The AM Forum

I just had a very good idea today, but have no idea how it would work. First things first, how exactly could I bias a GG amp to class C? My idea was to take a grounded grid amp (possibly even my current linear) and modify it to run in class C instead of linear operation, then cathode modulate it with a transformer between the filament transformer's center tap and ground. Ignoring that cathode modulation is lower efficiency than plate, by biasing the GG amp to class C, would the carrier have the same efficiency as in a grid driven design? Because when I would tune the amp, it would be dipped and loaded for max output, then loaded heavier to modulate linearly. Would the efficiency at max power (before loading it heavier) be the same as a grid driven plate modulated class C amp or would it be lower? When set up for cathode modulation, it will probably be only running about 50% efficiency. The other idea I came up with was what if I took a tube with an indirectly heated cathode, and applied the audio to the center tap of the filament transformer, but applied the RF drive to the cathode?

Shelby,

Regarding your first idea you're a bit late here however, I sent you an email of a
circuit that should work for modulating the GG Linear Amp...

As for the second idea, think about it...  "INDIRECTLY" heated cathode...
That means the filament is ISOLATED from the tubes operating elements.
This should tell you something...

Interesting, because I was reading about using GU-50 Russian pentodes (that have indirectly heated cathodes) in grounded grid and it said on the higher bands to feed the RF into the filaments if you had problems driving the cathode. So, in that case maybe it could be switched, audio on the cathode and RF on the filaments.

Several indirectly heated triodes, etc. have references to tying the filament to the cathode because of limited filament to cathode spacing with potential breakdown but this is in GG service where the grid is grounded, - 8874's etc.

So the cathode, along with the all the filament pins are grounded to RF, AND audio AC, through the filament center tap, with slightly offset DC bias voltage, say 8 volts or so. RF is isolated from the filament transformer with parallel winding chokes or three wire windings in the case of the cathode being kept separate from the filament windings.

If I'm thinking right (not necessarily so), impressing audio (AF) on the filament would have a "somewhat" heh, heh, hummy sound, say 60 watts audio impressed on 20 watts of 60 cycle AC in the case of an 8874 (6.3V @ 3A fil.) running 120 watts or so plate input. Now could a hum balance circuit be devised to minimize the 60 cycle AC and not the speech audio?  I need to think this through.  Oh, and don't forget to bypass the AF amplifier for RF in the filament circuit.

Modulation may approach 90% a/c fed through RF in a GG circuit.  I suppose you could modulate the driver too.  Hey, two hummies are better than one.

But then your getting back to simply modulating the driver's RF with less audio required for cathode modulation in a GG circuit.  Linears in AM service don't have nearly the fear factor that the ARRL promoted for so many years.  Statements such as" a 4x pep linear amp. is no amplifier at all" in AM service served to dissuade the 'washed' from even trying to use a linear for AM.
Little did they know or want to allow that 13 to 15 db single stage GG power amps even existed in AM service, or that terms such as headroom had significant meaning.

Covered in one instance by AB2EZ.

http://amfone.net/Amforum/index.php?topic=16683.0

Stu is driving the cathode with RF, but he is plate modulating the final. I've heard of a few other people cathode modulating directly heated cathodes with a audio transformer in the center tap. The center tap would still be grounded, but through the secondary of the audio transformer.

Oh yea. Got it now. I recall seeing an interesting circuit on this using a solid state amp as the modulator.

Ok, so use DC filament voltage at RMS equivalent voltage and all that, to eliminate all possibilities of AC hum?

thought id fire up the 32v on 7295, talking to Dave wb3etn , t368 now.
using plate mod. 4d32 here. 
so will check back later.

Well, another possibility I thought of was to make the tube grid driven, but run the filament off of a DC supply and modulate the supply, similar to how some people series modulate a solid state transmitter. The cathode would be directly heated. The only problem I see is the filament voltage would have to be kept above zero.