[Patrick Turner]

I didn't really want to proceed to using the Gilbert Cell for an AM
generator although I got it to work a bit, but not much better than
the pentode thing I built over 10 years ago.

I tried using 3 x MJE340 transistors with a +41V supply.

Two were placed in an LTP with CT coil of about 400uF and a two gang
tuning cap fitted to give 500kHz to 1,700 kHz for MW.

One base was taken to a fixed supply of +20V and the other biased 20V
but through a 100 ohm R.
The commoned emitters were taken via a 1K0 to the collector of a
common emitter gain stage with its base biased via 27k to a fixed
+8V.
The emitter of this bjt has a 4k7 to 0V.

The value of the Re determines the idle current through all devices.
1.6mAdc was found to be ideal and any more or less made severe
distortions of one kind or another to appear.

The LC tank has a secondary of about 1/8 of total P turns, with
grounded CT and two live ends for a balanced output if needed.
It was found that when tuned, the LC impedance was so high that
serious distortion was too likely, so the LC had 33k strapped across
it to load the LC. This also meant the sec ended up with about 700
ohms Rout, end to end, or 350 ohms for 1/2 the sec.


RF was fed to the live side of the LTP from a 1k pot. Only 30mV was
needed Rin to the base was quite low, less than 100 ohms, so if the
feed signal is large, say a volt or two as it is from my existing sig
gene then the 1k pot turned low was fine, and helped keep the signal
linear, due to low nonlinear bjt input impedance.

AF to modulate the RF was fed to the bottom amp base via cap coupling
to an AF gene.
The large Re of 4k7 means that the base Rin is quite high, and also
that the current change applied to the common emitters of the LTP is
also linear. The 1k R allows measurement of signals but is not really
needed because Rc collector impedance is effectively megohms.

The above set up needed 5.1Vrms applied to the bottom bjt base. AC
current change was about 1.2mA.

The AM thus generated looked slightly better than the AM produced by
one pentode, and I could go right up to 100% without wave flattening.

The max Vo at LTP collectors was nearly equal to Eb to supply, about
19V pk-pk which is the about the maximum possible swing. About 1/8 of
that appears across each 1/2 secondary.

The circuit worked better than the Gilbert, but was rather too
sensitive to control pot settings compared to the easier to use
pentode modulator. Distortion varied a lot depending on the tuning,
more Dn when tuned high, less THD when tuned low, but it works.
Once set up, RF coud be adjusted to any level below 30mV and then AF
set to modulate it at any level. The RF wave has virtually no
distortion because the tank Q was found to be over 200, and harmonics
are low. least envelope Dn is when both RF and AF produce a wave of
about 1/2 of the maximum possible, ie, between 0.5V and 1V rms max.

It seems to me the AF merely changes bias current in both LTP bjts. AF
cannot appear differentially between collectors because they are
shunted by 400uh, a very low Z at AF.

The linear change in Ic causes the differential gain of the bjts to
change where there is a differential input. In this case, one base is
grounded and the other is fed the RF carrier. There is a CCS common
emitter sink, so equal but opposite RF waves appear at each collector.
The change in modulator current applied to commoned emitters swings
the bjts each side of their square law non linearity, and this sums to
linearly alter the differential gain. Hence the AM has a good looking
envelope, and better than a Gilbert Cell. Setting up is much more
fiddly than for a single pentode or for a pair of PP pentodes because
a bjt only needs a very small Vb-Ve change to give a huge current
change. I found that gain between the one base input and Vc-c was over
200, even at 1.7MHz, and bjts are renowned for being slow at RF, but
the Ft for MJE340 is well above where I wanted to use them I may have
tried BF469 which are video transistors with Ft at 60MHz which would
have been better. But I only had a lot of BF472 with are PNP, and
which require me to build the circuit "upside down" which I didn't
fancy doing.

The LC was easy to make. I googled up an inductance calculator and
dialed in the dia of the 42mm od PVC former and wire size of 0.45mm Cu
dia and wound a coil with slightly more turns than needed, 162t. I put
5 taps on each end of the coil to allow inductance trimming if needed
The 20t sec was placed over 0.2mm insulation and centered over the
primary coil CT to give little capacitance effects, but good magnetic
coupling.
The CT of the primary is taken to +41V and the 2 gang cap has its
frame to 0V and its fixed plates to each end of the primary via 0.01uF
to prevent 41Vdc appearing across the tuning plates. If ever the
plates short, then the 41V supply is shunted and there's a problem.

I have an HP 606A oscillator which has a similar modulator using a
pair of 6CL6 pentodes with 2 gangs of a 4 gang tuning cap and CT tank
coil. It has a 6B4 to drive the commoned cathodes. There is a balanced
RF input to the 6CL6. The 606 also has a fancy PP tuned oscillator
using the other 2 gangs. And another tube which fiddles with oscilator
biasing to keep the output level constant. But that 606 oscillator is
such a huge thing. Must be 2 cubic feet. I suppose they made all that
sort of thing big way back then to make it look impressive so they
could sell if for squillions of bucks to military and Govt
institutions. Anyway, I think I will haul the old girl out and hooker
up nearer to my bench and try to get it working OK if it doesn't after
being asleep for the last 20 years. Its probably better than anything
I could build. I got it given to me well after I built the simpler
pentode modulator. Its also got meters on the front and nice knobs.
There are no ****y little bjts inside.

