suppressor grid modulation?

Post by Scott Dorsey
So, if I adjust the voltage on the suppressor grid of a pentode I can alter
the gain of the tube substantially, but in the process I now am moving most
of the current from the plate to the screen grid.
Are there any tubes out there with a sufficiently heavy screen grid that
this could be done with? I can always reduce the total current to prevent
screen grid damage but then linearity suffers.
Do any small signal pentodes out there exist with heavy ruggedized screen
grids?
--scott

Sure that was done plenty of times back in the old days.

I think some of the cheaper Heathkit transmitters use SGM

Scott Dorsey

2017-10-01 13:31:45 UTC

Sure that was done plenty of times back in the old days.
I think some of the cheaper Heathkit transmitters use SGM

No, other way around, I want to modulate the suppressor and not the screen.
Screen grid modulation is... not really very linear at all but requires
little drive, which makes it a win for inexpensive communications grade
radios.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

philo

2017-10-02 02:58:44 UTC

Sure that was done plenty of times back in the old days.
I think some of the cheaper Heathkit transmitters use SGM

Like I said it was very common in the old days

http://www.vias.org/basicradio/basic_radio_17_06.html

Scott Dorsey

2017-10-02 14:44:18 UTC

Sure that was done plenty of times back in the old days.
I think some of the cheaper Heathkit transmitters use SGM

Like I said it was very common in the old days
http://www.vias.org/basicradio/basic_radio_17_06.html

Weird! I've never seen that before! What tubes were common for that?
Can't be anything too delicate...
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

philo

2017-10-04 00:23:34 UTC

Sure that was done plenty of times back in the old days.
I think some of the cheaper Heathkit transmitters use SGM

Like I said it was very common in the old days
http://www.vias.org/basicradio/basic_radio_17_06.html

Weird! I've never seen that before! What tubes were common for that?
Can't be anything too delicate...
--scott

Too long ago for me to recall.

All I know was that I could not afford a plate modulator so from plans
another ham gave me a build a cathode modulator.

I could run my 807 final at about 15 watts but it was fully modulated

Graham

2017-10-24 18:58:57 UTC

I did something similar, while back, with 807 , 1970's but used a back to front output transformer , with small 4 ohm transistor amp to the speaker windings . old primary 6k ohm was in series with the screen supply.

I used a potential divider for the screen voltage, lower resistor open ctt , tune for max RF out . then , flick the switch , screen to half voltage and modulated with the audio drive , gave good upward modulation , there was a cap to ground from the centre tap of the divider to couple audio to ground .

Graham
G0NBD

Sure that was done plenty of times back in the old days.
I think some of the cheaper Heathkit transmitters use SGM

Like I said it was very common in the old days
http://www.vias.org/basicradio/basic_radio_17_06.html

Weird! I've never seen that before! What tubes were common for that?
Can't be anything too delicate...
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Fred McKenzie

2017-10-02 17:13:26 UTC

Sure that was done plenty of times back in the old days.
I think some of the cheaper Heathkit transmitters use SGM

Like I said it was very common in the old days
http://www.vias.org/basicradio/basic_radio_17_06.html

How old is the "old days"? I do not doubt it can be done, but I never
came across it before. The tubes I knew were the 807/1625 and 6V6/6L6.
I think the 6L6 had a Suppressor Grid, but it was internally tied to the
Cathode.

Heathkit definitely tried Screen Modulation. They went as far as
increasing carrier power during modulation, called "Controlled Carrier
Modulation".

I suspect Suppressor Modulation may not be as bad as Scott thinks, or
else the example given was done at a low power stage.

Fred
K4DII

Scott Dorsey

2017-10-02 18:34:53 UTC

Post by Fred McKenzie
I suspect Suppressor Modulation may not be as bad as Scott thinks, or
else the example given was done at a low power stage.

It would have to be done at a low power stage, because when you are reducing
the carrier, you're shifting current from the plate to the screen and the
screen on most tubes is not really designed to sink much current. But even
so, it would still require a tube with a hefty screen grid... the 6AU6
would not last very long in that service.

I don't know how nonlinear it is, but I'm prepared to set up a jig on the
bench and find out!
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Gareth's Downstairs Computer

2017-10-07 11:07:18 UTC

I am reminded of Series Gate Modulation, the trick
used on 144MHz AM to avoid the cost of the Woden modulation
transformer.

Hank

2017-10-26 15:53:27 UTC

Coming into this late, as I don't often read this newsgroup any more.

Scott, you need a power pentode with a separate supressor terminal.
RCA made one: 6AG7. Overall, similar characteristics to a 50L6, but a
pentode, not a beam tetrode.

You're on your own working out the Gm and Mu curves for G2 and G3.
RCA came up with the tube, along with the 1851,1852,and 1853, for prewar
TV video amplification.

Hank

Scott Dorsey

2017-11-10 19:18:42 UTC