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Is it possible, likely or common that a bad tube (shorted internally) causing the failure of a screen grid resistor, could result in high voltage B+ to be fed back through the circuit afffecting the low voltage outputs for heater filaments and rectifier heater?
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It could be possible to have such a condition, but would represent a catastrophic failure, given the mechanical nature of how a tetrode/pentode is built.. It's more likely an over current from the screen grid caused by the misalignment of the screen wires with respect from the control grid..
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Problem occurred when trying old junk 6v6 tubes. A bit of smoke and then loss of output. Now crazy high (540volts) readings on 5y3 heater pins. 18 volts found on filament heater pins.Comment
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The 5Y3's heater is also its cathode, which is the main high voltage supply for the amp. So of course there will be 540V on it.
Some amps put a DC bias onto the 6.3V heater supply, to reduce hum. So 18V from a heater pin to ground might be normal.
Others have two little 100 ohm resistors to reference the heater string to ground, and these can be burnt out by a really bad tube short that somehow puts the plate voltage on the heater.
Fix all burnt resistors, put in some known good tubes and see if it works."Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"Comment
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It is entirely possible for a tube to short and dump a bunch of current onto the heater supply. In common amps with a pair of resistors or a pot providing a ground reference for the heater supply the Rs or pot will be burned.Originally posted by hack View Post
The heater for the rectifier is a completely different winding and is connected to the B+ by design.Comment
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