nashvillebill
Super Member
It is indeed plausible that hum could be heard through the speakers without any power tubes in the amp. Here's a couple of possible ways:
1) The OT secondaries are typically tied to ground at one point, but another winding is typically a tap for the NFB. Since the power supply usually has some ripple, if any of the B+ somehow is making its way onto the NFB, then the AC from the ripple may cause hum. Or maybe bias voltage instead of B+. Voltage measurements could find this. Or disconnect the NFB from the winding.
2) The OT primary may have flyback diodes. If one of those is leaking, the ripple on the B+ may be creating hum. Or for that matter, if there's any other leakage from one of the plates to ground or some other part of the circuit, the ripple will cause hum.
Of course there's more exotic failure modes, that's just two ways that came to mind immediately. Oh I guess a failing OT could cause hum, shorting to ground, but that would likely be a catastrophic failure.
1) The OT secondaries are typically tied to ground at one point, but another winding is typically a tap for the NFB. Since the power supply usually has some ripple, if any of the B+ somehow is making its way onto the NFB, then the AC from the ripple may cause hum. Or maybe bias voltage instead of B+. Voltage measurements could find this. Or disconnect the NFB from the winding.
2) The OT primary may have flyback diodes. If one of those is leaking, the ripple on the B+ may be creating hum. Or for that matter, if there's any other leakage from one of the plates to ground or some other part of the circuit, the ripple will cause hum.
Of course there's more exotic failure modes, that's just two ways that came to mind immediately. Oh I guess a failing OT could cause hum, shorting to ground, but that would likely be a catastrophic failure.
