Gentlemen,

in my current project (EL86/6CW5 PP) I use a solid
state pass regulator screen voltage supply to keep
the screen voltage within ratings.

I simply connected all four (since PP stereo) screens
together and hooked them directly to the reg'd supply,
without using screen stopper resistors at all.

I left the screen stoppers out so far because:

- I didn't notice oscillations to fight,

- no reason for an additional voltage drop,
since this is done by the screen PSU reg.

I noticed that sometimes the screen "stopper"
resistors seem to be there just for that reason:
to drop B+ to keep screen voltage within ratings.

After some reading at the Dennis Grimwood web
site about screens (link was provided by Mr.
Yeager recently, but I don't recall its web
adress), I got the impression that decoupling
the screens from each other would be A Good
Thing To Do - e.g by adding screen resistors
(I would not call them "stoppers" for this
purpose/function).

Since the screen voltage supply is very well
regulated in my amp project, I think the screens
are decoupled somehow already, since the common
screen supply DC voltage will not change/drop
when one screen draws more current than the others.

I would be glad to read your opinions, if, and why,
and how, further decoupling between the screen grids
should be applied.

Tnx2u!

Tom

--
this is my favourite sig, since there is no reference to
Kibo, Discordianism or The Church of the Subgenius in it.

I might add that in many tube spec sheets
a "common screen resistor" value can be
found for PP circuitry, which obviously means
that decoupling the screens from each other
in PP wasn't considered necessary/important
for some reason.

--
Do not meddle in the affairs of wizards,
for you are crunchy and good with ketchup.

Hi Tom,

Screen resistors are more to limit current.

Since your regulator for the screen supply has a very low impedance, I
wouldn't worry about it at all if I were you.

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca

Hi Gregg,

Screen resistors are more to limit current.

Okay, that's one function of them, but that
was not the point of my question.

Let's try to formulate it another way:

Viewed from outside the tube, DC-wise:

with fixed Vg2, they act as variable
"current drains", with a function
following plate current, which itself
is a function of Vg1, and (the fixed)
Vg2

So, if regulation of Vg2 is less than ideal
and two or more screens are fed by the same
PS (and not decoupled from each other), they
will influence each other, since also being
input grids, they will influence plate current,
following the voltage drop of the PS under load,
thus flowing the other tube screen grid, which
loads the common screen PS.

Due to the nature of a PP circuit with a signal
applied, the ammount of current flowing to/through
each screen grid of a PP pair is always different
except for the case of Vg1tube1=Vg1tube2, since one
tube pushes (higher Vg1 = higher plate current = higher
Vg2 current) and the other one pulls (lower Vg1 = lower
plate current = lower Vg2 current).

So, if the "fixed" screens of a PP output tube
pair are fed by a less than "ideal" PS,
they will be influenced by each other, and
as I understand it, this is no "common mode"
case, which will cancel out in PP - just
the opposite.

I don't know if I am right, since I don't
know much about electronics and I am just
exploring PP technology on the breadboard.

So the intention of my question was, if I am
right, how much this scenario is of concearn,
and why, and eventually how to fix it.

As could be derived from tube data sheets
this seems to be no problem of much concearn,
since often the value of a common screen
resistor (for both screens of a PP pair
together) is given.

Tom

--
Okay, maybe i am paranoid. But that doesn't mean
they are not out to get me. - unknown

Tom Schlangen said:

I left the screen stoppers out so far because:

- I didn't notice oscillations to fight,
- no reason for an additional voltage drop,
since this is done by the screen PSU reg.
I noticed that sometimes the screen "stopper"
resistors seem to be there just for that reason:
to drop B+ to keep screen voltage within ratings.

In that case they cease to be "stopper" resistors.
Stopper resistors are only there to eliminate parasitic oscillation.
They're usually only necessary with high Gm tubes.
I'd advise you to put them in the grid's circuit (1k) and g2 (100
ohms).
They won't hurt, and they don't have sonical consequences.

