"Bill Cohn" wrote in message
...
Richard Knoppow wrote:
I'm taking a chance that this is an obvious and dumb
question. So be it:-)
In researching the S-20R I looked at the circuits
for the S-40A and B, which were its successors. I've
found a puzzle, namely the first audio grid bias was
changed from a standard cathode bias arrangement in both
the S-20R and early S-40A to a grid leak bias in the
later S-40A and S-40B. Why? Grid leak bias is pretty rare
in audio stages and can reputedly cause distortion. Does
anyone know what H's designers were up to?
I can't find any other examples of grid leak bias in
audio stages.


The use of contact bias in first audio stages was very
common in most AA% receivers. This is not grid leak bias..
This was normally done so that the cathode of the audio
tube was grounded as this was often the cathode of the AM
detector. If you don't ground the detector diode you have
a bias the signal must overcome. This can cause distortion
in the detector. I am not sure why Hallicrafters did this
on the S40B as the detector was removed from the first
audio tube.

73 de N9MHT
Bill Cohn

The last is one of the puzzles (that is separate first
audio and detector tubes). Evidently they had some reason
for it.
I am not sure what AA receivers means. Boy, it sure
looks like series type grid leak bias.


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Richard Knoppow
Los Angeles
WB6KBL