Patrick Turner.

[Patrick Turner]

On Nov 19, 3:20*pm, "Alex" wrote:
"Patrick Turner" wrote in message

...





I didn't really want to proceed to using the Gilbert Cell for an AM
generator although I got it to work a bit, but not much better than
the pentode thing I built over 10 years ago.

I tried using 3 x MJE340 transistors with a +41V supply.

Two were placed in an LTP with CT coil of about 400uF and a two gang
tuning cap fitted to give 500kHz to 1,700 kHz for MW.

One base was taken to a fixed supply of +20V and the other biased 20V
but through a 100 ohm R.
The commoned emitters were taken via a 1K0 to the collector of a
common emitter gain stage with its base biased via *27k to a fixed
+8V.
The emitter of this bjt has a 4k7 to 0V.

The value of the Re determines the idle current through all devices.
1.6mAdc was found to be ideal and any more or less made severe
distortions of one kind or another to appear.

The LC tank has a secondary of about 1/8 of total P turns, with
grounded CT and two live ends for a balanced output if needed.
It was found that when tuned, the LC impedance was so high that
serious distortion was too likely, so the LC had 33k strapped across
it to load the LC. This also meant the sec ended up with about 700
ohms Rout, end to end, or 350 ohms for 1/2 the sec.

RF was fed to the live side of the LTP from a 1k pot. Only 30mV was
needed Rin to the base was quite low, less than 100 ohms, so if the
feed signal is large, say a volt or two as it is from my existing sig
gene then the 1k pot turned low was fine, and helped keep the signal
linear, due to low nonlinear bjt input impedance.

AF to modulate the RF was fed to the bottom amp base via cap coupling
to an AF gene.
The large Re of 4k7 means that the base Rin is quite high, and also
that the current change applied to the common emitters of the LTP is
also linear. The 1k R allows measurement of signals but is not really
needed because Rc collector impedance is effectively megohms.

The above set up needed 5.1Vrms applied to the bottom bjt base. AC
current change was about 1.2mA.

The AM thus generated looked slightly better than the AM produced by
one pentode, and I could go right up to 100% without wave flattening.

The max Vo at LTP collectors was nearly equal to Eb to supply, about
19V pk-pk which is the about the maximum possible swing. About 1/8 of
that appears across each 1/2 secondary.

The circuit worked better than the Gilbert, but was rather too
sensitive to control pot settings compared to the easier to use
pentode modulator. Distortion varied a lot depending on the tuning,
more Dn when tuned high, less THD when tuned low, but it works.
Once set up, RF coud be adjusted to any level below 30mV and then AF
set to modulate it at any level. The RF wave has virtually no
distortion because the tank Q was found to be over 200, and harmonics
are low. least envelope Dn is when both RF and AF produce a wave of
about 1/2 of the maximum possible, ie, between 0.5V and 1V rms max.

It seems to me the AF merely changes bias current in both LTP bjts. AF
cannot appear differentially between collectors because they are
shunted by 400uh, a very low Z at AF.

The linear change in Ic causes the differential gain of the bjts to
change where there is a differential input. In this case, one base is
grounded and the other is fed the RF carrier. There is a CCS common
emitter sink, so equal but opposite RF waves appear at each collector.
The change in modulator current applied to commoned emitters swings
the bjts each side of their square law non linearity, and this sums to
linearly alter the differential gain. Hence the AM has a good looking
envelope, and better than a Gilbert Cell. Setting up is much more
fiddly than for a single pentode or for a pair of PP pentodes because
a bjt only needs a very small Vb-Ve change to give a huge current
change. I found that gain between the one base input and Vc-c was over
200, even at 1.7MHz, and bjts are renowned for being slow at RF, but
the Ft for MJE340 is well above where I wanted to use them I may have
tried BF469 which are video transistors with Ft at 60MHz which would
have been better. But I only had a lot of BF472 with are PNP, and
which require me to build the circuit "upside down" which I didn't
fancy doing.

The LC was easy to make. I googled up an inductance calculator and
dialed in the dia of the 42mm od PVC former and wire size of 0.45mm Cu
dia and wound a coil with slightly more turns than needed, 162t. I put
5 taps on each end of the coil to allow inductance trimming if needed
The 20t sec was placed over 0.2mm insulation and centered over the
primary coil CT to give little capacitance effects, but good magnetic
coupling.
The CT of the primary is taken to +41V and the 2 gang cap has its
frame to 0V and its fixed plates to each end of the primary via 0.01uF
to prevent 41Vdc appearing across the tuning plates. If ever the
plates short, then the 41V supply is shunted and there's a problem.

