Interesting idea on arranging the resistors. So you'd have the 22K pot in parallel to one resistor and then that would lead into another resistor in series? You got me thinking about it and I ended up doing some charts. I hope I got the calculations right on these!
First, this is what I did, with a 22K pot in parallel to a 7K5 resistor.
The chart shows the setting of the pot across the x-axis and the total resistance as the y-axis. Assuming an EL34 would want between 35mA and 45mA and a KT88 would want 45mA to 55mA I've added red lines to show the required resistance ranges (35mA=2K7, 45mA=3K9, 55mA=4K7):
So for my KT88s I'm setting the pot between 8K and 12K5. But with a 3K9 in parallel to the 22K and a 2K4 in series you get this:
While it sounds a better idea to have more range, you actually end up with less adjustment in the required ranges! But the idea is right, and if instead you use 3K3 in parallel with the 22K and then 2K in series it becomes this:
Although this loses a bit of current (max is now about 57mA) this gives the best amount of adjustment range and also has the advantage that the ABS already provides a 3K3 so you just need an additional 2K.
Great to hear that the whole thing is analog. No wonder you were so certain there would be no digital noise! Analog computers, now there is a reminder from the past. At university part of my course required us to run feedback/feedforward simulations using analog computers, which of course the university no longer had. So we used some software called LEANS - Lehigh Analog Simulator. Basically it was a program to run on a digital computer that mimicked the calculations of an analog computer.
