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RE: bias is set rather differently

Most amps set the output tubes bias point through one of two methods.

A) There is a resistor from cathode to ground. It may be shared between both tubes in one channel, or even all four tubes. Or there may be one for each tube. Typically there will be a bypass cap across said resistor. This is "cathode bias" and it does create some feedback so amp will have a little less power output.

B) The cathode is grounded (in some cases a low value resistor is inserted between ground and cathode to measure bias point) and negative voltage is feed to the grid of the output tube. This is called fixed bias despite the fact that it is often adjustable. You might have a pot for each tube or a pot for each pair of tubes.

Now what is different about your amp? In your amp the tubes immediately before your output tubes have their cathode's referenced to a negative voltage supply. They do have cathode resistors. The signal is taken off the cathode of those tubes (they are cathode followers). Because of the negative voltage supply this AC signal voltage is below ground potential (it is negative voltage). This keeps the grid of the output tubes negative relative to their (grounded) cathodes and sets the bias (idle current) point for the output tubes. This is different than most amps.

I don't know if any of this made sense. I could try and explain differently. It would also help if I could find an up to date schematic.


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  • RE: bias is set rather differently - Russ57 20:58:30 07/29/14 (0)

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