The second test with an 813 was somewhat discounted because I have no specs
from anyone on what the distortion products are when grid driven with a
regulated screen. Suppose they are -18 db 3rd order distortion. That would
make the test meaningless. At least with the 4CX250B, I have the published
standard of -25 db. That published standard is only 3 db less than the
results you arrived at with the G2DAF - which is close enough for me to
wonder about the test conditions and the standard.
A number of test questions arise. What was the time constant for charging
the screen of the tubes? If there was a delay, there would be distortion.
Suppose the screen voltage could follow the drive exactly? Would the
results have been different? What was the distortion of the amps in the
"normal" giid driven configuration?
I would never build a G2DAF amplifier and put it on the air without testing
it under all circumstances. Perhaps that is the big drawback. You can take
a published circuit or copy one of the fine commercial amplifiers and get
good results - even to the point of not testing it yourself. All you have
to do is follow the proven procedures. You could not do that with a G2DAF
amp. But, as an experimenter, I would like to know more.
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