"John Byrns" wrote in message news:byrnsj-
Why would one want to do this vs. simply driving the control grid as
intended? The only reason I can see is higher potential power output
due to increased peak plate current, assuming the tube can tolerate
abnormally high peak screen voltages while the plate voltage is low.
Regards,
John Byrns
Reply by Jim, WD5JKO,
Many years ago the Gonset company did exactly this with amateur radio AM
transmitters. One example is where a pair of 6DQ6's plate modulated a single
class C 6DQ5. The 6DQ6's ran with G1 and the K grounded. The audio drive
came from a 6CM6 (similar to 6AQ5/6V6 but 9 pin), and drove either the
speaker (receive) or the 6DQ6 G2's (transmit).
Why would they do this? This was a transceiver designed for base or mobile
operation. Therefore size, weight, and power consumption were all big
considerations. Running the 6DQ6's this way with zero bias eliminated the
need for a bias supply and a screen supply. Since several watts of audio was
already available, why not use that to drive the 6DQ6's into G2 and have
them act like a hi - mu power triode (kind of like a baby 811a). Running the
modulators in class B also boosts the efficiency.
This modulator was capable of an easy 50 watts RMS of audio with a plate
voltage of about 600v.
This circuit in the Gonset with a few simple modifications is pretty clean,
and when compared to other designs of that AM ham era, this was sometimes
cleaner. Running screen grid tubes class AB2 and G1 driven with high
impedance drive, and high impedance G1 and G2 supplies can result in far
more distortion.
My experience with the G76 is that if you maintain the 6DQ6 G2 waveform
clean, then the plate waveform is almost an exact replica of the input. This
takes more NFB, and beefing up the 6CM6 audio driver (lower Rk, and higher
Ck). I raise the B+ to the 6CM6 on transmit to about 300v, and reduce it to
about 225 during receive. This keeps that tube cooler during extended
receive periods.
For such a simple circuit the modified G76 sounds quite good, and the Gonset
topology of driving the modulators as they do is very effective and
efficient.
Jim
see schematic 3 at the following link:
http://bama.edebris.com/manuals/gonset/g76/