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NOS 6V6 tubes used to be scarce, and matching them was not easy due to high cost and low selection. I modified the bias control on a Deluxe Reverb so each tube could have it's own pot to set its bias voltage. This allows mismatched tubes to be biased with less variance than one bias pot allows.
The unmodified bias control (a) has about -41v across 20K ohms, a 10K fixed resistor and a 10K pot in series. Its wiper feeds two 220K grid leak resistors for the 6V6 tubes.
The modified control (b) presents the same load, but it's a parallel circuit with 40K ohms in each branch. The 40K is achieved with a 15K fixed resistor and a 25K pot in series. The pots are linear taper. Each wiper feeds one 220K grid leak resistor. These resistors are normally joined in a "Y", and have to be split apart to allow independent control.
There is an interaction between the tubes, so you need to go back and forth between the bias controls several times to bring the tubes into line. In retrospect, it's probably better to use a pot to set the bias level, and a balance pot to distribute it to the tubes
The amp has been reliable, and I've talked with someone who did this on a Twin Reverb, so it seems my assumptions are not too far out of line.
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