I have an HP 606A oscillator which has a similar modulator using a
pair of 6CL6 pentodes with 2 gangs of a 4 gang tuning cap and CT tank
coil. It has a 6B4 to drive the commoned cathodes. There is a balanced
RF input to the 6CL6. The 606 also has a fancy PP tuned oscillator
using the other 2 gangs. And another tube which fiddles with oscilator
biasing to keep the output level constant. But that 606 oscillator is
such a huge thing. Must be 2 cubic feet. I suppose they made all that
sort of thing big way back then to make it look impressive so they
could sell if for squillions of bucks to military and Govt
institutions. Anyway, I think I will haul the old girl out and hooker
up nearer to my bench and try to get it working OK if it doesn't after
being asleep for the last 20 years. Its probably better than anything
I could build. I got it given to me well after I built the simpler
pentode modulator. Its also got meters on the front and nice knobs.
There are no ****y little bjts inside.

Patrick Turner.

You are on the right track Partick. As I promised, the simple differential
pair (you like to call it "long tail" for some reason) is more adequate for
AM.

The tail of a differential amp is often a high value resistance, or a
constant current sink from common cathodes.
The CCS is in effect an infinite resistance to an infinite negative
voltage. Hence the tail is long.
LTP has been in common electronic parlance for donkey's years.



Now some improvements.
1. Add common base cascodes on top (3...5V above) of each of your
transistors -- you will get rid of LC tank tuning affecting the setup. Mr..
Miller will simply go away. It is better to use small generic BC847 or any
crap in the diff pair (they have far better beta), and make your favourite
MJE340 become the cascodes.

Well maybe. Starting to get complex. But the load on top of the
cascode collectors is what?
Just an untuned L with CT? if so I thought maybe 200mH at least, and
then it has self capacitance with layer winding of many turns and much
more hard to do than a 400uH winding on a bitta PVC pipe.

2. AM envelope will be distorted by definition if the LC tank is not tuned
to carrier.

Indeed. I wouldn't use it unless it was tuned.

For the sinusoidal envelope both sidebands must be equal. It is
not the fault of the circuit.


3. For linear AM it important to maintain DC balance of the diff pair in the
whole range of current -- in your case from 0.1 to 3.2mA.



To improve the balance or rather 50/50 current distribution at all levels
you might try the following:
a) Insert another 100R in the base of the other diff transistor;

I tried that and distortion increased. The base input resistance is
not linear. Maybe a darlington pair would help but maybe they are
slower.


b) Increase drive from 30mV to say 100mV to get them work more like switches
(however it might make things worse at 1.6MHz);

Any increase beyond the small amount of about 30mV made distortions
worse on all F.

Alternatively you can use self-balancing circuit:
a) Disconnect emitters.
b) Instead of 1K which you are currently using in the common emitter point,
now use two 2K resistors. Each of them goes from the emitter of the
respective diff transistor to the collector of the third transistor (working
as AF amplifier). Connect 0.033uF capacitor between the emitters of the diff
transistors. Thus for DC and AF the current will be equally split by virtue
of two 2K 1% resistors, but for RF the emitters will be virtually joined,
maintaining differential switch operation.

I tried all that. it made little difference, and I could connect
shunts across the R and there was no change to operation so its OK to
have emitters joined.

I'm not so sure anything switches, but the range of non linear
transistor turn on/off voltage is exploited, and unlike a tube that
voltage range between base and emitter is a small one The CRO
certainly showed this to be the case.

I dusted off the HP 606A and turned it on. It gave out AM for awhile,
with lots of intermittent noise and it has a distorted audio generator
for the modulation After about 12 minutes it fizzled to a flat line on
the CRO. Its probably fulla problems and has never been serviced
properly. It has a NASA sticker on it. But its here in Oz, so don't
ask me how it got here. Was made about 1955.
The CRO showed a flat line, but the meter for modulation swung when I
varied mod %, so there was a signal still in there. There is hope yet
for the old girl.

Anyone got a clean printable schematic for the HP606A? I couldn't find
one in HP online archives.
I do have a set of schematics but I can hardly read them and most
values and numbers are illegible, even when printed up.

But inside the metalwork is a wonder. It does 50kHz to 65MHz in 6
ranges. I have it on the bench ready to diagnose tomorrow.

Regards,
Alex
PS Your posts are inspiring me to build an AM modulator-transmitter for
listening music of choice on a retro AM radio. But I will be using a
frequency doubler. Without the doubler it is difficult to avoid spurious FM
due to leakage of the strong emission from the antenna (about 100V RF) to
the oscillator tank.

Well, could you not screen the oscillator tank well? Isn't there many
a ham who operates a transceiver with oscillator and transmitter both
having the same F.

For local retro, you don't want high modulation % because many old AM
radios have such poor performing detectors and don't like high % mod.



Also an instantaneous compressor, but not clipper is a
must to avoid overmodulation and make the system not critical to the
different recording levels found on different CDs. Fortunately I have
experience with these compressors from guitar gear (sustain effect).

I think you should find no need for compression as the signal will be
as good as astrong local AM station or better, so if the radio is
quiet, then low average mod % can be used.

There was a guy here who made FM short distance stereo modules based
around BA1404 plus opamps with a j-fet in the shunt FB path to act as
clipper limiters to peaks to keep FM deviation under +/- 75kHz.

Many ways to skin the monkey.

Patrick Turner.

