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03-17-2020, 12:32 AM
Hi Noel
The splitter in some Mesa amps uses a negative supply for the tail resistor, which allows the grids to be ground-referenced. This has nothing to do with the bias of the output tubes except that the only negative supply is for the output tube bias so both circuits share it.
Adjustable bias is definite;y something to add - and in this case retain - on any Mesa amp. The pot(s) should be panel types NOT trimmers. Trimpots only have 200-cycle lives where even the cheapest panel pot is good for 15k-cycles. Mount the pot(s) on the tube plane of the chassis so they are accessible and protected like the tubes. Add meter jacks on the rear apron to measure the voltage drop across cathode current-sense resistors that you have to add as well.
Remember to add safety resistors across the bias pot(s) from the pot-0 (cold, negative supply input) to the wiper. Pot-X should be the end grounded via the range resistor. Pot-X is then the correct orientation for 'hot' bias.
Have fun
The splitter in some Mesa amps uses a negative supply for the tail resistor, which allows the grids to be ground-referenced. This has nothing to do with the bias of the output tubes except that the only negative supply is for the output tube bias so both circuits share it.
Adjustable bias is definite;y something to add - and in this case retain - on any Mesa amp. The pot(s) should be panel types NOT trimmers. Trimpots only have 200-cycle lives where even the cheapest panel pot is good for 15k-cycles. Mount the pot(s) on the tube plane of the chassis so they are accessible and protected like the tubes. Add meter jacks on the rear apron to measure the voltage drop across cathode current-sense resistors that you have to add as well.
Remember to add safety resistors across the bias pot(s) from the pot-0 (cold, negative supply input) to the wiper. Pot-X should be the end grounded via the range resistor. Pot-X is then the correct orientation for 'hot' bias.
Have fun
