good emmision and they may "bias up OK" and have dc set up conditions
that look healthy. But they may have endured a few "red hot anode"
events where the Pda has been allowed to considerably exceed the max
rated amount for longer than a minute. This can occur where there is a
sick low Z speaker used, or there has been a shorted speaker cable.
Ths kinds **** happens even amoung audiophiles, and more often than
they would ever admit. The results of such excessively high Pda
"trauma events" can result in permanent damage to the screen grid
wires which will bend and change position with the excessive heat. If
the screen wires have significanly become mis-aligned with the control
grid wires the Ig2 dc is usually rather too high, but this goes un-
noticed in many Ultralinear amps.
Usually when the screen wires are badly bent the tubes only can make a
fraction of the maximum clipping power which should be possible.
The testing I am talking about will reveal all.

OK, so let us say we have set up the KT88 with Ea = Eg2 = 470V and
bias current gives idel Pda = 25W.

The signal testing is done to find out the maximum pure class A1 power
available and for this the tube tester id rather useless but we would
need a couple of good meters and an OSCILLOSCOPE, and a sine wave
signal source. The sine wave source should at least be a fixed
frequency type giving 500Hz or therabouts which allows accurate
measurements using a cheap digital multimeter. The oscilloscope need
only be a low bandwidth single trace job.

Suitable arrangements MUST be made to make safe measurements with no
risk of allowing unsafe or damaging ac voltage pulses into delicate
inputs of solid state type oscilloscopes or cheap DMM. Much knowledge
and trade practice must be so far learnt in order to get to this stage
of the procedure.
The Vac anode swing during testing below clipping levels will range
between +/- 470Vpeak = 332Vrms, and if we are to measure this voltage
from the anode circuit we will need to reduce its level by say a
factor of 1/10 to a harmless maximum of 33Vrms. For this we would use
a 1,000Vdc rated cap off the anode followed by a resistance divider
of 90k + 10k to 0V, so that the Vac is measured across the 10k. The
divider will slightly load the anode circuit as well as the primary
inductive reactance, and we must remember such things as we proceed to
measure voltage gains with various added resistance loads to calculate
the tube parameters of Ra, µ, and gm.

Just be warned, testing vacuum tubes can kill the person testing the
tubes - easily!


Now it should be known that the maximum anode efficiency of any power
beam tetrode or pentode is about 45%.
This assumes the screen is kept at a fixed +Vdc. Triode operation
would give maybe 30% efficiency maximum.
Let us consider beam or pentode operation.....
In this test case, the idle power drawn from the PSU = 470Vdc x 45mAdc
= 21.15W. If efficiency = 45%, then we should extract a maximum audio
output power = 0.45 x 21.1W = 9.45W and measured as coming directly
from the anode, and this will be without clipping, although we will
find the distortion may well be up to 12% with multiple harmonics just
before obvious clipping.
Such distortion and onset of clipping will be obvious on the
oscilliscope.

Let us assume we could get 9.45W of output power and with Ia = 45mA.

From Ohm we know P = I squared x R.

In a single device class A amp the load I swing will be between 0.0
amps and twice the idle current. In effect Load IRL = 0.707 x Idc at
idle and in this case = 0.0318Arms.

So we have 9.45W = 0.0318 x 0.0318 x RL.

So RL = 9,337 ohms.

Now let us say we have a test signal frequency = 500Hz and the loading
choke inductance is say 20H, then the reactance = 500 x 6.28 x 20 =
62.8k ohms. This may be the calculated value, but in the real world we
would try a small signal with the tube and without any added RL and
measure the Iac current which equals the choke ac current, and measure
the anode Vac and then the choke load = Vac across choke / Iac flow
through choke.

If we have resistance divider of 100k to measure anode Vac, then we
have maybe a summed total of approximately 50k ohms which loads the
anode in the absense of any other added loading. If we were to add a
20W rated 10k resistance across the choke to load the anode, the
actual load = 10k in parallel with say 50k, giving a total of 8.33k,
and maybe a bit too low for an ideal load for maximum power, bearing
in mind that the maximum PO of 9.45W is only available at ONE value of
load and that at all other load values the PO max at clipping will be
LESS.

So in this case we might use a 12k RL across the choke and total load
= 9.677k, and it is close to what we expect to find should produce
maximum PO at the Ea and Ia idle conditions.

Also from Mr Ohm we know E = I x R, so that if the P = 9.4W, and RL =
9.33k, we should expect to measure Vac = 0.0318A x 9,330 ohms =
296Vrms, which is +/-420Vpeak.

The distortion will affect the figures slightly, but when testing the
tube the above described voltages should be seen, and if the anode Vac
is only half what is expected when the distortion becomes maybe 30%,
and using the right load value then the tube is STUFFED, and should be
binned. A comparison should be made with a couple of known good tubes.

But we ain't finished yet.

We want to find out the Ra, µ, and gm. The KT88 data suggests Ra =
18k ohms, gm = 11mA/V, and µ is not even mentioned.
I can tell you all that you will never ever measure any KT88 and get
the Ra and gm as per data unless you have Ea and EG2 at around 250Vdc
and Ia at around 140mAdc, and this info is rather useless because we
never run any such tubes in amps at these Ea and Ia conditions.

We can find out Ra and gm and µ and at the conditions we typically use
as in out test case conditions I have described above.

For these tests we would use the original load value of whatever it
actually was, say 9.67k, and a second load value of say half this =
4.54k which would be a 5.0k across the choke plus the other divider
and choke load.

We will apply an input signal and bring up the anode voltage to say
50Vrms with the higher load. We will record the grid input signal and
anode signal and the voltage gain.
The test is repeated using the same grid input Vac but with RL =
4.54k, and the anode Vac and gain all recorded.
The anode Vac and voltage gain will be lower with the lower RL.

A good exercize book and sharp pencil is needed. Those who expect to
use a keyboard only and a PC and arm chair may leave the classroom now
and proceed to a kindergarten where neat and tidy use of pencil and
paper may still be taught.

For all tubes, Voltage gain, A, = Anode Vac / Grid Vac = µ x RL / (Ra
+ RL).

Therefore µ = A x ( Ra + RL ) / RL.

BTW, µ is greek "mu" and is amplification factor and equals the
voltage gain with utterly no RL loading the anode circuit.

