I have a Gibson GA-15RVT tube amp here, circa mid 1960's, with a hum problem that seems like a no-brainer, but I'm completely stuck on it. The reverb recovery stage is adding a significant amount of noise, more than is acceptable. It is there even with the reverb tank disconnected and/or the grid of V1B grounded. One of the footswitch buttons grounds the junction of C15 and R15 when toggled. When that junction is grounded (reverb effect OFF state) the hum in question is not present. When the footswitch is toggled (or disconnected) the junction is no longer grounded and the hum is present in the output. I've done a number of things in the way of troubleshooting, e.g. the grounding of the tube grid that I mentioned, also removing the wire that goes from C15/R15 to the footswitch, replacing the V1 tube, etc. but have not been able to eliminate the hum other than grounding C15/R15. So my question is. looking at the schematic, is it a given that this design will result in noise being introduced? In other words, is it a bad design and there's nothing that can be done to eliminate the noise?
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