I have a ST-120 you built for me last spring that has started to hum. When the amp warms up after about a minute it has a steady but low hum. I replaced all the power tubes (Gold Lion T-88's)and it does the same thing with the hum. I have not replaced the 12AX's but they do not appear to be microphonic when lightly tapped and appear to light up normally.About two moths ago I had a KT-88 die on me and it looked like the tube was frosted inside. I replaced the pair and it weas fine until this hum began. (I'm trying to give you as much info to go on as possible here).
There have been the occasional sharp cracking sounds before this began when I was playing music.
It also blew a couple of fuses just before the hum began but it then began to operate normally except for the hum I now have.
My question is what do I check first and how do I logically proceed to isolate and repair this hum.
Can you give me a list of things that might be the problem. Should I be using the amp at all until this is resolved?
Hope you can help.