If we have TWO loads, RL1, and RL2, and we have two sets of recorded
anode voltages and voltage gains, then we can establish two equations.
For each equation the µ and Ra will be identical and so we will get...

µ = A1 x ( Ra + RL1) / RL1 = A2 x ( Ra + RL2 ) / RL2.

From here on the person wanting to find out actual working values of
Ra and µ can proceed to an answer because they will understand the use
of very basic high school mathematics. I don't have time for coaching
you in basics right now.

Now for all tubes, µ = gm x Ra, so gm , or transconductance, = µ / Ra.

Now I can tell you that an approximate set of values you should see if
the emission is OK and there isn't much wrong with the KT88. BTW, the
values will be similar to those gained with Russian KT88, or 6550.

You should see that Ra will be around 32,000 ohms, and gm = 5mA/V, and
µ = 160.

These are radically different from quoted data figures, but you must
realise that as Ia decreases the gm decreases, with Ea change making
little difference to either µ or gm as Ra is such a high value.

The ratio of Ra to RL in a class A1 SE tetrode amp will be about 3:1
or more depending on load value and Ia and Ea conditions.
For loading knowledge go to my website, http://www. turneraudio.com.au
and follow your nose.

Of course many PP amps will have tubes set up as I have described
above but the RLa-a is maybe only 6k.
This means the class A load at low levels for each tube is 3k and the
ratio of Ra to RL is thus around 10:1 which is in effect a very poor
damping factor of 0.1. Hence the need for at least 20dB of global or
other NFB to get the effective ratio between Ra and the anode load to
be 1:10, which is a damping factor of 10.

But we only want to know if the KT88 are good or not good, and I don't
have this info at my site. Here it is, and if you really wanna know
about it all, you must get real busy.

Patrick Turner.

[Richard[_12_]]

On 05/09/2010 12:33, Patrick Turner wrote:




Most people have utterly no clue about what the tube tester tests
mean.

Patrick Turner.

Let's just say that I wanted to make a dedicated tube tester for my
KT88's. Not just test the tubes, but to enjoy the experience.

Okay, let's take this step by step and let's just concentrate on each
step. Let's just deal with steps 1 and 2 for now.

Of course, if the tube is faulty it may turn up in the early stages of
testing and testing would stop early on.

STEP 1: Measure heater continuity.

If an open condition - stop testing.


STEP 2: Electrode leakage (Cold). If failure stop test.

What electrodes are tested? And what voltages? AC or DC?

a) Heater to Cathode (What Voltage?)

b) Cathode to G1 (What voltage?)

c) Cathode to G2 (What voltage?)

d) Cathode to G3 (What voltage?)

e) Cathode to Anode (What voltage?)

etc.

I'm not actually sure which electrodes should be tested in relation to
other electrodes. I believe some electrodes would be connected together
in the leakage tests. Anyone know all about the leakage part of testing?

Can anyone give me an idea of a suitable circuit (components) just to
test KT88's for leakage? A circuit that protects a meter.

After leakage is sorted, then I can go to another specific step in testing.

[Richard[_12_]]

On 05/09/2010 15:55, Richard wrote:
On 05/09/2010 12:33, Patrick Turner wrote:
[..]
I'm not actually sure which electrodes should be tested in relation to
other electrodes. I believe some electrodes would be connected together
in the leakage tests. Anyone know all about the leakage part of testing?

Can anyone give me an idea of a suitable circuit (components) just to
test KT88's for leakage? A circuit that protects a meter.

After leakage is sorted, then I can go to another specific step in testing.

What should STEP 3 be?

After testing for leakage, I think it should be a test to establish that
the tube is not gassy. What do folks say about this? And if so, what is
the test set-up?

[[email protected]]

On Sep 5, 11:55*am, Richard wrote:
On 05/09/2010 15:55, Richard wrote:

On 05/09/2010 12:33, Patrick Turner wrote:
[..]
I'm not actually sure which electrodes should be tested in relation to
other electrodes. I believe some electrodes would be connected together
in the leakage tests. Anyone know all about the leakage part of testing?

Can anyone give me an idea of a suitable circuit (components) just to
test KT88's for leakage? A circuit that protects a meter.

After leakage is sorted, then I can go to another specific step in testing.

What should STEP 3 be?

After testing for leakage, I think it should be a test to establish that
the tube is not gassy. What do folks say about this? And if so, what is
the test set-up?

Since this is becoming a little project (to say the least), building a
dedicated test apparatus for KT88 is certain to be exciting,
refreshing and most importantly- fun. I don't recommend using
voltages of 500V,though. Electrocution is a fact of life.

[Richard[_12_]]

On 05/09/2010 15:55, Richard wrote:
On 05/09/2010 12:33, Patrick Turner wrote:




Most people have utterly no clue about what the tube tester tests
mean.

Patrick Turner.

Let's just say that I wanted to make a dedicated tube tester for my
KT88's. Not just test the tubes, but to enjoy the experience.

Okay, let's take this step by step and let's just concentrate on each
step. Let's just deal with steps 1 and 2 for now.

Of course, if the tube is faulty it may turn up in the early stages of
testing and testing would stop early on.

STEP 1: Measure heater continuity.

If an open condition - stop testing.


STEP 2: Electrode leakage (Cold). If failure stop test.

What electrodes are tested? And what voltages? AC or DC?

a) Heater to Cathode (What Voltage?)

b) Cathode to G1 (What voltage?)

c) Cathode to G2 (What voltage?)

d) Cathode to G3 (What voltage?)

e) Cathode to Anode (What voltage?)

etc.

I'm not actually sure which electrodes should be tested in relation to
other electrodes. I believe some electrodes would be connected together
in the leakage tests. Anyone know all about the leakage part of testing?

Can anyone give me an idea of a suitable circuit (components) just to
test KT88's for leakage? A circuit that protects a meter.

After leakage is sorted, then I can go to another specific step in testing.

To try to answer my own questions:

I see that the data sheet mentions 4 maximum voltage ratings (ignoring
triode connection):

Va 800V (presumably between k and a)
Vg2 600V (presumably between k and g2)
Vh-k 250V
-Vg1 200V (presumably betwen k and g1)

So, we have:

TEST LEAKAGE VOLTAGES

a) Heater to Cathode 250V

b) Cathode to G1 200V

c) Cathode to G2 600V

d) Cathode to Anode 800V

But I think there are some few other leakage tests connections that I've
missed.