- Hide quoted text -

- Show quoted text -

[Alex]

"Patrick Turner" wrote in message
...
I didn't really want to proceed to using the Gilbert Cell for an AM
generator although I got it to work a bit, but not much better than
the pentode thing I built over 10 years ago.

I tried using 3 x MJE340 transistors with a +41V supply.

Two were placed in an LTP with CT coil of about 400uF and a two gang
tuning cap fitted to give 500kHz to 1,700 kHz for MW.

One base was taken to a fixed supply of +20V and the other biased 20V
but through a 100 ohm R.
The commoned emitters were taken via a 1K0 to the collector of a
common emitter gain stage with its base biased via 27k to a fixed
+8V.
The emitter of this bjt has a 4k7 to 0V.

The value of the Re determines the idle current through all devices.
1.6mAdc was found to be ideal and any more or less made severe
distortions of one kind or another to appear.

The LC tank has a secondary of about 1/8 of total P turns, with
grounded CT and two live ends for a balanced output if needed.
It was found that when tuned, the LC impedance was so high that
serious distortion was too likely, so the LC had 33k strapped across
it to load the LC. This also meant the sec ended up with about 700
ohms Rout, end to end, or 350 ohms for 1/2 the sec.


RF was fed to the live side of the LTP from a 1k pot. Only 30mV was
needed Rin to the base was quite low, less than 100 ohms, so if the
feed signal is large, say a volt or two as it is from my existing sig
gene then the 1k pot turned low was fine, and helped keep the signal
linear, due to low nonlinear bjt input impedance.

AF to modulate the RF was fed to the bottom amp base via cap coupling
to an AF gene.
The large Re of 4k7 means that the base Rin is quite high, and also
that the current change applied to the common emitters of the LTP is
also linear. The 1k R allows measurement of signals but is not really
needed because Rc collector impedance is effectively megohms.

The above set up needed 5.1Vrms applied to the bottom bjt base. AC
current change was about 1.2mA.

The AM thus generated looked slightly better than the AM produced by
one pentode, and I could go right up to 100% without wave flattening.

The max Vo at LTP collectors was nearly equal to Eb to supply, about
19V pk-pk which is the about the maximum possible swing. About 1/8 of
that appears across each 1/2 secondary.

The circuit worked better than the Gilbert, but was rather too
sensitive to control pot settings compared to the easier to use
pentode modulator. Distortion varied a lot depending on the tuning,
more Dn when tuned high, less THD when tuned low, but it works.
Once set up, RF coud be adjusted to any level below 30mV and then AF
set to modulate it at any level. The RF wave has virtually no
distortion because the tank Q was found to be over 200, and harmonics
are low. least envelope Dn is when both RF and AF produce a wave of
about 1/2 of the maximum possible, ie, between 0.5V and 1V rms max.

It seems to me the AF merely changes bias current in both LTP bjts. AF
cannot appear differentially between collectors because they are
shunted by 400uh, a very low Z at AF.

The linear change in Ic causes the differential gain of the bjts to
change where there is a differential input. In this case, one base is
grounded and the other is fed the RF carrier. There is a CCS common
emitter sink, so equal but opposite RF waves appear at each collector.
The change in modulator current applied to commoned emitters swings
the bjts each side of their square law non linearity, and this sums to
linearly alter the differential gain. Hence the AM has a good looking
envelope, and better than a Gilbert Cell. Setting up is much more
fiddly than for a single pentode or for a pair of PP pentodes because
a bjt only needs a very small Vb-Ve change to give a huge current
change. I found that gain between the one base input and Vc-c was over
200, even at 1.7MHz, and bjts are renowned for being slow at RF, but
the Ft for MJE340 is well above where I wanted to use them I may have
tried BF469 which are video transistors with Ft at 60MHz which would
have been better. But I only had a lot of BF472 with are PNP, and
which require me to build the circuit "upside down" which I didn't
fancy doing.

The LC was easy to make. I googled up an inductance calculator and
dialed in the dia of the 42mm od PVC former and wire size of 0.45mm Cu
dia and wound a coil with slightly more turns than needed, 162t. I put
5 taps on each end of the coil to allow inductance trimming if needed
The 20t sec was placed over 0.2mm insulation and centered over the
primary coil CT to give little capacitance effects, but good magnetic
coupling.
The CT of the primary is taken to +41V and the 2 gang cap has its
frame to 0V and its fixed plates to each end of the primary via 0.01uF
to prevent 41Vdc appearing across the tuning plates. If ever the
plates short, then the 41V supply is shunted and there's a problem.

I have an HP 606A oscillator which has a similar modulator using a
pair of 6CL6 pentodes with 2 gangs of a 4 gang tuning cap and CT tank
coil. It has a 6B4 to drive the commoned cathodes. There is a balanced
RF input to the 6CL6. The 606 also has a fancy PP tuned oscillator
using the other 2 gangs. And another tube which fiddles with oscilator
biasing to keep the output level constant. But that 606 oscillator is
such a huge thing. Must be 2 cubic feet. I suppose they made all that
sort of thing big way back then to make it look impressive so they
could sell if for squillions of bucks to military and Govt
institutions. Anyway, I think I will haul the old girl out and hooker
up nearer to my bench and try to get it working OK if it doesn't after
being asleep for the last 20 years. Its probably better than anything
I could build. I got it given to me well after I built the simpler
pentode modulator. Its also got meters on the front and nice knobs.
There are no ****y little bjts inside.