In your typical pentode amp, you're already using a well regulated G2
voltage, so there wouldn't be any need to use resistors for
attenuating Vg2. 100 ohms hardly reduces Vg2.
The need for grid stoppers is still there, even if you didn't notice
any obvious oscillations. They could be there when exciting the power
stage with a msucial signal!
BTW: what circuit are you using? The EL86s were intended for
"ironless" output stages that were loaded with 800 ohm speakers.
Are you using a 800/8 tranny to drive a "normal" impedance speaker?
If so, there are some benefits to be had there, e.g. no DC on the
tranny, so no problems with saturation of the core.
Much like the use of loading chokes we see today. If only that output
cap was of sufficient quality ;-)
The original circuit had a simple screen grid supply solution by using
a double winding choke, feeding G2 from the upper tube from va of the
lower and vice versa.
This cancelled out some harmonic distortion and provided feedback,
very clever!

--
Sander deWaal
Vacuum Audio Consultancy

Hi Sander,

I'd advise you to put them in the grid's
circuit (1k)

Already there ...

and g2 (100 ohms). They won't hurt, and
they don't have sonical consequences.

Okay, I'll include screen stoppers, too,
just to be on the safe side.

But all this doesn't adress my primary question ;-)

BTW: what circuit are you using?

I use my own circuit, which includes lots of detail
help from other RATs regarding the diff amp PI stage,
which has to cope with the low B+ available
and was the hardest part to get right in this amp
so far.

It is a simple 3 stage amp; first stage serving
as gain stage and injection point for NFB, second
stage being a diff amp with solid state cathode CCS,
power stage generic fixed bias / reg'd screens UL op.

The more or less up to date schematic is at:

http://www.ndh.net/home/schlangen/ro...p_amp_full.gif

Please note that this schematic doesn't include
the stepped attenuators I am currently trying
on the breadboard for improved stability with
global voltage NFB applied.

The EL86s were intended for "ironless" output stages
that were loaded with 800 ohm speakers.

That was what Philips did with them, indeed, but
besides this, they are just quite normal noval pentodes,
for lowish B+ and highish plate current operation. They
make nice pass regulators for e.g gain stages, too :-)

Please have a look at the 6CW5 spec sheet from GE,
at Frank Philipse's site: This spec sheet, contrarily
to the Philips one, contains mostly data for plain
normal pentode SE & PP operation of EL86/6CW5.

NOS/NIB/sealed Telefunken EL86 are much cheaper on
(german) Ebay - if one can wait for an offer - than even
the cheapest new production EL84/6BQ5 from China, but
please don't spread word on this ;-)

Are you using a 800/8 tranny to drive a "normal"
impedance speaker?

No; I use a pair of them into Hammond 1645 iron (Raa 5k),
giving a very healthy dozen watts (at least) of output
power. Plates at 250V/40mA, screens at 195V regulated.

According to GE specs, a PP pair could produce twice
the Po (25 watts) of my circuit, which would force
any other noval socketed output pentode (like EL84)
to give up. But I am not hunting for watts.

It is possible, and was done commercially with the
Klein + Hummel VS70 amp, to use EL86 in PP/UL with
plate and screen voltage at ~275 volts, but I dislike
to exceed spec sheet ratings.

Tom

--
Okay, maybe i am paranoid. But that doesn't mean
they are not out to get me. - unknown

"Tom Schlangen" wrote

Screen resistors are more to limit current.

Okay, that's one function of them, but that
was not the point of my question.

Let's try to formulate it another way:
....

Perhaps you don't realise that Gregg has answered your question.
Your voltage-regulated common screen supply should have a very low
output resistance. That means that small currents drawn by the
screens will not change the voltage significantly. If you continue
to be worried, then you could use more regulators. They are cheap.

It is worth noting the point about limiting screen current. You may
or may not wish to do this, depending on your design. Grid resistors
limit current dynamically, because current causes a voltage drop at
the screen, which increases the screen resistance.

cheers, Ian

Gregg wrote:

Hi Tom,

Screen resistors are more to limit current.

Since your regulator for the screen supply has a very low impedance, I
wouldn't worry about it at all if I were you.