Any comments?

[[email protected]]

On Sep 6, 3:26*am, Richard wrote:
On 05/09/2010 15:55, Richard wrote:





On 05/09/2010 12:33, Patrick Turner wrote:

Most people have utterly no clue about what the tube tester tests
mean.

Patrick Turner.

Let's just say that I wanted to make a dedicated tube tester for my
KT88's. Not just test the tubes, but to enjoy the experience.

Okay, let's take this step by step and let's just concentrate on each
step. Let's just deal with steps 1 and 2 for now.

Of course, if the tube is faulty it may turn up in the early stages of
testing and testing would stop early on.

STEP 1: Measure heater continuity.

If an open condition - stop testing.

STEP 2: Electrode leakage (Cold). If failure stop test.

What electrodes are tested? And what voltages? AC or DC?

a) Heater to Cathode (What Voltage?)

b) Cathode to G1 (What voltage?)

c) Cathode to G2 (What voltage?)

d) Cathode to G3 (What voltage?)

e) Cathode to Anode (What voltage?)

etc.

I'm not actually sure which electrodes should be tested in relation to
other electrodes. I believe some electrodes would be connected together
in the leakage tests. Anyone know all about the leakage part of testing?

Can anyone give me an idea of a suitable circuit (components) just to
test KT88's for leakage? A circuit that protects a meter.

After leakage is sorted, then I can go to another specific step in testing.

To try to answer my own questions:

I see that the data sheet mentions 4 maximum voltage ratings (ignoring
triode connection):

Va 800V *(presumably between k and a)
Vg2 600V (presumably between k and g2)
Vh-k 250V
-Vg1 *200V (presumably betwen k and g1)

So, we have:

* TEST LEAKAGE VOLTAGES

a) Heater to Cathode 250V

b) Cathode to G1 200V

c) Cathode to G2 600V

d) Cathode to Anode 800V

But I think there are some few other leakage tests connections that I've
missed.

Any comments?- Hide quoted text -

- Show quoted text -

Va is measured to ground,as far as I always knew. Also known as B+.
And doubt if 800 VDC will do any good for a 50 year-old tube !

[Richard[_12_]]

On 06/09/2010 23:18, François Yves Le Gal wrote:
On Mon, 06 Sep 2010 08:26:45 +0100, wrote:
Any comments?

Don't go above 450 or 500v using standard tube sockets unless you love
fireworks.

Download some manuals from Bama or similar sources and have a look at how
Hicock, B&K, Eico and others measured leakage. IMO the B&K 650 (or above)
and Eico 667 approaches are worth serious consideration.

Model 650 manual does not tell what the leakage tests are exactly.

Eico manual on page 10 (as per print on manual) shows some leakage
tests. It seems that leakage tests with this tester is simply in the
main, to test the electrode in question, against all the other
electrodes as per the diagrams.

c to heater (all others strapped together)

g1 to all

g2 to all

a to all

Is this satisfactory?

As to voltages, yes, no need to test at maximum voltages as per KT88
data sheet.

As to a leakage tester, I'm mulling the idea making a motor-driven one
using an Evershed and Vignoles Megger:

http://www.richardsradios.co.uk/megger.html

That will give me an instrument with a continuous test voltage range or
whatever steps I choose. From about 100V right up to about 600V. I just
need a geared dc motor.

[Patrick Turner]

On Sep 6, 12:55*am, Richard wrote:
On 05/09/2010 12:33, Patrick Turner wrote:





Most people have utterly no clue about what the tube tester tests
mean.

Patrick Turner.

Let's just say that I wanted to make a dedicated tube tester for my
KT88's. Not just test the tubes, but to enjoy the experience.

Well, feel free to enjoy the experience.

Tube testers used to be a common peice of quite expensive test gear
owned by service dudes.

If a television arrived on their bench they could test all 27 vacuum
tubes to see if they were serviceable within an hour.
The alternative was difficult because it involved lots of probings of
the working circuit and use of much other gear and the analysis took
much longer.

But audio circuits are dead simple compared to much other TV circuitry
operating at RF and with weird signals within.

Now that nearly all service ppl have retired, expired, died, their
piles of old test gear are for sale as old cheap junk ( if their
surviving wives have not chucked it all in the dumpster bin ). Not
many folk value the old junk, except for some audio amateurs or
audiophiles who can afford such junky old gear which sells for a price
that is a tiny fraction of the real cost of the unit in 1960.


Okay, let's take this step by step and let's just concentrate on each
step. Let's just deal with steps 1 and 2 for now.

Of course, if the tube is faulty it may turn up in the early stages of
testing and testing would stop early on.

STEP 1: Measure heater continuity.

If an open condition - stop testing.

A DMM can test the heater continuity faster than you can say "vacuum".

STEP 2: Electrode leakage (Cold). If failure stop test.

This is unusual. Usually leakage mostly happens when a tube is
heated.


What electrodes are tested? And what voltages? AC or DC?

When testing any audio tube in a circuit prepared for the purpose or
in an existing amplifier all electrode voltages applied are recorded,
both Vac and Vdc, and all current flows anywhere are analysed and
recorded. This is done in a manner where the person testing is on
guard for any wayward tube conducting too much current anywhere. So
one has to learn to be alert and measure the rise of cathode currents
right after turn on and all test circuits should feature suitable
sensitive fuse values to protect the testing circuits and PSU from
undue stress, and most certainly against the antics of a bamboozled
audiophile wondering just why the power tranny of his tester PSU or
precious amplifier is getting do damn hot.
After some practice, with some possible clouds of smoke and fused test
gear parts, the audio amateur might get to understand what he is
testing and what he is doing so that when he probes or measures
anything, he KNOWS what he SHOULD SEE on his meters and on his CRO,
( oscilloscope ).

Electronics testing is a great teacher of mental discipline and
applied logic, and one should learn to ask many questions of the gear
and of yourself and to never assume anything. Departure from
discipline leads to smoke.

a) Heater to Cathode (What Voltage?)

b) Cathode to G1 (What voltage?)

c) Cathode to G2 (What voltage?)

d) Cathode to G3 (What voltage?)

e) Cathode to Anode (What voltage?)

etc.

I'm not actually sure which electrodes should be tested in relation to
other electrodes. I believe some electrodes would be connected together
in the leakage tests. Anyone know all about the leakage part of testing?