Patrick Turner.

You are on the right track Partick. As I promised, the simple differential
pair (you like to call it "long tail" for some reason) is more adequate for
AM.

Now some improvements.
1. Add common base cascodes on top (3...5V above) of each of your
transistors -- you will get rid of LC tank tuning affecting the setup. Mr.
Miller will simply go away. It is better to use small generic BC847 or any
crap in the diff pair (they have far better beta), and make your favourite
MJE340 become the cascodes.
2. AM envelope will be distorted by definition if the LC tank is not tuned
to carrier. For the sinusoidal envelope both sidebands must be equal. It is
not the fault of the circuit.
3. For linear AM it important to maintain DC balance of the diff pair in the
whole range of current -- in your case from 0.1 to 3.2mA.

To improve the balance or rather 50/50 current distribution at all levels
you might try the following:
a) Insert another 100R in the base of the other diff transistor;
b) Increase drive from 30mV to say 100mV to get them work more like switches
(however it might make things worse at 1.6MHz);

Alternatively you can use self-balancing circuit:
a) Disconnect emitters.
b) Instead of 1K which you are currently using in the common emitter point,
now use two 2K resistors. Each of them goes from the emitter of the
respective diff transistor to the collector of the third transistor (working
as AF amplifier). Connect 0.033uF capacitor between the emitters of the diff
transistors. Thus for DC and AF the current will be equally split by virtue
of two 2K 1% resistors, but for RF the emitters will be virtually joined,
maintaining differential switch operation.

Regards,
Alex
PS Your posts are inspiring me to build an AM modulator-transmitter for
listening music of choice on a retro AM radio. But I will be using a
frequency doubler. Without the doubler it is difficult to avoid spurious FM
due to leakage of the strong emission from the antenna (about 100V RF) to
the oscillator tank. Also an instantaneous compressor, but not clipper is a
must to avoid overmodulation and make the system not critical to the
different recording levels found on different CDs. Fortunately I have
experience with these compressors from guitar gear (sustain effect).

[Patrick Turner]

snip,

and I had said...

, and bjts are renowned for being slow at RF, but
the Ft for MJE340 is well above where I wanted to use them I may have
tried BF469 which are video transistors with Ft at 60MHz which would
have been better. But I only had a lot of BF472 with are PNP, and
which require me to build the circuit "upside down" which I didn't
fancy doing.

You said.....

Sound a lot like a sand box 6ME8 beam deflection circuit.

I've been thinking of doing it in reverse, though. Self excite G1 with
carrier. Feed single ended AF to one deflection plate and use bias
(offset) on the other to control modulation depth.

I'm not sure what a 6ME8 beem deflection circuit looks like.

Anyway, I checked out all the PSU voltages on the HP 606A oscillator
and all was well. Nothing wrong.

But the 0.15A fuse which blew protects the PSU from excessive current
going to two 6CL6 pentodes used for the RF amp after the oscillator.
Before the fuse blew, the CRO trace was wobbly, noisy, and varied in
amplitude. Its rather difficult to get such low F disturbances to pass
through a tuned RF transformer, but I figured one of both 6CL6 was
arcing internally, ie, ****ed.
So pulled the 606A apart to get to the 6CL6 and replaced both 6CL6
with newies I happened to have. Everything calmed right down and is
now OK. I checked out all voltages and all seemed normal. at teh
bottom of the lowest range and at below 100kHz there is a bit of
excessive envelope distortion at 100% mod, but at 455 kHz or across
the broadcast band the distortions of any kind are minimal, so I'll
use the beast for testing AM radios. But one stange thing does happen.

OK, I set up with 100% AM and say 3V output into a 1 metre lead to my
CRO. The attenuator has multiple positions. Whatever range of output
level is selected, the RF output swings a meter just fully, with the
fine adjustment pot set to max so that on the 3V range it means you
have 3V output and you read that on the meter which has scales to suit
1-3, and 3-10, between 3V and 3 uV, or whatever it is on the lowest
setting.
If set Vo range for 3V max Vo, then you can adjust the level down from
3V to anything shown on the meter scale for 1V - 3V.

But if I have a signal modulated at 100% and I adjust it down using
the pot for fine adjustment, the modulation % increases, and you just
get a straight line with bulbs of modulated RF along the line. Its a
queer effect and and of course the output signal attenuator is
anything but simple, as it has many positions and a fair amount of
jiggery pockery goes on. To compensate, one must re-adjust the mod %
level until the CRO tells you you have a nice 100% mod and no more
than that. The meter which tells you your Mod % is anything but
accurate, and only approximate even with atten pot at max position.
The Mod% meter merely measures the amount of AF being sent to the 12B4
modulator triode grid, and it does not measure the actual RF modulated
wave. Its a silly shortcoming but then ppl were expected to always use
a CRO to monitor the AM modulated RF wave and ppl were expected to
make the adjustments to mod % as the output level was adjusted with a
pot. Or else something is wrong with the attenuator. So the
attenuator does not work like a simple rotary switch with a string of
low resistances which I know would not **** up the % of modulation. So
I have to investigate further, and that looks difficult because the
attenuator guts is so buried among other stuff.

