Motorboating after tube change

It happens to the best of us, Jim :-)

I note that two of the absolute maximum ratings of the 5751 would most likely have been exceeded by a considerable margin as a result of inserting it into the 5755 socket. The absolute maximum negative grid bias rating of 55 volts (presumably relative to the cathode, which had what should have been the voltage applied to the plate applied to it instead, in one section of the tube), and the absolute maximum heater-to-cathode voltage rating of 100 volts. I'm not sure what the results of that might have been, but it seems very conceivable to me that it could have caused some kind of breakdown of the tube, in turn resulting in secondary damage to something in the power supply or other circuitry.

Best regards,
-- Al

Thanks for all the responses. It's apparent to me that the fix is beyond my capabilities. Therefore, it's going in for repair. It'll take a while for an answer since the unit needs to be shipped. I will post the results of that repair here when I get it back.

Again, thanks for all the responses. I truly appreciate the information that everyone offered.

Regards,
GP

From what I saw from pin locations there could be some protection involved and maybe some resistor is blown that provides voltage drop for the offset.

I agree with Bjbcab, that's the only reason why I read this thread :)

Lou, b aware that Bj-bcab already sounds wierd!