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.


--

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Richard Knoppow
Los Angeles
WB6KBL

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

"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.


--

--
Richard Knoppow
Los Angeles
WB6KBL

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.

"Paul P" REMOVE paul @ REMOVE ppinyot . REMOVEcom wrote in
message ...
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, All American is obvious once pointed out:-) I
found the S-38B has the same arrangement.


--

--
Richard Knoppow
Los Angeles
WB6KBL

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

"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.


--

--
Richard Knoppow
Los Angeles
WB6KBL