Series Modulation of a 807 CW Transmitter

[wd8pcu]

Hi Everyone,

I have been setting here looking at Steves Circuit.
I have a question for the group.
Suppose that I place another 807 in a box outside my
807 Final CW Transmitter. Now suppose I connect the Plate
of the outboard 807 to the Cathode of the Final 807 and I ground
the Cathode of the outboard 807.
Now you would think that if I apply ONLY enough bias to park it
in Class B where NO Plate Current Flows without Modulation that
this would be a down right sweet little circuit.
Understood that I believe that I would have to double the NORMAL
Plate voltage from 500vdc to 1000 or slightly above.
What do you think?
Is it worth the effort?
I have all of the parts.
Dan

[The Slab Bacon]

If you are serious about series cathode modulation, you should look for
Frank C. Jones' book on it. He had a lot of different scheems for it.

However with cathode mod you are basically using the series mod tube as a variable switch to control the current flow through the final tube. You set the biass in the mod tube for the resting carrier level, and then swing the grid from cutoff to full on with the audio voltage.

I have often pondered doing a big power cathode mod transmitter using one of those high voltage
horizontal output transistors from a modern tv instead of a tube. (keeping the tube final) ! or 2 of them could easily mod a "legal limit" rig with a minimum of parts.  Keep in mind that in cathode mod service the mod device is operating in a very low impedance environment.

                                                                                  The Slab Bacon 

[WA1GFZ]

Frank,
I've done it. Those transistors have crap for Beta / gain. You would be better off with an IGBT or high voltage FET. Transistor will go into thermal runaway when it heats upo so bias would need to be temperature compensated.

[W3FJJ]

Dan, should work nice.
Check out am window, theres a nice schematic of series modulated
807.
http://www.amwindow.org/tech/htm/wb9eckseriesmod.htm

73, Chuck

[The Slab Bacon]

When series cathode modding a transmitter You want to use a tube wuth the lowest plate resistance possible. I cant believe no one has ever used something like a 6080 or a 6AS7G or similar low Mu triode. It just seems like a natural for the job. 1 or 2
6080's with both sections in parallel could prolly modulate a "legal limit" rig!
These toobz are now very cheap and plentiful at hamfesters.

                                                                      The Slab Bacon

[Rob K2CU]

Keep in mind that for series modulation you will need separate filament transformers as the tube with its cathode at +500 or more will need its filament supply at about the same value. Cathode to filament insulation is good for only about 100 volts or so. You could let the filament supply winding float, but stray capacitance of the transformer winding may result in unpredictable values. Might be best to tie one side to cathode or use a center tapped winding with tap tied to cathode. You might look at K1JJ's series 6AQ5 pw unit.