Both are a yes. Would it be safer to preset the trimmer while its off the PCB to lets say 0.5Komhs or is it okay to adjust it while measuring bias untill DMM reads something at all? Thanks a lot you have helped a bunch so far.
Goldie99
Super Member
You can do it where it is - while it's measuring 0mV, there's no current flowing through it, so just monitor it with the DMM as you adjust the trimmer, at a certain point it'll start to measure mV's and slowly increase as you adjust it (there's plenty of adjustment on a 25 turn trimmer, so it shouldn't overshoot quickly).
If you're concerned, put the amp on a DBT while you do it.
If you're concerned, put the amp on a DBT while you do it.
So
So yep, the trimmers are way off. I have just adjusted one of the biases to 30.5-31mV (well it isnt very steady, is it?). So I think I will manage to do the same with the other one! @Goldie99, you are awesome!
Goldie99
Super Member
Good - that sounds better 
The 'not very steady' is probably thermal - once you have them set at ca. 30mV, let it sit like that for an hour or so, to reach thermal equilibrium, then tweak the settings again (helps to have 2x DMM's). Air currents / draughts will affect it as well - try blowing on the amp and see what happens to the 30mV... it can help to have the extension wires run out of the amp, and the amp cover loosely on, just to get the temp. as stable as possible.
The 'not very steady' is probably thermal - once you have them set at ca. 30mV, let it sit like that for an hour or so, to reach thermal equilibrium, then tweak the settings again (helps to have 2x DMM's). Air currents / draughts will affect it as well - try blowing on the amp and see what happens to the 30mV... it can help to have the extension wires run out of the amp, and the amp cover loosely on, just to get the temp. as stable as possible.