Can anyone give me an idea of a suitable circuit (components) just to
test KT88's for leakage? A circuit that protects a meter.

After leakage is sorted, then I can go to another specific step in testing.

Leakage is only one thing might worry about before, during and after
one tests a given OP tube.

All the other basics must be learnt.

I started by building a preamp from scratch but froma circuit a good
bloke I knew gave me. Then I found myself asking 1,001 questions and I
could not rest easily unless the full answers came as a result of
reading good books on tube audio and carrying out experiments in my
shed behind my house for a couple of year fulls of frustration and
very late evenings.

I acquired the shelf of good books, and I done my home work, and all
before ever getting a PC and talking about the learning process with
others. I just DID IT. I moved onto repairing giutar amps for a
couple of years and this taught me about every possible way a vacuum
tube could misbehave.

But musicians are always broke, and I had had a dream about my future.
I dreampt I would get to know all there was to know about tubed audio
gear and that I would build and sell new gear second to none
elsewhere. So I soldered onwards, and I am still learning, although
sometimes I think now I am trying not to forget what I have begun to
understand, before I become senile, or before my hard drives crash yet
again, and take with them a nice new website page which is better than
one I have aleardy established.

Unless an audiophile has a visionary mission plan he may dither around
and never seem to get anywhere. I know guys who have made large
complex stereo systems with 4 amplifiers per channel and with active
crossovers and they are constantly changeing things and they
constantly complaining about noise and distortion and bad channel
balance and they don't want to spend money on a tradesman to fix
things and even after 5 years they cannot use a digital voltmeter and
nor do they seem to have the ability to think about things concisely
or logically. I recall the joke about Californians. "How is a light
bulb changed in Carlifornia?" Well, it takes 20 people gathered for a
happening and a pile of LSD tabs, and man, wow, what an experience -
especially when the house caught fire.

Testing a KT88 is not rocket science and not terribly dangerous IF ONE
TAKES PRECAUTIONS.

But one has to gradually work up to testing a KT88, IMHO.

I would suggest those interested should first spend time with basic
transistor circuits using supply rails less than +/- 40Vdc. Build a
preamp. Get it working. Analyse why it works. find out why the first
three transistors you connected failed and fused so easily. Write out
your thesis about why so someone else can read it and understand it.

Then move onto a triode preamp with the dangerous voltages. Almost
nobody makes such a move without getting an electrical shock or
without wrecking something. But the man who makes no mistakes never
builds anything. And the man who reckons he knows it all really knows
nothing.

Finally, move up to power tubes and power amps and testing output
tubes.

Patrick Turner.

[[email protected]]

On Sep 7, 7:31*am, Patrick Turner wrote:
On Sep 6, 12:55*am, Richard wrote:

On 05/09/2010 12:33, Patrick Turner wrote:

Most people have utterly no clue about what the tube tester tests
mean.

Patrick Turner.

Let's just say that I wanted to make a dedicated tube tester for my
KT88's. Not just test the tubes, but to enjoy the experience.

Well, feel free to enjoy the experience.

Tube testers used to be a common peice of quite expensive test gear
owned by service dudes.

If a television arrived on their bench they could test all 27 vacuum
tubes to see if they were serviceable within an hour.
The alternative was difficult because it involved lots of probings of
the working circuit and use of much other gear and the analysis took
much longer.

But audio circuits are dead simple compared to much other TV circuitry
operating at RF and with weird signals within.

Now that nearly all service ppl have retired, expired, died, their
piles of old test gear are for sale as old cheap junk ( if their
surviving wives have not chucked it all in the dumpster bin ). Not
many folk value the old junk, except for some audio amateurs or
audiophiles who can afford such junky old gear which sells for a price
that is a tiny fraction of the real cost of the unit in 1960.

Okay, let's take this step by step and let's just concentrate on each
step. Let's just deal with steps 1 and 2 for now.

Of course, if the tube is faulty it may turn up in the early stages of
testing and testing would stop early on.

STEP 1: Measure heater continuity.

If an open condition - stop testing.

A DMM can test the heater continuity faster than you can say "vacuum".



STEP 2: Electrode leakage (Cold). If failure stop test.

This is unusual. Usually leakage mostly happens when a tube is
heated.

What electrodes are tested? And what voltages? AC or DC?

When testing any audio tube in a circuit prepared for the purpose or
in an existing amplifier all electrode voltages applied are recorded,
both Vac and Vdc, and all current flows anywhere are analysed and
recorded. This is done in a manner where the person testing is on
guard for any wayward tube conducting too much current anywhere. So
one has to learn to be alert and measure the rise of cathode currents
right after turn on and all test circuits should feature suitable
sensitive fuse values to protect the testing circuits and PSU from
undue stress, and most certainly against the antics of a bamboozled
audiophile wondering just why the power tranny of his tester PSU or
precious amplifier is getting do damn hot.
After some practice, with some possible clouds of smoke and fused test
gear parts, the audio amateur might get to understand what he is
testing and what he is doing so that when he probes or measures
anything, he KNOWS what he SHOULD SEE on his meters and on his CRO,
( oscilloscope ).

Electronics testing is a great teacher of mental discipline and
applied logic, and one should learn to ask many questions of the gear
and of yourself and to never assume anything. Departure from
discipline leads to smoke.





a) Heater to Cathode (What Voltage?)

b) Cathode to G1 (What voltage?)

c) Cathode to G2 (What voltage?)

d) Cathode to G3 (What voltage?)

e) Cathode to Anode (What voltage?)

etc.

I'm not actually sure which electrodes should be tested in relation to
other electrodes. I believe some electrodes would be connected together
in the leakage tests. Anyone know all about the leakage part of testing?

Can anyone give me an idea of a suitable circuit (components) just to
test KT88's for leakage? A circuit that protects a meter.

After leakage is sorted, then I can go to another specific step in testing.

Leakage is only one thing might worry about before, during and after
one tests a given OP tube.

All the other basics must be learnt.

I started by building a preamp from scratch but froma circuit a good
bloke I knew gave me. Then I found myself asking 1,001 questions and I
could not rest easily unless the full answers came as a result of
reading good books on tube audio and carrying out experiments in my
shed behind my house for a couple of year fulls of frustration and
very late evenings.

