Need help on Bassman AB165

[StratTone]

Something is up. The bass channel is fine but low volume. I would say 1/4 volume. The Guitar channel is the problem. You can increase the volume ever so slightly and the bias changes into relating and mad occilation occurs. I'm thinking I have a bad cap passing DC. Any ideas just off the top of your head. My voltages are all right on the money but I just have a low volume bass channel and bad lead channel. Any help would be appreciated.

[tubeswell]

yep DC on a coupling cap is a possible cause of volume drop. Lift the 'tail' end of the suspect cap and measure it for DC.

[StratTone]

Ok, this one has me chasing my tail. The amp just is basically an oscilating redplating Bassman at the moment. I have everything wired to spec and still can't figure it out. The volume controls on the Normal channel actually change the bias. I've replaced pots tested "most" coupling caps and all voltages on tubes are good. Anyone got a shot in the dark on this one?

This is a home build with MM trannies and all good parts. Never had this problem before on a build. All "usually" seems to go well. I think I just may have a brand new bad component.

[Enzo]

Swap ends on your output transformer promary winding.

[StratTone]

Swap ends on your output transformer promary winding.

Ok, I'll give that a shot. I did try it before but why not haha. By the way how can you tell which is which when you are installing your OT from MM?

[StratTone]

Also an odd thing that I noticed on my Bassman MM PT. It has a white wire with a ground lug on it from MM. There is not mention of it on the diagram. What is that for?

[Enzo]

I have no idea, other than it is a shield to be grounded. I would ask MM about it.

WIndings on transformers always have a "start " end and a "finish " end - often marked on schematics by a black dot. It is up to the builder to know the phase relationship of his output NFB and the rest of his circuitry. Or 50/50 chance by guessing.