Followed your recipe of mods on my 69 YBA-1 and single channeled it by lifting a 100k resistor, changed the common cathode resistor to 1.5k and tried lowering the capacitance there from 100uFd but the amp wanted to regenerate at high gain settings, so I put back the 100u cap.
(i) … in the YBA-1 bias circuit you can replace the load resistors by an equivalent “variable” circuit, it won’t disturb existing voltages because the total load resistance is maintained (15k + 56k close to 25k + 47k) … the same reasoning can be applied to any other Traynor amp with similar circuit … I might use a pot that’s 5k to 10k larger than the top load resistor … then take the difference away from the bottom load resistor …
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(ii) … the capacitor C14 of value 8uF cannot be changed too much unless the whole bias circuit is re-designed (10uF to 16uF is ok) … a much larger value would produce a very slow charging bias circuit … because the next leg of the bias circuit will likely contain a resistance that’s somewhat close to 15k we can hike the next cap up a decade (80uF = 10 * 8uF) so that time constants jive in both RC circuits (by gross rule of thumb) … either way, if you increase C14 expect your bias circuit to take a little longer to charge; in which case you might have to use the standby switch when powering up …
(iii) … for extra low “hum” in your tube amp make sure the filament wires are very tight against one another – you might have to unsolder one wire to give it an extra twist around the other in each link of the filament wire chain … pull them against one another so they stand up high above the signal wires … make sure grid wires run as orthogonally as possible in the viscinity of, and in relation to, the filament wire chain …
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