Biasing an Acoustic 361

[filament]

Could anyone tell me how to bias an old Acoustic 361? There are two pots. Bias voltage and bias current. Any help will be greatly appreciated. Thanks.

[Enzo]

Link a schematic?

Without the print in front of my, I would tend the think the voltage labelled adjustment was to center the offset, while the current wouldbe what we consider the bias adjustment. I could be wrong.

[J M Fahey]

And to begin from the beginning, why would you want to bias it?
The answer depends on your goal.

[filament]

Here is the schematic. (BassPoweredCabinet361 < Acoustic < Wiki). Only pages 1 and 2 are needed. On my board there are also two diodes (1N914) of opposite polarity between the bases of Q1A and Q1B. There is a 2.2k resistor between the collector of Q1A and the junction of R6, D6, C8, and Q2's base. And there are two 470pF ceramic caps between Q5's base to the 80 volt supply and Q5's emitter to the 80 volt supply. Everything on the board is new except for the three ceramic caps. I don't know enough about SS power amps to feel good about plugging it in blindly. FYI, I prefer tubes. But it would be nice to get the idle current right and crossover distortion minimized. Thanks.

[rf7]

It's in the service manual. R5 is the Bias voltage. Run the amp with a 4 ohm load and turn the input up until the amp just clips. Adjust R5 so that clipping occurs on the negative and positive waveforms simultaneously.

R14 is the bias current. Run the amp at 100Vac, with a 4 ohm load and an output of 1Vp-p on your scope. Start with R14 fully CCW and turn it up until crossover distortion just disappears on the waveform.

[rf7]

Bring the amp up initially on a variac, monitoring the current draw. It should not draw more than an 1A at 120Vac, and will draw about 2.5A at full clip. Or else use a light bulb limiter to bring it up. I'm doing this myself today.