But they don't limit current except by the R fusing.
If the tube got a short between screen and cathode,
then 300 volts is across say 470 ohms, and lotsa current,
and poof goes the R, and that's how the R limits current.

The regulated screen supply should start to go
to non regulation just when clipping occurs, when IG2 becomes the maximum
anticipated level, so that if a faulty tube drawing 10 times normal IG2
will
drag the Eg2 to a low level, and bias the tubes off,
The reg should be designed so an R in series with a reg transistor
burns out, instead of an OPT or something else.

Patrick Turner.

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca

Seems you are aware of the most common intention & application of the
pentode's screen grid resistor - to supress oscillations (especially if you
mount the R right at the socket) and to limit Ig2. However - just because
your g2 supply is regulated doesn't mean that your tube's screen grid won't
short out to another element in the tube at some point and possibly damage
your solid state pass reg as well as maybe a socket & who knows what else. I
have observed that the screen grid resistor also applies itself very smartly
as a safety fuse - for this reason I prefer to use metal oxide flame proof
types. When there is a screen grid failure (some circuits/tubes are more
prone than others to screen grid related failures) a metal oxide FP will
open up pretty quick and also it won't spew conductive carbon all over the
place.

I say use individual g2 resistors - 100 ohms 2W should be good for the
EL86 - you'll never hear the difference and it's much easier to replace than
the whole SS supply and the socket, etc, if you ever have a failure.

Todd

Hi Ian,

That means that small currents drawn by the
screens will not change the voltage significantly.

If it is of no significance, then it is obviously okay.
I just didn't know _how_ significant it is :-)

It is worth noting the point about limiting screen
current.

This is done by the regulator IC, which limits current
to roughly the ammount of screen current drawn by the
four screens under full amp load condition, which is
within specs of screen power dissipation (of all
four screens together).

This obviously doesn't "fuse" a single tube screen,
but saves the OPTs (which themselves are overrated
3 times in current and power handling capabilities).

Tnx2u!

Tom

--
When in doubt, use brute force.
- Ken Thompson

Hi Todd,

I say use individual g2 resistors - 100 ohms 2W
should be good for the EL86 - you'll never hear
the difference and it's much easier to replace than
the whole SS supply and the socket, etc, if you ever
have a failure.

I appreciate your practical approach, thank you!

Tom

--
Anyone who considers protocol unimportant has
never dealt with a cat. - R. Heinlein

"Patrick Turner"

Gregg wrote:

Hi Tom,

Screen resistors are more to limit current.


But they don't limit current except by the R fusing.


** More ***utterly stupid crap*** from the autistic Turneroid.


The presence and value of the screen resistors is crucial in most class AB
output stages. If you increase their value - output power is lost. If you
reduce their value then screens will over dissipate at full output, glow
white hot and the tubes blow up.

Current limiting is just what they do - by dropping voltage under load
when the screen is more positive than the plate.

Omit them at your peril.



............. Phil

Phil Allison wrote:

"Patrick Turner"

Gregg wrote:

Hi Tom,

Screen resistors are more to limit current.


But they don't limit current except by the R fusing.

** More ***utterly stupid crap*** from the autistic Turneroid.

The usual attempt to win by insult and repetion won't get Phool Arsillilon
anywhere.



The presence and value of the screen resistors is crucial in most class AB
output stages. If you increase their value - output power is lost. If you
reduce their value then screens will over dissipate at full output, glow
white hot and the tubes blow up.

The usual value of screen R in many pentode/tetrode amps is 470 ohms.
The current limiting effect is minimal.
even if the screen current went from 5 mA at idle to 50ma
for 20 watts screen dissipation, where Eg2 was 400v, the
voltage drop across the 470 ohms is only 23 volts.
The 470 ohms dissipates 1.15 watts, so of the R
was a 1/2 watt R, it would fry when a serious fault occurs, and screen current
went high.




Current limiting is just what they do - by dropping voltage under load
when the screen is more positive than the plate.

