"Bill Cohn" wrote in message
...
Paul P wrote:
I am not sure what AA receivers means. Boy, it sure
looks like series type grid leak bias.

That would be All American 5 tube line up. And not %. I
do that my self some times. The hot chassis tube line up
and circuitry patented by RCA.

Did I get that right?

Paul P.
Thanks for the correction as I meant AA5 or All American 5
like the S38.

Contact bias is the slight negative voltage developed on
the control grid if the grid resistor is kept very high
like in the range above 2.2 megohms. This voltage will be
about -1 to -2 volts sufficient for low level audio
signals.

N9MHT-Bill

Thanks Bill, and to the others who responded. I've now
discovered this is a pretty common arrangement. However, it
doesn't seem to be mentioned in any of the older texts I
looked at and neither is a gas gate tube, another feature of
the S-20R and S-40 series. Later receivers don't seem to use
them. I suspect they were necessary in receivers designed
around the early metal type octal tubes, which, if I
remember right, had a reputation for sometimes being a
little gassy.
Even though the S-20R did not originally use the
contact bias arrangement I will try it and compare the
results with the original circuit which is the conventional
cathode bias sort. The values in the S-40A should do since
otherwise the circuits are nearly identical other than the
RF and mixer stages. As I mentioned, the circuit diagram for
the very early S-40 shows the same cathode bias as in the
earlier model but its revised in later production to the
contact bias circuit. Since this would not have resulted in
any cost saving it must have been done to improve
performance.


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Richard Knoppow
Los Angeles
WB6KBL