I read somewhere early 606 had troubles with their tuning caps. The
schematic I have is an early model one and the the fixed plates were
at +300Vdc while the earthy body and moving plates are grounded. But
in my 606A, body and moving plates are at 150Vdc, with resistance
divider biasing and shunt capacitor grounding. With less Vdc across
the small gap between the beautifully made cadmium plated tuning gang
plates, the tuning cap was much less likely to arc, and be the cause
of any problems which ppl thought were because of a poor cap quality.

All the tubes in the 606A had made for HP labels on them, and were
originals. I doubt the unit has ever been serviced or re-calibrated.
The F ranges are changed by a rugged rotating platter with 6 coils
which are brought close to the 6CL6 anodes and there are silver
contacts everywhere. Beautiful craft work. Mine has not the slightest
sign of corrosion and is in mint condition internally. Not one browned
resistance. Probably the unit wasn't used much and Nasa auctioned it
off when they would have upgraded well before anything failed.

What the 606A does not do is have an internal variable AF oscillator
with AF range between say 2Hz and 20kHz to thouroughly test the AF
range available from a given receiver while tuned on one RF carrier F.
But this can be done using an external AF oscillator. Now I don't know
what the Q of the RF amp transformer is. I have to measure that.

But suppose one is tuned at 600kHz. If we wanted audio BW of 20kHz,
-3dB, then the -3dB points of carrier level each side of 600kHz is
580kHz and 620kHz, and RF BW = 40kHz, so Q = 600 / 40 = 150. Looking
at the coils, maybeQ is much higher which would cut down sidebands
more and give less AF bandwidth, but then there maybe a damping
resistance across the primary LC, or I could put one across the 50 ohm
rated output.
I need to do further measurements, and maybe figure how to avoid the
output attenuator or fix it if its broken.

Patrick Turner.

[Mike Coatham]

On 19/11/2010 2:15 a.m., Patrick Turner wrote:
On Nov 19, 3:20 pm, wrote:
"Patrick wrote in message

...





Snip

Anyone got a clean printable schematic for the HP606A? I couldn't find
one in HP online archives.
I do have a set of schematics but I can hardly read them and most
values and numbers are illegible, even when printed up.

Snip


The 70 page Operating/Service Manual is available he

http://bama.edebris.com/manuals/hp/606a/


Mike

[Patrick Turner]

On Nov 20, 3:54*am, flipper wrote:
On Fri, 19 Nov 2010 04:58:00 -0800 (PST), Patrick Turner





wrote:
snip,

and I had said...

, and bjts are renowned for being slow at RF, but
the Ft for MJE340 is well above where I wanted to use them I may have
tried BF469 which are video transistors with Ft at 60MHz which would
have been better. But I only had a lot of BF472 with are PNP, and
which require me to build the circuit "upside down" which I didn't
fancy doing.

You said.....

Sound a lot like a sand box 6ME8 beam deflection circuit.

I've been thinking of doing it in reverse, though. Self excite G1 with
carrier. Feed single ended AF to one deflection plate and use bias
(offset) on the other to control modulation depth.

I'm not sure what a 6ME8 beem deflection circuit looks like.

OK, well, it looks a lot like your sand box circuit. hehe

Oh hell. The folks I ordered the 19BR5s from just called and they only
have three, enough for me but it screws the Christmas presents.

But I'm getting interested in this so I threw in some 6ME8s, which
they have 'on sale' for a buck each.

Anyway, 6ME8 datasheet

http://www.mif.pg.gda.pl/homepages/f...049/6/6ME8.pdf

As you can see, from cathode moving up we have G1 and a cathode
connected G2 with G3 acting like pentode folks would call a screen. I
suppose beam folks might call it an accelerator.

That's going to act like your 'tail' transistor.

Above G3 are the deflection plates steering the 'beam' between the two
respective anodes for a 'differential pair'.

See here for some SSB circuits (what the 7360 cousin was designed for)

http://jlandrigan.com/files/Receiver...ircuits%20Usin...

None of those are AM transmitters but it gives a flavor of their use.

Anyway, what I was thinking is a G1-cathode oscillator and, like your
circuit, a CT transformer on the anodes.

Say, for the moment, there's no AF but equal bias on both deflections.
You should have 'nothing' out of the PP transformer, right? Because
the same RF, in phase, is going to both sides of the PP.

Now, offset the bias on one deflection to 'half way'. The PP is no
longer balanced so we get carrier out, right?

Well, if we apply AF to the other deflection then we should be
modulating the carrier between 0 and maximum, if things are adjusted
right. I.E. AF peak equal to the other deflection bias makes for '0'
out, as that's 'balanced', and AF peak equal but opposite polarity to
the other deflection bias makes for 'maximum', since twice 'half way'
is all the way.

If AF is a smaller amplitude then we could adjust deflection bias to
move the carrier over so it idles at AF peak. Or, conversely, wherever
we set deflection bias determines the AF peak for 100% modulation.

