[AMPS] G2DAF amps

[Peter Chadwick]

There was a lot of discussion in the pages of the then RSGB Bulletin
when G2DAF first described the circuit. G6JP was extremely dubious about
it. I guess that it is possible to analyse it: it will depend upon exact
transfer ratio of input volts to screen grid current. In a conventional
tube amplifier, the Ia is proportional to Vg to the power 3/2.
Similarly, the Ia is proportional to Vg2 to the 3?2, so driving both
suggests a transfer characteristic of Ia proportional to Vin to the
power 2.25. The actual characteristic is, I think, also going to depend
on the mu g1 - g2. It should be possible to do a Chaffee analysis. or
even a straightforward translation from the tbe data sheet to get a
graph of the theoretical plate current against input volts, and predict
from that the expected IMD performance.

I would be very inclined to measure the IMD with W8JI's 3 tone test,
with 2 of the tones about 40 or 50 Hz apart - that will show up any
problems with the time constants in the SG circuit as well as in the HV
PSU.

The alternative IMD measurement is to use white noise.

There does seem to be a variation in performance depending on the tubes
used - the original G2DAF used 4-125A's.

73

Peter G3RZP

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