Bogen MT-250 PA Power Amp Schematic

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This is a 250 W Bogen PA amp that my son would like to use for his bands vocal PA. They are on a very limited budget. I changed the 3 electrolytic caps other than the main PSU caps and am powering it up slowly on a Variac. One side of the output stage idles with a lot more current (370 mA) than the other and I believe that this is due to a small DC offset current. There is actually an output transformer so what would normally be a DC voltage offset turns into mainly CD current.

There is one trim pot on the main board, anyone know if this is the DC offset control?

Pete B.
 
Thanks very much, I did look when I first posted, don't recall finding that. It looks to be a newer model, not exactly the same but still might be helpful. This unit runs on a split supply and there is an output that bypasses the output transformer.

Idle current is 300 mA, supply is only +/- 28 volts so this is about 16.8 W total in the output stage. Should be fine.

Pete B.
 
I'd expect this amp to be running close to class B, not class A. This is not an esoteric stereo amp, this is a utilitarian PA amp. 100 watts of idle through the output stage sounds extreme to me. I'd expect your amp to operate about like the one in the schematic above, and that spec was 30 watts idle.
 
I agree with all your points, however, my guess is that with the slow 2N3055 output devices, they had to bias it up to get reasonable speed and stability. There is a lot of heat sink area and it seems perfectly fine at this bias level. There is no bias pot and it is now balanced. Ran it for hours like this. I'd guess that one of the goals of the newer design was to get the power down.

Pete B.
 
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