One of my M-125s (output tubes KT-120) has developed an unstable bias, with 4 tubes in place, I can increase bias to about 0.4v and then the reading becomes unstable. Tubes all tested (e-tracer) and good. With speakers connected I get a 4 beats a second sound. Quad cap is good an no other obvious faults - thoughts welcome!
2 posters
unstable bias M-125 monoblock
Gerry Borghesi- Posts : 1
Join date : 2024-08-07
Location : Melbourne Australia
Seamus- Posts : 90
Join date : 2020-03-17
What happens with one tube per side?
Did you build the amp?
Have you checked for a poor solder connection on the driver board?
If you fiddle with bias a lot, the pot could be shot.
Cermet pots have a rated life of about 200 cycles.
If you replace the pots, use multi-turn as they are easier to adjust
Bourns 3339P-1-503LF
Did you build the amp?
Have you checked for a poor solder connection on the driver board?
If you fiddle with bias a lot, the pot could be shot.
Cermet pots have a rated life of about 200 cycles.
If you replace the pots, use multi-turn as they are easier to adjust
Bourns 3339P-1-503LF
Last edited by Seamus on Sun Sep 08, 2024 5:14 pm; edited 1 time in total (Reason for editing : add line)