I acquired the shelf of good books, and I done my home work, and all
before ever getting a PC and talking about the learning process with
others. I just DID IT. *I moved onto repairing giutar amps for a
couple of years and this taught me about every possible way a vacuum
tube could misbehave.

But musicians are always broke, and I had had a dream about my future.
I dreampt I would get to know all there was to know about tubed audio
gear and that I would build and sell new gear second to none
elsewhere. So I soldered onwards, and I am still learning, although
sometimes I think now I am trying not to forget what I have begun to
understand, before I become senile, or before my hard drives crash yet
again, and take with them a nice new website page which is better than
one I have aleardy established.

Unless an audiophile has a visionary mission plan he may dither around
and never seem to get anywhere. I know guys who have made large
complex stereo systems with 4 amplifiers per channel and with active
crossovers and they are constantly changeing things and they
constantly complaining about noise and distortion and bad channel
balance and they don't want to spend money on a tradesman to fix
things and even after 5 years they cannot use a digital voltmeter and
nor do they seem to have the ability to think about things concisely
or logically. I recall the joke about Californians. "How is a light
bulb changed in Carlifornia?" Well, it takes 20 people gathered for a
happening and a pile of LSD tabs, and man, wow, what an experience -
especially when the house caught fire.

Testing a KT88 is not rocket science and not terribly dangerous IF ONE
TAKES PRECAUTIONS.

But one has to gradually work up to testing a KT88, IMHO.

*I would suggest those interested should first spend time with basic
transistor circuits using supply rails less than +/- 40Vdc. Build a
preamp. Get it working. Analyse why it works. find out why the first
three transistors you connected failed and fused so easily. Write out
your thesis about why so someone else can read it and understand it.

Then move onto a triode preamp with the dangerous voltages. Almost
nobody makes such a move without getting an electrical shock or
without wrecking something. But the man who makes no mistakes never
builds anything. And the man who reckons he knows it all really knows
nothing.

Finally, move up to power tubes and power amps and testing output
tubes.

Patrick Turner.- Hide quoted text -

- Show quoted text -

And that's the way it is...

[Richard[_12_]]

On 08/09/2010 05:34, wrote:
On Sep 7, 7:31 am, Patrick wrote:
On Sep 6, 12:55 am, wrote:

On 05/09/2010 12:33, Patrick Turner wrote:

Most people have utterly no clue about what the tube tester tests
mean.

Patrick Turner.

Let's just say that I wanted to make a dedicated tube tester for my
KT88's. Not just test the tubes, but to enjoy the experience.

Well, feel free to enjoy the experience.

Tube testers used to be a common peice of quite expensive test gear
owned by service dudes.

If a television arrived on their bench they could test all 27 vacuum
tubes to see if they were serviceable within an hour.
The alternative was difficult because it involved lots of probings of
the working circuit and use of much other gear and the analysis took
much longer.

But audio circuits are dead simple compared to much other TV circuitry
operating at RF and with weird signals within.

Now that nearly all service ppl have retired, expired, died, their
piles of old test gear are for sale as old cheap junk ( if their
surviving wives have not chucked it all in the dumpster bin ). Not
many folk value the old junk, except for some audio amateurs or
audiophiles who can afford such junky old gear which sells for a price
that is a tiny fraction of the real cost of the unit in 1960.

Okay, let's take this step by step and let's just concentrate on each
step. Let's just deal with steps 1 and 2 for now.

Of course, if the tube is faulty it may turn up in the early stages of
testing and testing would stop early on.

STEP 1: Measure heater continuity.

If an open condition - stop testing.

A DMM can test the heater continuity faster than you can say "vacuum".



STEP 2: Electrode leakage (Cold). If failure stop test.

This is unusual. Usually leakage mostly happens when a tube is
heated.

What electrodes are tested? And what voltages? AC or DC?

When testing any audio tube in a circuit prepared for the purpose or
in an existing amplifier all electrode voltages applied are recorded,
both Vac and Vdc, and all current flows anywhere are analysed and
recorded. This is done in a manner where the person testing is on
guard for any wayward tube conducting too much current anywhere. So
one has to learn to be alert and measure the rise of cathode currents
right after turn on and all test circuits should feature suitable
sensitive fuse values to protect the testing circuits and PSU from
undue stress, and most certainly against the antics of a bamboozled
audiophile wondering just why the power tranny of his tester PSU or
precious amplifier is getting do damn hot.
After some practice, with some possible clouds of smoke and fused test
gear parts, the audio amateur might get to understand what he is
testing and what he is doing so that when he probes or measures
anything, he KNOWS what he SHOULD SEE on his meters and on his CRO,
( oscilloscope ).

Electronics testing is a great teacher of mental discipline and
applied logic, and one should learn to ask many questions of the gear
and of yourself and to never assume anything. Departure from
discipline leads to smoke.





a) Heater to Cathode (What Voltage?)

b) Cathode to G1 (What voltage?)

c) Cathode to G2 (What voltage?)

d) Cathode to G3 (What voltage?)

e) Cathode to Anode (What voltage?)

etc.

I'm not actually sure which electrodes should be tested in relation to
other electrodes. I believe some electrodes would be connected together
in the leakage tests. Anyone know all about the leakage part of testing?

Can anyone give me an idea of a suitable circuit (components) just to
test KT88's for leakage? A circuit that protects a meter.

After leakage is sorted, then I can go to another specific step in testing.

Leakage is only one thing might worry about before, during and after
one tests a given OP tube.

All the other basics must be learnt.

I started by building a preamp from scratch but froma circuit a good
bloke I knew gave me. Then I found myself asking 1,001 questions and I
could not rest easily unless the full answers came as a result of
reading good books on tube audio and carrying out experiments in my
shed behind my house for a couple of year fulls of frustration and
very late evenings.

I acquired the shelf of good books, and I done my home work, and all
before ever getting a PC and talking about the learning process with
others. I just DID IT. I moved onto repairing giutar amps for a
couple of years and this taught me about every possible way a vacuum
tube could misbehave.

But musicians are always broke, and I had had a dream about my future.
I dreampt I would get to know all there was to know about tubed audio
gear and that I would build and sell new gear second to none
elsewhere. So I soldered onwards, and I am still learning, although
sometimes I think now I am trying not to forget what I have begun to
understand, before I become senile, or before my hard drives crash yet
again, and take with them a nice new website page which is better than
one I have aleardy established.

