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