Marshall JCM-800 2204 Amplifier problems.

This is an old version Marshall and the bias pot is 10k instead of 22k. With a 10k bias pot biased for -54mA the valves ran at 103% this would lead to a shorter life. I put a 12k resistor in series with the 10k pot which meant I could adjust to a very low -24 mA, I was able to adjust up to 34mA which is 60% of output; Now the valves should last 12 months instead of 2 months!