When the G2DAF circuit was first published (in the old RSGB 'Bulletin'),
there was a degree of vituperative correspondance, with some people claiming
that Dick's signal was poor, and others saying it was good! G6JP did some
analysis (he was a professional in tube manufacturing at the M-O Valve
company, where the original KT66 came from) and was very dubious.
I would expect the performance on 2 tone test to be very critically
dependent upon the time constant in the rectifier filter for the screen
grid. To some extent, the same would apply on speech, while the regulation
of the screen rectifier will also affect matters - if it such that the 3/2
law is applying, but the resultant plate current has gone up linearly
because of poor regulation, you could well have something very linear, but
critical on set up. The best test would a white noise test with modulation
at syllabic rate - say 10 to 15Hz. Alternatively, the 3 tone test that W8JI
advises, with two tones very close together. I guess that I go along with
G3SEK that it all depends on the tubes, the rectifier (whole circuit
including filter) etc. F'r instance, I'd expect 4CX250s with their negative
screen current to be awful.
73
Peter G3RZP
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