I have a weird situation. One of my Line magnetic 503pa s.e.t. amp mono-blocks is making a squealing sound (goes into oscillation I guess) but I do not think the reason is wrong wiring of primary leads of transformers or tubes. It does mostly go away (not completely) when I put my hand between tubes or close the cage of the tubes.
The amp is s.e.t. with 310a driving 300b and 300b driving the 845 and has 274b rectification. They are mono blocks and only one has this problem.
I changed the tubes, change the cables, change the placement to no avail. Changing feedback level from -3db to -6db has no effect either. The only thing that has any effect is the cage or putting something in between the tubes.
What to do?
The amp is s.e.t. with 310a driving 300b and 300b driving the 845 and has 274b rectification. They are mono blocks and only one has this problem.
I changed the tubes, change the cables, change the placement to no avail. Changing feedback level from -3db to -6db has no effect either. The only thing that has any effect is the cage or putting something in between the tubes.
What to do?
If you pass the hands near the tubes and circuitry/wiring and noise stops or changes the kind of sound generated, clearly the amp is oscillating perhaps in a frequency above the audio band (100KHz, 1MHz perhaps). It would be to see the schematic(s) or pic(s) of the device having this trouble.
It was the 845 tube. This was the second pair I got and both had one faulty tube. By chance I put the faulty ones on the same channel (right) on both times and this time I thought it was the amps fault. I thought I changed it, I have changed all other tubes from right to left but I double checked and found out I forgot swapping the 845! I did and the noise is transferred to the other channel. I took off 310a and 300b one by one leaving only 845 and the noise persisted. I am sending the tube for change.
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