Unless an audiophile has a visionary mission plan he may dither around
and never seem to get anywhere. I know guys who have made large
complex stereo systems with 4 amplifiers per channel and with active
crossovers and they are constantly changeing things and they
constantly complaining about noise and distortion and bad channel
balance and they don't want to spend money on a tradesman to fix
things and even after 5 years they cannot use a digital voltmeter and
nor do they seem to have the ability to think about things concisely
or logically. I recall the joke about Californians. "How is a light
bulb changed in Carlifornia?" Well, it takes 20 people gathered for a
happening and a pile of LSD tabs, and man, wow, what an experience -
especially when the house caught fire.

Testing a KT88 is not rocket science and not terribly dangerous IF ONE
TAKES PRECAUTIONS.

But one has to gradually work up to testing a KT88, IMHO.

I would suggest those interested should first spend time with basic
transistor circuits using supply rails less than +/- 40Vdc. Build a
preamp. Get it working. Analyse why it works. find out why the first
three transistors you connected failed and fused so easily. Write out
your thesis about why so someone else can read it and understand it.

Then move onto a triode preamp with the dangerous voltages. Almost
nobody makes such a move without getting an electrical shock or
without wrecking something. But the man who makes no mistakes never
builds anything. And the man who reckons he knows it all really knows
nothing.

Finally, move up to power tubes and power amps and testing output
tubes.

Patrick Turner.- Hide quoted text -

- Show quoted text -

And that's the way it is...

Okay. But - I'm just interested in a very small area of tube testing
right now. Static leakage tests between the electrodes with cathode hot
and cold, no other tests at all. Nothing else right now is on my mind
apart from current leakage between electrodes in other words - shorts.

I believe I can use a megger to tell me about shorts between electrodes.

Now, it seems to me that to really test for shorts we ought to put
serious voltages on the electrodes. And from what I gather the tests
should be for a KT88:

a) c to h 250V

b) g1 to all 200V

c) g2 to all 600V

d) a to all 800V

I can motorise my megger and set these test voltages.

I can test cold clearly, but what gets me though is testing for shorts
hot. Can I actually test for short with a megger when I'm running the
tube hot?

[Richard[_12_]]

On 08/09/2010 17:52, Richard wrote:
On 08/09/2010 05:34, wrote:
On Sep 7, 7:31 am, Patrick wrote:
On Sep 6, 12:55 am, wrote:

On 05/09/2010 12:33, Patrick Turner wrote:

Most people have utterly no clue about what the tube tester tests
mean.

Patrick Turner.

Let's just say that I wanted to make a dedicated tube tester for my
KT88's. Not just test the tubes, but to enjoy the experience.

Well, feel free to enjoy the experience.

Tube testers used to be a common peice of quite expensive test gear
owned by service dudes.

If a television arrived on their bench they could test all 27 vacuum
tubes to see if they were serviceable within an hour.
The alternative was difficult because it involved lots of probings of
the working circuit and use of much other gear and the analysis took
much longer.

But audio circuits are dead simple compared to much other TV circuitry
operating at RF and with weird signals within.

Now that nearly all service ppl have retired, expired, died, their
piles of old test gear are for sale as old cheap junk ( if their
surviving wives have not chucked it all in the dumpster bin ). Not
many folk value the old junk, except for some audio amateurs or
audiophiles who can afford such junky old gear which sells for a price
that is a tiny fraction of the real cost of the unit in 1960.

Okay, let's take this step by step and let's just concentrate on each
step. Let's just deal with steps 1 and 2 for now.

Of course, if the tube is faulty it may turn up in the early stages of
testing and testing would stop early on.

STEP 1: Measure heater continuity.

If an open condition - stop testing.

A DMM can test the heater continuity faster than you can say "vacuum".



STEP 2: Electrode leakage (Cold). If failure stop test.

This is unusual. Usually leakage mostly happens when a tube is
heated.

What electrodes are tested? And what voltages? AC or DC?

When testing any audio tube in a circuit prepared for the purpose or
in an existing amplifier all electrode voltages applied are recorded,
both Vac and Vdc, and all current flows anywhere are analysed and
recorded. This is done in a manner where the person testing is on
guard for any wayward tube conducting too much current anywhere. So
one has to learn to be alert and measure the rise of cathode currents
right after turn on and all test circuits should feature suitable
sensitive fuse values to protect the testing circuits and PSU from
undue stress, and most certainly against the antics of a bamboozled
audiophile wondering just why the power tranny of his tester PSU or
precious amplifier is getting do damn hot.
After some practice, with some possible clouds of smoke and fused test
gear parts, the audio amateur might get to understand what he is
testing and what he is doing so that when he probes or measures
anything, he KNOWS what he SHOULD SEE on his meters and on his CRO,
( oscilloscope ).

Electronics testing is a great teacher of mental discipline and
applied logic, and one should learn to ask many questions of the gear
and of yourself and to never assume anything. Departure from
discipline leads to smoke.





a) Heater to Cathode (What Voltage?)

b) Cathode to G1 (What voltage?)

c) Cathode to G2 (What voltage?)

d) Cathode to G3 (What voltage?)

e) Cathode to Anode (What voltage?)

etc.

I'm not actually sure which electrodes should be tested in relation to
other electrodes. I believe some electrodes would be connected together
in the leakage tests. Anyone know all about the leakage part of
testing?

Can anyone give me an idea of a suitable circuit (components) just to
test KT88's for leakage? A circuit that protects a meter.

After leakage is sorted, then I can go to another specific step in
testing.

Leakage is only one thing might worry about before, during and after
one tests a given OP tube.

All the other basics must be learnt.

I started by building a preamp from scratch but froma circuit a good
bloke I knew gave me. Then I found myself asking 1,001 questions and I
could not rest easily unless the full answers came as a result of
reading good books on tube audio and carrying out experiments in my
shed behind my house for a couple of year fulls of frustration and
very late evenings.

I acquired the shelf of good books, and I done my home work, and all
before ever getting a PC and talking about the learning process with
others. I just DID IT. I moved onto repairing giutar amps for a
couple of years and this taught me about every possible way a vacuum
tube could misbehave.

But musicians are always broke, and I had had a dream about my future.
I dreampt I would get to know all there was to know about tubed audio
gear and that I would build and sell new gear second to none
elsewhere. So I soldered onwards, and I am still learning, although
sometimes I think now I am trying not to forget what I have begun to
understand, before I become senile, or before my hard drives crash yet
again, and take with them a nice new website page which is better than
one I have aleardy established.

