Ok - my amps have been sitting quite a while unused (6 months) and I went to install a new cart/tonearm and found one of my output tubes reddish. Amp was only one for 8 or so minutes. I then shut the amp off - and turned back on and checked the bias (I'm running two output tubes per amp) - bias was high .6+ on the redplate tube - and .499 or so on the OK tube. So I reset the bias to just below .5 on both and turned the amps back on. Note - I had to turn the bias all the way to get it down to .5. Anyhow after being on a while - the bias is back to creeping up .699 today and climbing.
Things I've done so far (replaced both driver tubes - RCA clear tops) - same issue.
Swapped the GZ37's rectifier tubes - same issue (I'm somewhat relieved as these are beautiful $$$ tubes.)
I'll swap the phase splitters next - but I'm thinking I might have a fundamental problem with the amp.
The output tubes are practically new - bought from Jim McShane - new Sovtek 6550s - bought in March 2019. My original Sovtek KT88's lasted close to 6 years - no redplating.
So far for the life of the amps - problems have always been a tube failure - I've never had to dig into it.
I'll have to dig out how to check all the voltages I presume.
Amps built in 2011.
Last edited by costerdock on Wed Feb 05, 2020 6:35 pm; edited 1 time in total