03-25-2013, 11:08 AM
Something does not seem right in the 57 detector stage. If there is 16V on the cathode, there is effectively -16V on the control grid which would put the tube way beyond cutoff. The tube should be biased just before cutoff (5 or 6V) so that positive peaks of the signal will cause the tube to conduct and rectify the RF, producing audio in the plate circuit. With 16V of bias it takes a very large RF signal to produce any audio, which seems to be what you are experiencing.
If the 40K resistor in the cathode circuit is ok, then I would probably say the 57 tube is bad ( gassy) which is causing the tube to conduct more than it should, allowing more current to flow through the cathode resistor. This would increase the bias past what it would be under normal operating conditions.
Do you have another 57 you can try?
If the 40K resistor in the cathode circuit is ok, then I would probably say the 57 tube is bad ( gassy) which is causing the tube to conduct more than it should, allowing more current to flow through the cathode resistor. This would increase the bias past what it would be under normal operating conditions.
Do you have another 57 you can try?
