For some time my "red" M-125 has been making a soft, intermittent, crackling, hiss. Think initial hit of needle to vinyl on a 10 to 40 second frequency.
I swapped tubes, one at a time from offending amp to the other until all tubes swapped. The noise stayed with the "red' amp. At some point I felt that by slightly wiggling the output tube in the V-2 position I was able to make the noise go away. Hard to tell with the intermittence. Cleaned and tightened socket.
So....... Ignore it........
Now I have lost the tube in the V-2 position, no fanfare, just quit. Running amps on two output tubes now while waiting on replacement with readjusted bias. Next the sound quality dropped off on that amp. Check bias, and it will not hold steady. Replace GZ34 with the Webber WZ68. Bias comes back everything is cool, until the Webber goes TU and the bias is all over the board again.
Can't really ignore this ..........
Any help will greatly appreciated as my degree is mechanical, not electrical. Please be specific, this is all new to me.