I have an m125 autobias board installed on my amp an have noticed since I put this in about 18 months ago the amp has terrible hum. This is new to before I used the autobias, anyway I had stopped using the amp, but since I had a little time, went back to reexamine, I assume there must be a ground issue anyway when I check grounds, I note that on v3 and v4 ground my reading to chassis are high, about 0.9 mOhm, The autobias board ground is 0, are the v3 and v4 levels abnormal? I did have difficulty removing the 50k ohm trim pots and resoldering wire to the autobias unit. and wonder if I am getting a good connection although seem secure. If these are abnormal could I attach the ground leads elsewhere? All other connections look OK, any help appreciated
3 posters
M125 autobias hum, a ground issues? solved !
ericoto- Posts : 26
Join date : 2013-12-11
LeGrace- Posts : 389
Join date : 2016-08-07
Location : Ontario, Canada
Does it still hum when the corresponding tubes are removed? ie with only V1 and V2 populated?
ericoto- Posts : 26
Join date : 2013-12-11
Problem solved, it was the AC being too close to the autobias, rerun the wires and moved the 6.3V input to a higher level away from the board, now sounds great.
Roy Mottram- Admin
- Posts : 1838
Join date : 2008-11-30
Problem solved, it was the AC being too close to the autobias, rerun the wires and moved the 6.3V input to a higher level away from the board, now sounds great.
A common problem and easily solved, hope this post finds it's way to some others who thought the auto-bias was noisy.
A common problem and easily solved, hope this post finds it's way to some others who thought the auto-bias was noisy.