Unless an audiophile has a visionary mission plan he may dither around
and never seem to get anywhere. I know guys who have made large
complex stereo systems with 4 amplifiers per channel and with active
crossovers and they are constantly changeing things and they
constantly complaining about noise and distortion and bad channel
balance and they don't want to spend money on a tradesman to fix
things and even after 5 years they cannot use a digital voltmeter and
nor do they seem to have the ability to think about things concisely
or logically. I recall the joke about Californians. "How is a light
bulb changed in Carlifornia?" Well, it takes 20 people gathered for a
happening and a pile of LSD tabs, and man, wow, what an experience -
especially when the house caught fire.

Testing a KT88 is not rocket science and not terribly dangerous IF ONE
TAKES PRECAUTIONS.

But one has to gradually work up to testing a KT88, IMHO.

I would suggest those interested should first spend time with basic
transistor circuits using supply rails less than +/- 40Vdc. Build a
preamp. Get it working. Analyse why it works. find out why the first
three transistors you connected failed and fused so easily. Write out
your thesis about why so someone else can read it and understand it.

Then move onto a triode preamp with the dangerous voltages. Almost
nobody makes such a move without getting an electrical shock or
without wrecking something. But the man who makes no mistakes never
builds anything. And the man who reckons he knows it all really knows
nothing.

Finally, move up to power tubes and power amps and testing output
tubes.

Patrick Turner.- Hide quoted text -

- Show quoted text -

And that's the way it is...

Okay. But - I'm just interested in a very small area of tube testing
right now. Static leakage tests between the electrodes with cathode hot
and cold, no other tests at all. Nothing else right now is on my mind
apart from current leakage between electrodes in other words - shorts.

I believe I can use a megger to tell me about shorts between electrodes.

Now, it seems to me that to really test for shorts we ought to put
serious voltages on the electrodes. And from what I gather the tests
should be for a KT88:

a) c to h 250V

b) g1 to all 200V

c) g2 to all 600V

d) a to all 800V

I can motorise my megger and set these test voltages.

I can test cold clearly, but what gets me though is testing for shorts
hot. Can I actually test for short with a megger when I'm running the
tube hot?

Actually I can reduce some of those voltages.

Also, if the tube is tested for shorts cold, is the only reasonable test
for shorts hot between cathode and heater?

[Richard[_12_]]

On 08/09/2010 17:56, Richard wrote:

I believe I can use a megger to tell me about shorts between electrodes.

Now, it seems to me that to really test for shorts we ought to put
serious voltages on the electrodes. And from what I gather the tests
should be for a KT88:

a) c to h 250V

b) g1 to all 200V

c) g2 to all 600V

d) a to all 800V

I can motorise my megger and set these test voltages.

I can test cold clearly, but what gets me though is testing for shorts
hot. Can I actually test for short with a megger when I'm running the
tube hot?

Actually I can reduce some of those voltages.

Also, if the tube is tested for shorts cold, is the only reasonable test
for shorts hot between cathode and heater?

When I think of "short circuits" I think in terms of testing insulation,
such as insulation between a tag-strip and earth, or a cable and earth.
That's in my minds eye when I think of testing for shorts.

The AVO tube testers test for shorts when cold.

I see tube data and see maximum voltages table and I see these voltages
as possible test voltages for shorts between electrodes.

So I quote for KT88:

c to h 250V

g1 to all 200V

g2 to all 600V

a to all 800V

So, I'm minded to test the tube at these voltages with my megger when
tube cold.

Then I think the same tests should be made when tube is hot. Because the
AVO tests for hot as well as cold, although I don't have an AVO tube
tester, so I'm not sure what the hot tests are.

Of course, when hot there is a potential problem, can you test the tube
for shorts when hot?

I believe you can, but it's clear that the test polarity is important.

I'm thinking, that if the polarity is right, I can test the tube hot as
cold with my megger at the above voltages.

Now when I've seen some circuit for shorts there has been nowhere near
the voltages I quote above.

Take the Eico 667 shorts tests, page 10 b), plate to all leakage test -
is that -70v the actual test voltage for all tubes, or what?

Is the Eico testing for shorts/leakage using -70v for all tubes? I sorta
doubt it but don't know for sure.

Is leakage tests cold a completely different animal from leakage tests hot?

[[email protected]]

On Sep 8, 1:45*pm, Richard wrote:
On 08/09/2010 17:56, Richard wrote:





I believe I can use a megger to tell me about shorts between electrodes.

Now, it seems to me that to really test for shorts we ought to put
serious voltages on the electrodes. And from what I gather the tests
should be for a KT88:

a) c to h 250V

b) g1 to all 200V

c) g2 to all 600V

d) a to all 800V

I can motorise my megger and set these test voltages.

I can test cold clearly, but what gets me though is testing for shorts
hot. Can I actually test for short with a megger when I'm running the
tube hot?

Actually I can reduce some of those voltages.

Also, if the tube is tested for shorts cold, is the only reasonable test
for shorts hot between cathode and heater?

When I think of "short circuits" I think in terms of testing insulation,
such as insulation between a tag-strip and earth, or a cable and earth.
That's in my minds eye when I think of testing for shorts.

The AVO tube testers test for shorts when cold.

I see tube data and see maximum voltages table and I see these voltages
as possible test voltages for shorts between electrodes.

So I quote for KT88:

c to h 250V

g1 to all 200V

g2 to all 600V

a to all 800V

So, I'm minded to test the tube at these voltages with my megger when
tube cold.

Then I think the same tests should be made when tube is hot. Because the
AVO tests for hot as well as cold, although I don't have an AVO tube
tester, so I'm not sure what the hot tests are.

Of course, when hot there is a potential problem, can you test the tube
for shorts when hot?

I believe you can, but it's clear that the test polarity is important.

I'm thinking, that if the polarity is right, I can test the tube hot as
cold with my megger at the above voltages.

Now when I've seen some circuit for shorts there has been nowhere near
the voltages I quote above.

Take the Eico 667 shorts tests, page 10 *b), plate to all leakage test -
is that -70v the actual test voltage for all tubes, or what?

Is the Eico testing for shorts/leakage using -70v for all tubes? I sorta
doubt it but don't know for sure.

