M125 - Red plating output tube

costerdock wrote:Hey folks been a long time.

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.

Thanks!

Chris

Bob Latino wrote:

costerdock wrote:Hey folks been a long time.

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.

Thanks!

Chris

Hi Chris,

OK - You are running just two output tubes in this amp. Try swapping the two output tubes side to side on this amp. If the problem moves over to the other side, then that tube has a problem (even though the tubes are fairly new). If a DIFFERENT tube in that socket does the same thing, then there is a problem on the socket.

Bob