Motorboating after tube change

The only screwup was that when initially replacing the tubes, I accidentally installed a 5751 into the spot for a 5755.

The pinout for the two tubes are not the same.

5751

5755

You need to let the guys on AA know about the tube mix up.
I am sure you will get a different set of answers and hopefully some new answers.

You say motor boating? Is the music cutting in and out at a regular pattern?
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Motorboating is an indication of a power supply problem.

My theory is that the different tube somehow was able to do some damage, but it is not obvious how upon looking up the two different tubes. So it could be coincidence, but at any rate you are experiencing a power supply problem.

My theory is that the different tube somehow was able to do some damage, but it is not obvious how upon looking up the two different tubes.

Atmasphere,

I was wondering the same thing as neither heater of the tubes would have been energized. But what if the designer of the amp connected one lead of the filament secondary transformer winding to chassis to save wire or trace and at each of the tube sockets connected one heater socket pin terminal to chassis? If that were the case I can see a completed circuit where plate voltage would have been directed through one section of a triode through the heater to power supply B- chassis. Not sure what that load/s could do to the power supply. Just a thought.

On another note, as a kid I remember if a coupling/blocking capacitor was bad it would not block the high plate DC voltage from a preceding tube stage from entering the grid of the next tube along with the signal. Just going from memory the music was heard through the speaker only in very short segments of on and off repeated sequences repeatedly. Back then that was also called motor boating.
Jim