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Hallicrafters S-40 Puzzle
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 |
Hallicrafters S-40 Puzzle
"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 |
Hallicrafters S-40 Puzzle
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. |
Hallicrafters S-40 Puzzle
"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 |
Hallicrafters S-40 Puzzle
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 |
Hallicrafters S-40 Puzzle
"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 |
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