Is leakage tests cold a completely different animal from leakage tests hot?- Hide quoted text -

- Show quoted text -

And the least of tube parameters is so much longer than solid state.
Now , speaking of leakage, most bipolar devices seem to develope that
illness with old age,in general. Vacuum tubes,on the contrary take
incredibly much more electrical abuse,deteriorate in a totally
different way and are capable of withstanding an EMP. Tubes ,in
general, suffer from: low emission,heater to cathode short (those
that have indirect filaments), opens (any internal element can go
bad),other internal short cicuits- most of these may be tested with a
simplest Ohmmeter. Once power is applied, the voltage on the heater
will make an incredible difference in performance- a well-known trick
used to be to increase the emission substantially and nearly always
obtain verr-ryspectacular results! This always shortened the tube-
life,as well. Plate voltage will drastically alter the performance.
Don't forget Vg1- equally as important. A tube biased for class A will
have different measurements from other classes,as well. What about
frequency responce? I don't recall any tube testers having such
function at all...

[Richard[_12_]]

On 08/09/2010 18:45, Richard wrote:
On 08/09/2010 17:56, Richard wrote:

I believe I can use a megger to tell me about shorts between electrodes.

Now, it seems to me that to really test for shorts we ought to put
serious voltages on the electrodes. And from what I gather the tests
should be for a KT88:

a) c to h 250V

b) g1 to all 200V

c) g2 to all 600V

d) a to all 800V

I can motorise my megger and set these test voltages.

I can test cold clearly, but what gets me though is testing for shorts
hot. Can I actually test for short with a megger when I'm running the
tube hot?

Actually I can reduce some of those voltages.

Also, if the tube is tested for shorts cold, is the only reasonable test
for shorts hot between cathode and heater?

When I think of "short circuits" I think in terms of testing insulation,
such as insulation between a tag-strip and earth, or a cable and earth.
That's in my minds eye when I think of testing for shorts.

The AVO tube testers test for shorts when cold.

I see tube data and see maximum voltages table and I see these voltages
as possible test voltages for shorts between electrodes.

So I quote for KT88:

c to h 250V

g1 to all 200V

g2 to all 600V

a to all 800V

So, I'm minded to test the tube at these voltages with my megger when
tube cold.

Then I think the same tests should be made when tube is hot. Because the
AVO tests for hot as well as cold, although I don't have an AVO tube
tester, so I'm not sure what the hot tests are.

Of course, when hot there is a potential problem, can you test the tube
for shorts when hot?

I believe you can, but it's clear that the test polarity is important.

I'm thinking, that if the polarity is right, I can test the tube hot as
cold with my megger at the above voltages.

Now when I've seen some circuit for shorts there has been nowhere near
the voltages I quote above.

Take the Eico 667 shorts tests, page 10 b), plate to all leakage test -
is that -70v the actual test voltage for all tubes, or what?

Is the Eico testing for shorts/leakage using -70v for all tubes? I sorta
doubt it but don't know for sure.

Is leakage tests cold a completely different animal from leakage tests hot?

This is what I think on shorts:

The maximum voltage ratings of tubes are NOT meant to be taken as test
voltages for leakage/insulation tests/shorts.

As to leakage tests when cold, it's not meant to be clear what test
voltages you would use. But, you could use the maximum voltage ratings
if you wish. And these tests could be made with a megger.

But in fact, you could ignore shorts test when cold and just rely on
shorts test hot. And in that case, you can use a circuit employing a low
voltage of -70 volts as per page 10 of Eico 667 manual.

So, you can test for electrode leakage for tubes using -70v.

Correct?

[Richard[_12_]]

On 09/09/2010 18:39, Richard wrote:
On 08/09/2010 18:45, Richard wrote:
On 08/09/2010 17:56, Richard wrote:

I believe I can use a megger to tell me about shorts between
electrodes.

Now, it seems to me that to really test for shorts we ought to put
serious voltages on the electrodes. And from what I gather the tests
should be for a KT88:

a) c to h 250V

b) g1 to all 200V

c) g2 to all 600V

d) a to all 800V

I can motorise my megger and set these test voltages.

I can test cold clearly, but what gets me though is testing for shorts
hot. Can I actually test for short with a megger when I'm running the
tube hot?

Actually I can reduce some of those voltages.

Also, if the tube is tested for shorts cold, is the only reasonable test
for shorts hot between cathode and heater?

When I think of "short circuits" I think in terms of testing insulation,
such as insulation between a tag-strip and earth, or a cable and earth.
That's in my minds eye when I think of testing for shorts.

The AVO tube testers test for shorts when cold.

I see tube data and see maximum voltages table and I see these voltages
as possible test voltages for shorts between electrodes.

So I quote for KT88:

c to h 250V

g1 to all 200V

g2 to all 600V

a to all 800V

So, I'm minded to test the tube at these voltages with my megger when
tube cold.

Then I think the same tests should be made when tube is hot. Because the
AVO tests for hot as well as cold, although I don't have an AVO tube
tester, so I'm not sure what the hot tests are.

Of course, when hot there is a potential problem, can you test the tube
for shorts when hot?

I believe you can, but it's clear that the test polarity is important.

I'm thinking, that if the polarity is right, I can test the tube hot as
cold with my megger at the above voltages.

Now when I've seen some circuit for shorts there has been nowhere near
the voltages I quote above.

Take the Eico 667 shorts tests, page 10 b), plate to all leakage test -
is that -70v the actual test voltage for all tubes, or what?

Is the Eico testing for shorts/leakage using -70v for all tubes? I sorta
doubt it but don't know for sure.

Is leakage tests cold a completely different animal from leakage tests
hot?

This is what I think on shorts:

The maximum voltage ratings of tubes are NOT meant to be taken as test
voltages for leakage/insulation tests/shorts.

As to leakage tests when cold, it's not meant to be clear what test
voltages you would use. But, you could use the maximum voltage ratings
if you wish. And these tests could be made with a megger.

But in fact, you could ignore shorts test when cold and just rely on
shorts test hot. And in that case, you can use a circuit employing a low
voltage of -70 volts as per page 10 of Eico 667 manual.

So, you can test for electrode leakage for tubes using -70v.

Correct?


To answer my own question - correct.

But, this is not the test really required. Should be tesying leakage at
say 500V.