gu81m G3 modulation?

[pe1pal]

hello OM, I have build a Gu81 amplifier for AM 160 and 80 meter.
At this time I am running the amplifier with 5W drive power at 2KV giving around 200W.
I have modulation in the driver. Now I wonder if someone have experients with G3 modulation! Is this a solution for receiving full modulation without a big modulator?
73'Paul  PE1PAL

[Opcom]

The ARRL handbook says so. Tune the stage to normal maximum power with zero volts on the supressor. Increase the negative bias to the suppressor until the plate current (and RF ammeter reading) drops to half. now apply modulation. The supressor is modulated by a transformer winding in series with the supressor and the negative voltage supply. The book says 100V is the most you should need.

[The Slab Bacon]

Paul,
       Although I'm not familiar with the GU81 tube, this should apply to most pentode and tetrode applications:

It is usually much easier to attain better modulation swing if you modulate the screen grid (G2) instead of the supressor. Since the Screen grid is wound with a finer mesh than the supressor it will cause the plate current to swing much easier than the supressor. It is very important to bias the control grid (G1) to get the tube way down into class C. The further down it is biased the less negative you have to drive the screen to get 100% negative (cutoff). the plate efficiency will also be a little better with screen modulation.

You can do it several ways, A cathode follower to swing the screen voltage, a large interstage transformer (class B driver type) or a small plate mod transformer in series with the screen voltage feed. You could also probably do it with a choke coupling scheem ala Heising.

I know that it does work, because I use a home brewed screen modulated 4-1000A transmitter that has no trouble making copious power and well in excess of 125%+ positive peak modulation. Some folks will say that a screen modulated transmitter wont make 100%, dont believe them. The only downfall is the reduced plate efficiency compared to plate modulation.

                                                          The Slab Bacon