I have a Sixty Five that I replaced the tubes in and then biased because it was sounding muddy, but it started to sound muddy again recently so I took the amp out of the chassis and tested the bias voltage and it was lower than I had set it before. Not by much, probably around 0.07 volts lower. When I raised the bias back to 0.5 volts i sounded clear and as beautiful as ever, but after a few times playing it I found it to sound muddy again, and when I went to check the bias it had dropped again. Any clue what would cause this? Maybe a faulty resistor somewhere? Just wondering where to start checking, or if someone has had this problem in the past.
Thanks!
More likely is the tubes are
More likely is the tubes are just burning in. Good vendors burn them for 24 hours before they sell them but most don't. Let them cook for a few hours and then set them and try again. Mike.