Rich:
In any screen grid amplifier, plate current is determined by the combination
of control grid and screen grid voltage. Varying both the control grid and
the screen grid certainly makes testing much more difficult and also adds
another variable that can screw things up, but these variables do not mean
that it cannnot work.
What happens if the screen grid is zero volts. No distortion will be heard,
because there will be no plate current. Splatter is most likely to occur
when the input signal causes a flattening of the plate curve. We have
previously discussed the spikes that are transmitted in many exciters at the
beginning of transmission - especially when alc is used.. These spikes
saturate the plate and severe splatter occurs.
However, in a G2DAF amplifier, screen voltage is at zero during voice lulls.
When drive is applied, voltages to both grids increase. If the screen
voltage lags a bit, that would not cause plate saturation. Your friend Norm
hung a 1 mf cap on the screen - thus keeping the screen fully charged.
Then, he ran a substantial amount of grid current. Splatter would and did
occur. I do not know how your other friend hooked his up, but I expect it
was an "improved" model of the original circuit.
A few years ago I heard a new extra class ham, almost 50 miles away,
splattering 200 khz away from his fundamental. His transceiver was a
FT-1000D and his amplifier was a new Alpha. The splattering was caused by
the fact that he was relying on his ALC to limit everything, and the spikes
at the beginning of his speech overloaded everything. This was not the
fault of the equipment, it was operator error. Should I condemn all
grounded grid amplifiers because they can be used improperly? That is what
you are doing to the G2DAF circuit.
Distortion is measured against peak output, and is caused when the plate
current does not faithfully follow the input. I wrote a hypothesis on how
the screen voltage can vary in a screen grid grounded cathode amplifer yet
have linear output. I confirmed my hypothesis on my test bench. Lars has
run tests on his amplifier, which were also consistent. But, you have
ignored those results.
The Collins KWM2 and 32S1 does not use a "regulated" screen supply, yet it
produces an excellent signal. Bill Orr regulated his and reduced distortion
even further, but because of rf feedback, The Collins provides an excellent
signal even though it does not use the "regulated screen" that you say is
essential.
I should get one of the QBL tubes and copy the circuit of Lars and put it on
the air over here, and see if you can tell when I have my grounded grid amps
on or when I am using the "other" circuit.
Colin K7FM
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