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Tube amps like these should never be cycled on and off repeatedly. I usually let mine sit minimum 15 minutes before turning it on again. Remember, it has a high voltage power supply. It’s very stressful on the rectifier. The yellow sheet mod is highly recommended as it will protect the tube rectifier. Your tube rectifier is likely damaged and needs to be replaced. It only takes one bad thing to go wrong to kill it. I went through a bunch of them before I learned about the yellow sheet mod. Haven’t blown a tube rectifier since.
Dynaco style tube amps all use slow blow fuses. ST120 uses 5 amp slow blow. Don’t use anything else or you’re just asking for trouble. In some ways tube amps can be very robust devices and in other ways they can be fragile and require more care than solid state electronics. Expect to blow a tube from time to time. Sometimes it’s just a bad tube. Respect your amp and you might go years with no problems. But you have to treat them right.
Always have speakers connected when powered on. When it starts distorting it’s likely a power tube that needs to be replaced. Toss the bad one (you can tell by the bias which will probably drift way up) keep the other one from that channel as a spare and buy a new pair of matched tubes.. if it’s distorting but bias is good and stable then it’s likely the small center tube going bad. Replace it. If your not sure what’s wrong with it come back here or shoot an email to Roy at firstname.lastname@example.org