Seems to me it has the ability to act like Alex's idea of lowering
modulation depth (to the most linear region) and then subtracting out
'excess' carrier to get effective modulation of 100%. And there
shouldn't be a carrier phase problem because both carriers are coming
down the same pike from the same cathode though the same grids in the
same tube.

So, looking at the curves, one might decide to limit maximum
deflection to 50% so you'd offset the deflection bias 25% and adjust
AF amplitude *for -+ 25% deflection.

That's just an idea and there would, no doubt, be other things to work
out. Like, just off hand we've got a bias current offset too so would
we need to choke load the anodes and cap couple into the transformer?
Or maybe it doesn't matter air core? I'm not versed in RF
transformers, which is one reason I've been reluctant to try this
stuff.

Anyway, it's a thought.

If you want maximum output, Scotty I need more power, you could center
the carrier and just single end one output into a single primary. In
which case whatever the deflection curve is from 0 to 100 is what the
curve is and, as I joked earlier, you call it 'automatic dynamic peak
compression rather than distortion.

That might not seem any better than the 'normal' way but the center of
those deflection curves look straight as an arrow so distortion should
be pretty dern low till you get to 'peak compression'.

snip of stuff to think about

The ARRL books and UK ham books have use for 7360 for generating SSB
signals.

But nothing about their use for plain old AM which went right out of
fashion among radio hams because a little bit of SSB went just as far
but was a lot cheaper to do.

Hams have wives, and wives hate husbands hobby expenses. OK if he
fiddles at night without costs but pity help the ham who spends the
holiday dough on a nice new pair of 813.


Anyway, all the AM generator schematics generaly advise PLATE
modulation as the best form of generating AM. It could be done in a
signal gene and I could easily have used a second pentode as a power
tube to work in series with the anode RF primary of my little R
tranny, which BTW is a very easy thing to wind for BC band and SE use,
only 200uH needed, and an old radio tuning gang. The AF powrer amp
must be linear though, so there must be driver tube and NFB, and a
6BM8 could be used.
But the OPT should be about 1:1 so the voltage supplied to anode
circuit of RF stage is quite high.
I baulked at winding such a coupling tranny, but tubed modulators use
such things routinely, but you can't always buy one anywhere.

The books then move along to screen modulation where AF is applied to
the screen of an RF tube excited with RF at g1.
This needs high voltage AF and linear.

Then there is cathode modulation which can be done using a normal SE
amp with 6BM8 and speaker tranny using the 16 ohm winding at low Z to
put AF into the cathode of an RF class A SE amp. I built a transmitter
this way when I was 15. Parts from two unwanted mantle radios were
used which had 6V6 in them.
My sister kept speaking into the transmitter built while I walked up
the road to see how far mum's new 7 transistor portable radio could
receive it. The signal was dirty, as it appeared on several places on
the band. Then my dear sister started saying "these broadcasts are
being brought to you by Patrick Turner and are highly illegal
transmissions..."

Patrick Turner was then seen breaking all athletic running records in
an effort to seek discourse with dear sister.

Of course we got away with it. And I learnt more, and buried my
illegal activities in the vastness of lower short wave bands where few
ppl were listening, except my mate, for we had an illegal radio
telephone using 2 different frequencies, and headphones to prevent AF
fedback. Only 16 then.


Recently I found with cathode mod it was less easy to get a full
linear range of mod between 0% and 100% when using an SE pentode.
But with PP pentodes it works flawlessly, as it does in my HP 606A sig
gene, right up to 65MHz.

Finally the books mention grid modulation which appears to be the
least efficient for a transmitter.

But carefully adjusted I get good modulation linearity with one triode
for an oscilator, 1/2 12AT7, one buffer, other 1/2 12AT7, and one
pentode, 6BX6 or 6EJ7, and one double gang tuning cap, and two RF
coils, mine are different, one is air cored for PA, other is ferrite
cored, less turns, but same L and low stray self C. The oscillator and
PA track each other closely, and you don't get all the parasitics and
bull**** fiddles needed with solid state.

Audio needs to be about 7Vrms, and from a source less than 1k if
possible.

The same could be done using transistors, but why? the tubes give
better results IMHO.

Patrick Turner.



Patrick Turner.- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

[Patrick Turner]

On Nov 20, 1:13*pm, flipper wrote:
On Fri, 19 Nov 2010 04:58:00 -0800 (PST), Patrick Turner





wrote:
snip,

and I had said...

, and bjts are renowned for being slow at RF, but
the Ft for MJE340 is well above where I wanted to use them I may have
tried BF469 which are video transistors with Ft at 60MHz which would
have been better. But I only had a lot of BF472 with are PNP, and
which require me to build the circuit "upside down" which I didn't
fancy doing.

You said.....

Sound a lot like a sand box 6ME8 beam deflection circuit.

I've been thinking of doing it in reverse, though. Self excite G1 with
carrier. Feed single ended AF to one deflection plate and use bias
(offset) on the other to control modulation depth.

I'm not sure what a 6ME8 beem deflection circuit looks like.

