I have a pair of M125 mono blocks, circa 2010, that have been working great. Each has four Tungsol 6550s. The right amp biases perfectly at 1.2v for each pair of tubes (amp warm, outputs connected but no signal applied). However, the bias on the left amp wanders up from 1.2v to over 1.3 volts on the left pair of tubes, and from 1.2v down to 1.05 volts on the right side of tubes. Time for new tubes? Should I try to swap all the power tubes between amps?
3 posters
Wandering Bias on M125
bcinnamon- Posts : 27
Join date : 2011-05-07
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
bcinnamon wrote:I have a pair of M125 mono blocks, circa 2010, that have been working great. Each has four Tungsol 6550s. The right amp biases perfectly at 1.2v for each pair of tubes (amp warm, outputs connected but no signal applied). However, the bias on the left amp wanders up from 1.2v to over 1.3 volts on the left pair of tubes, and from 1.2v down to 1.05 volts on the right side of tubes. Time for new tubes? Should I try to swap all the power tubes between amps?
Hi Barry,
Your amps are from May of 2011 .. If you still have the original tubes, you should change at least all the output tubes on the left amp. Depending on how many hours are on your amps, you might also consider changing the output tubes on the good right amp also and keep the right amp's output tubes as spares.
I know I have said this before on the forum here but tubes do not last forever. They do wear out in time. Anytime the tube is "on" it is wearing out. You don't have to be playing music through the amp to have tube wear. A sign that tubes are wearing out is (as you have on your amp) a "wandering bias". Red plating, noise when playing and lower perceived output are other symptoms.
Bob
rebellovw- Posts : 78
Join date : 2022-01-23
I can't believe you ran M125s with bias at 1.2 v with 6550s since 2011. That is quite a feat.
I found longevity reducing the bias to 1.0 and .5 for two output tubes (a VTA recommendation a few years back) - I too built in 2011 - I got 6 years with Sovtek KT88s and after that lots of hit and misses with Gold Lion KT88s which for me don't last at all.
What is your wall voltage? I'm building a bucking transformer to help bring mine down if I ever get them working again.
I found longevity reducing the bias to 1.0 and .5 for two output tubes (a VTA recommendation a few years back) - I too built in 2011 - I got 6 years with Sovtek KT88s and after that lots of hit and misses with Gold Lion KT88s which for me don't last at all.
What is your wall voltage? I'm building a bucking transformer to help bring mine down if I ever get them working again.
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