The amount of voltage drop across a 470 ohm to a screen is
negligible, but if the anode is shorted to 0v, and Eg2 stays high, if it
comes from a separate supply, you then would get a high Ig2,
and hopefully the resistor will burn itself open before the
screen heats up and melts.

Omit them at your peril.

I have seen 2 watt screen R of 470 ohms in guitar amps, where
the stress on the tubes is far higher than hi-fi amps when routinely
over driven, and the screens will blow especially if a dude uses a 4 ohm
speaker instead
of a specified 8 ohm.
Hi-fi amps are more forgiving, but most are UL or triode, or
acoustical, with lower Eg2 supplies than B+ anode supplies, and
with the rated load, the max power can be maintained indefinately
with class AB amps at 1 dB under clipping into the rated load,
which stresses screens far more than normal,
and the tubes don't burn out their screens without a series R
like Phool says they will.

Patrick Turner.



............ Phil

"Patrick Turner"

Phil Allison wrote:


Gregg wrote:

Hi Tom,

Screen resistors are more to limit current.

But they don't limit current except by the R fusing.

** More ***utterly stupid crap*** from the autistic Turneroid.

The usual attempt to win by insult and repetion won't get Phool Arsillilon
anywhere.


** Go **** yourself you stinking, lying POS.

The public need warning about you in the strongest terms.




The presence and value of the screen resistors is crucial in most class
AB
output stages. If you increase their value - output power is lost. If
you
reduce their value then screens will over dissipate at full output,
glow
white hot and the tubes blow up.

The usual value of screen R in many pentode/tetrode amps is 470 ohms.
The current limiting effect is minimal.


** The Turneroid fool just repeats his Bull****.




Current limiting is just what they do - by dropping voltage under load
when the screen is more positive than the plate.

The amount of voltage drop across a 470 ohm to a screen is
negligible,


** No it aint, and who said it was 470 ohms ????????



Omit them at your peril.


I have seen 2 watt screen R of 470 ohms in guitar amps, where
the stress on the tubes is far higher than hi-fi amps when routinely
over driven, and the screens will blow especially if a dude uses a 4 ohm
speaker instead of a specified 8 ohm.


** It is what this complete fool has not seen that matters most.



Hi-fi amps are more forgiving, but most are UL or triode, or
acoustical, with lower Eg2 supplies than B+ anode supplies, and
with the rated load, the max power can be maintained indefinately
with class AB amps at 1 dB under clipping into the rated load,
which stresses screens far more than normal,
and the tubes don't burn out their screens without a series R
like Phool says they will.


** The Turneroid autistic postulates arbitrary conditions - to suit his
stupid error.

Here is is again: " But they don't limit current except by the R
fusing."

See what it says - screen resistors are not current limiters - not
ever.

Take any AB amp that uses EL34s and has a 450 volts or more supply for
plates and screens.

Short out the ( typically 1 kohm) screen resistors.

Drive the amp to clipping for 30 seconds.

Bang.




............... Phil

Its totally pointless discussing Phil's errors of perception
about tube amps.

There *will be* some occasions where series screen R
will save an AB amp from blowing screens, because the R blows open,
which is why I always have some series R between the G2 supply,

But a class AB amp will not always blow up in all cases where there is no series
R to the screens, and Phool takes no account of variations in set up conditions.

Phil Allison should **** off out of the news group,
because he ridicules and insults without reason, rather than enquire why
an alternative viewpoint has been stated to his own.
And even when someone is incorrect about some electronic issue,
there is still no justification for insults.

Patrick Turner.





Phil Allison wrote:

"Patrick Turner"

Phil Allison wrote:


Gregg wrote:

Hi Tom,

Screen resistors are more to limit current.

But they don't limit current except by the R fusing.

** More ***utterly stupid crap*** from the autistic Turneroid.

The usual attempt to win by insult and repetion won't get Phool Arsillilon
anywhere.

** Go **** yourself you stinking, lying POS.

The public need warning about you in the strongest terms.