Well, it appears my observation of similarity was no 'accident' as,
according to this site, the Gilbert cell derives directly from the
beam deflection tubes, being essentially a SS 'emulation' of it.

http://www.analog-rf.com/mixer.shtml

According to that fellah the solid state versions suffered from what I
suggested, device mismatch, and what you mentioned, Vbe non linearity.- Hide quoted text -


I'd read the James Long site http://www.analog-rf.com/mixer.shtml and
that bloke has a loud grumble about just about everything.
OK, maybe he's mainly right but its all history now.

But for a discrete part circuit without any NFB to keep the SS devices
linear, the gilbert cell I made up wasn't too bad a performer even
without the tweaks I may have done in addition to what I did do.
But I have a HP 606A, and it was worth getting that going.

I tested the 606A for the audio bandwidth capabilities. Seems as
though if modulation is set at 100%,
then for a 455kHz carrier signal, I got a flat and undistorted
envelope from about 50Hz to 2kHz. Above 2kHz the envelope suffered bad
distortion with the AF wave shape looking like bad slew distortion.
But with mod set at 50%, the AF bandwidth was 23Hz to 50kHz, -3dB, so
quite god enough for testing an AM radio AF pass band. Same for 550kHz
to 1.7MHz, So the Q of the tuned LC tank in the 606A was not too high
to cut sidebands. This makes evaluating the receiver BW easy, just
tune to the 606A, and record the detector AF bandwidth.

I have yet to evaluate the signal path of external modulation signals
to the 606A. The input resistance for external mod is 1k0, and perhaps
its my oscillator gene which can't give a 2.5Vrms undistorted sine
wave into only 1k0. It should be OK as I have a 6AR5 CF output from a
completely gutted then re-designed old Japanese AF oscillator someone
donated to me.
It does 20Hz to 220 kHz, flat, wien bridge with tuning gangs.

Its possible to avoid the whole existing 606A attenutator unit because
the box with tubes, coils and tuning caps has ONE signal lead out from
the secondary on the OPT. the other end of the secondary is grounded.
Its an SE output.
The lead out has a 50 ohm plug and socket, so an alternative lead out
to a different attenuator, maybe an external switch in a box can be
arranged to avoid the existing which wrecks the modulation % in a
queer manner afaiac if the fine tuning pot is used.

I don't have the schematic of the 606A attenuator. But its output
measures 50R DC resistance on each range and its constant for any
level, so basically its done right. It may have compensation caps in
there, and things I cannot estimate.

Patrick Turner.

[lyly10388]

Originally Posted by flipper

On Fri, 19 Nov 2010 04:58:00 -0800 (PST), Patrick Turner
wrote:

snip,

and I had said...

, and bjts are renowned for being slow at RF, but
the Ft for MJE340 is well above where I wanted to use them I may have
tried BF469 which are video transistors with Ft at 60MHz which would
have been better. But I only had a lot of BF472 with are PNP, and
which require me to build the circuit "upside down" which I didn't
fancy doing.

You said.....

Sound a lot like a sand box 6ME8 beam deflection circuit.

I've been thinking of doing it in reverse, though. Self excite G1 with
carrier. Feed single ended AF to one deflection plate and use bias
(offset) on the other to control modulation depth.

I'm not sure what a 6ME8 beem deflection circuit looks like.

Well, it appears my observation of similarity was no 'accident' as,
according to this site, the Gilbert cell derives directly from the
beam deflection tubes, being essentially a SS 'emulation' of it.

http://www.analog-rf.com/mixer.shtml

According to that fellah the solid state versions suffered from what I
suggested, device mismatch, and what you mentioned, Vbe non linearity.

Such a very amazing link!

[Patrick Turner]

On Nov 27, 2:37*am, lyly10388
wrote:
flipper;920344 Wrote:

http://www.analog-rf.com/mixer.shtml


Such a very amazing link!

--
lyly10388-

Not really that amazing, but there are the multiple claims or
fraudulant intellectual behaviours by ppl like Charles Darwin and many
others. OK, its a very human thing to misbehave to get ahead. Such
behaviours began when we stole bananas off each other in the jungles
some millions of years ago. Animals always have found a surprising
number of options to survive, and they find food on their own, or
perhaps they steal what some other has just found, or to pillage what
some other entity has stored up after a lot of work which is thus
avoided. Hence the evolution of making war. The evolution of peace
took rather longer and for a long time was rarely a viable option for
survival amoung competing members of our own species. So from a very
long time ago our little puny brains evolved to be larger and we have
simply become more devious about farnarkling with intellectual and
acedemic matters. We found ways to convince other people that the
banana we found was what we found out there in Natureland and that
nobody else has found that particular banana, and that banana was ours
and ours only, and we could patent the ****in thing. Generally
speaking, mountains of disagrable bull**** are generated in the
intellectual processes of knowledge expansion. One lot of bull****
disagrees with another, because one banana has a slightly different
shape and colour to another.

But that site does not cast any fresh light upon the function of a
Gilbert cell or upon the OT of AM modulation, hence the site is not
all that amazing.

I am going to do more fiddling around with the bias on a PP output amp
circuit to see how much AM can be generated because maybe that's the
best way to generate vibrato in guitar amps.

Patrick Turner.