High-pitched squeal from amp - with gain and volume

OK, if the master volume can eliminate it, then the output is feeding back into the circuit before that control. So ignore those components I highlighted - they were the feedback circuit for the output stage.
 
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Just a thought. When you reduce the master volume, does the sound just reduce along with everything else, or does it actually stop happening when you get to a certain spot on the control?
 
DonP said:
Just a thought. When you reduce the master volume, does the sound just reduce along with everything else, or does it actually stop happening when you get to a certain spot on the control?
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The latter. It is there at a consistent drone at a reducing volume until it stops.
 
Gain or volume reduction of the pedals itself will resolve the issues, but to a point of a whisper or low gain, making the whole endeavor irrelevant
 
OK, if you leave the master volume at a position that produces the noise, then back off the other volume control, do you get the same sudden stop effect?
 
DonP said:
OK, next test. Pull the input valve and see if it still happens.
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Which position is the input valve?

I am headed home now, so may not be able to respond or test until later, but I am greatly appreciative.
 
Tap test yeilded no results. But, there was an unexpected circumstance.

In order to acces the tubes, I moved the amp and reconnected the native 1x12 speaker. The issue is far reduced and only occurs at much higher pedal gain setttings, alhough there is a louder general hum. FWIW the native cab runs at 8 ohms and the 2x12 at 16 ohms - the head has a selector.
 
Sorry - was thinking. This is sounding like acoustic feedback again. Any chance your speaker cable is long enoough to move the speaker into another room and close the door?
 
Swapped tubes yet? Start with the power tubes.

From left to right:
V1 – 12AX7/ECC83 (clean and overdrive stages 1 and 2)
V2 – 12AX7/ECC83 (overdrive stages 3 and 4)
V3 – 12AX7/ECC83 (phase inverter for power amplifier)
V4 – 6V6
V5 – 6V6
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