The presence and value of the screen resistors is crucial in most class
AB
output stages. If you increase their value - output power is lost. If
you
reduce their value then screens will over dissipate at full output,
glow
white hot and the tubes blow up.

The usual value of screen R in many pentode/tetrode amps is 470 ohms.
The current limiting effect is minimal.

** The Turneroid fool just repeats his Bull****.


Current limiting is just what they do - by dropping voltage under load
when the screen is more positive than the plate.

The amount of voltage drop across a 470 ohm to a screen is
negligible,

** No it aint, and who said it was 470 ohms ????????

Omit them at your peril.


I have seen 2 watt screen R of 470 ohms in guitar amps, where
the stress on the tubes is far higher than hi-fi amps when routinely
over driven, and the screens will blow especially if a dude uses a 4 ohm
speaker instead of a specified 8 ohm.

** It is what this complete fool has not seen that matters most.

Hi-fi amps are more forgiving, but most are UL or triode, or
acoustical, with lower Eg2 supplies than B+ anode supplies, and
with the rated load, the max power can be maintained indefinately
with class AB amps at 1 dB under clipping into the rated load,
which stresses screens far more than normal,
and the tubes don't burn out their screens without a series R
like Phool says they will.

** The Turneroid autistic postulates arbitrary conditions - to suit his
stupid error.

Here is is again: " But they don't limit current except by the R
fusing."

See what it says - screen resistors are not current limiters - not
ever.

Take any AB amp that uses EL34s and has a 450 volts or more supply for
plates and screens.

Short out the ( typically 1 kohm) screen resistors.

Drive the amp to clipping for 30 seconds.

Bang.

.............. Phil

"Patrick Turner" wrote in message
...

Its totally pointless discussing Phil's errors of perception
about tube amps.


** Go get stuffed you pig ignorant turd.

The Turneroid has a congenital brain defect that causes cognitive
catatonia.




But a class AB amp will not always blow up in all cases where there is no
series
R to the screens,


** The Turneroid posts lie after lie after lie after lie ....

There is no logical fallacy or deceit that he does not practise.



Phil Allison should **** off out of the news group,


** The Turneroid gets no say in who has the right to point out his gross
errors.

The Turneroid needs to be exposed as a dangerous fraud as often as
possible.





.............. Phil

Tom Schlangen wrote

If it is of no significance, then it is obviously okay.
I just didn't know _how_ significant it is :-)

Well, you could calculate it. Let's say at max power an output valve
anode current varies by 60mApk, and its screen current by 10mApk. With
a screen source resistance of say 2ohms, that would give a screen
variation of 20mVpk. Now assuming a triode-strapped anode impedance of
say 3k, 20mVpk at screen will give as a guesstimate 6.7uA current
variation at anode (not entirely sure of my argument here.... That is
more or less 0.1 percent. That is only for the peaks of the waveform
and not all of it is non-linear, so the total distortion would measure
less. All the same it is clear you need very low source resistance,
and this raises the question of how your voltage regulator senses its
current. If it does so without a penalty of high output resistance,
you are OK.

It is worth noting the point about limiting screen
current.

This is done by the regulator IC, which limits current
to roughly the ammount of screen current drawn by the
four screens under full amp load condition, which is
within specs of screen power dissipation (of all
four screens together).

See above, but also consider how sharply the regulator reacts to peak
current. Even when the mean output is considerably below full power,
parts of the signal are likely to exceed your limit. I guess a softish
limit is in order.

This obviously doesn't "fuse" a single tube screen,
but saves the OPTs (which themselves are overrated
3 times in current and power handling capabilities).

Dunno about the safety thing. I know if I drive my amps too hard the
screens will burn, so I don't do it.

cheers, Ian

Prego

"Tom Schlangen" wrote in message
...
Hi Todd,

I say use individual g2 resistors - 100 ohms 2W
should be good for the EL86 - you'll never hear
the difference and it's much easier to replace than
the whole SS supply and the socket, etc, if you ever
have a failure.

I appreciate your practical approach, thank you!

Tom

--
Anyone who considers protocol unimportant has
never dealt with a cat. - R. Heinlein