Hi All,
I have a Tubelab SE with 300B tubes that I have been happily running for a few years, but recently have been having an odd issue with holding bias. It all started when I was getting squawks in one channel which I traced down to a noisy 5842, so I replaced the two 5842s.
Since then, it's very tricky to hold the 300B bias steady at any values higher than 60ma. Once the amp has warmed up, if I set both tubes to 65ma, and check an hour later, the right one will have shot up to 85ma, the left staying around 65ma. If I turn the right down to 65ma, the left will start creeping up and the right will float down to 60ma. It seems to hold bias around 60ma in each tube, but any attempt to bring it higher then that results in the slow floating around. Even just now I set each tube to 62ma, and 5 min later the right tube is down to 61ma and the left is up to 64ms. It's not that I need the values always perfect, but my worry is that in 2 hours that left tube will have crept up to un unsafe value. Even as I have been writing this the right is down to 60ma now and the left at 65ma.
All measurements are in mV across R18 & R29, with the amp warmed up but without music playing. I have the Edcor XPWR-008 transformer, with B+ around 360-370VDC.
Anyway, I'm not sure what parts I should be checking, or any advice in troubleshooting would be much appreciated!
I have a Tubelab SE with 300B tubes that I have been happily running for a few years, but recently have been having an odd issue with holding bias. It all started when I was getting squawks in one channel which I traced down to a noisy 5842, so I replaced the two 5842s.
Since then, it's very tricky to hold the 300B bias steady at any values higher than 60ma. Once the amp has warmed up, if I set both tubes to 65ma, and check an hour later, the right one will have shot up to 85ma, the left staying around 65ma. If I turn the right down to 65ma, the left will start creeping up and the right will float down to 60ma. It seems to hold bias around 60ma in each tube, but any attempt to bring it higher then that results in the slow floating around. Even just now I set each tube to 62ma, and 5 min later the right tube is down to 61ma and the left is up to 64ms. It's not that I need the values always perfect, but my worry is that in 2 hours that left tube will have crept up to un unsafe value. Even as I have been writing this the right is down to 60ma now and the left at 65ma.
All measurements are in mV across R18 & R29, with the amp warmed up but without music playing. I have the Edcor XPWR-008 transformer, with B+ around 360-370VDC.
Anyway, I'm not sure what parts I should be checking, or any advice in troubleshooting would be much appreciated!