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

On 09/30/2017 10:47 AM, Scott Dorsey wrote:
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

In article , philo wrote:
On 09/30/2017 10:47 AM, Scott Dorsey wrote:
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?

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."

On 10/01/2017 08:31 AM, Scott Dorsey wrote:
In article , philo wrote:
On 09/30/2017 10:47 AM, Scott Dorsey wrote:
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?

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




Like I said it was very common in the old days

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

In article , philo wrote:
On 10/01/2017 08:31 AM, Scott Dorsey wrote:
In article , philo wrote:
On 09/30/2017 10:47 AM, Scott Dorsey wrote:
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?

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.

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."

In article , philo
wrote:

On 10/01/2017 08:31 AM, Scott Dorsey wrote:
In article , philo wrote:
On 09/30/2017 10:47 AM, Scott Dorsey wrote:
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?

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




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

Fred McKenzie wrote:

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."

On 10/02/2017 09:44 AM, Scott Dorsey wrote:
In article , philo wrote:
On 10/01/2017 08:31 AM, Scott Dorsey wrote:
In article , philo wrote:
On 09/30/2017 10:47 AM, Scott Dorsey wrote:
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?

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.

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

On 30/09/2017 16:47, Scott Dorsey wrote:
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


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

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



On Monday, 2 October 2017 15:44:20 UTC+1, Scott Dorsey wrote:
In article , philo wrote:
On 10/01/2017 08:31 AM, Scott Dorsey wrote:
In article , philo wrote:
On 09/30/2017 10:47 AM, Scott Dorsey wrote:
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?

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.

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."