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Simple SE help desperately needed

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The SSE is as close to an unconditionally stable amplifier, as there is. Oscillation is highly unusual - I can only recall one instance in a stock amp, and it was very recent:

1 channel fades after a few minutes of playing - SSE

The 12AT7 is a high gain tube, made for VHF mixing and amplification. It is more likely that if your amp is in fact oscillating, it is the front end, and not the PA tubes. The PA tubes would quickly fail if they were oscillating at any significant power level.

Good quality pictures of your board and wiring could be very helpful. By failing to post pictures, you are making it unnecessarily difficult for folks to try to help you solve this ( likely simple ) problem.
 
I now have pictures. But, I need to get a chance to upload them to my computer for posting. Hopefully this afternoon.

In the meantime, I checked the 5v taps (yellow wires) going to the board. On is putting out over 5 volts the other less than 2.

So, I switched the yellow wires and the tube began to glow. However, without any inputs or speakers connected, I got a terrible squeal coming from the amp after a few seconds. This does not happen with the solid state rectifier.

I have decided to fix the rectifier tube issue before working on the UL taps.
 
The observation you posted re the 5 volt winding voltages isn’t clear to me. Measure the voltage between the two wires. Red lead on one wire and the black lead on the other. Please report your findings as vdc or vac as applicable so we can be sure we are on the same page. Keep one hand in your pocket when dealing with live circuits.
 
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You are correct. The first board was bought a year ago. The second last month. I did find the information regarding D3, D4 and T1. All have been installed and best I can tell appear to be working.

Work and family obligations have prevented me from testing the 5volt taps as you suggested. What numbers should I be getting if they are working correctly?
 
Thanks for all the suggestions along the way.

I had a chance to measure the 5volt taps this evening. I show 5vac when the rectifier tube installed and the switch open.

It appears that I also solve the squeal when using the rectifier tube. When I disconnected the inputs, I inadvertently disconnected the board’s ground. Once I grounded the board, the squeal disappeared.

This weekend, I will take another look at the UL taps.
 
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Good to hear you are making progress, ejr. Doesn't that bug you when things just randomly start working? I hate it. I am never sure whether I should be happy or be worried that it will randomly stop working again and I won't know how to fix it.

It is almost worse than it not working at all, but not quite.
 
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