Sosijdog wrote: ↑Sat Apr 03, 2021 7:23 am
...If the diode fails momentarily AC will be passed onto the grids of to output tubes activating them. Also any signal will pass through the coupling cap backwards into the preceding stage and appear at the pins. The coupling caps are not polarised so this is possible...
That hypothesis seems unlikely to me. If the bias supply diodes D23 shorted intermittently, its reservoir ecap C28 100uF would
a/ present a very low impedance to the 60VAC winding and rapidly overheated from the current flow.
b/ being an ecap, its insulation is unipolar, eg it might be thought of as being a diode with a very high junction capacitance, and hence it would overheat rapidly.
So as you've tested the ESR of all such caps without any oddballs, and extreme overheating might be expected to damage the cap and affect its performance, it seems unlikely.
Even if it did happen, AC noise would be heavily filtered out by R84 5k1 and C56 22uF, so very little high frequency noise of the type exhibited might be expected to pass through to the bias pots.
And what does pass would be common mode to a balanced system, and so would tend toward being cancelled out.
Suppose alternatively what if D23 was intermittently going open, rather than short, circuit?
C28 would be unaffected, which aligns with the findings, but any noise, eg from rapidly filling open circuit and back, would still get filtered out and be common mode.
Hence I am skeptical that D23 is the root cause of the noise issue.
Also, how does whether the coupling caps being polarised or non polarised types affect how they might pass such noise signal?
I suggest to scope TV4 and 5, the power valve current sensing resistors, and monitor how the noise manifests.
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