VTA ST120 Bias

General question regarding the biasing of the ST120:

Have had my ST120 for well over 3 years now and have been checking the bias about once a month making adjustments when necessary. My question is how much drift from .550 up or down is allowable before adjustment is necessary, i.e. what is to high or to low? Sometimes I feel obsessive compulsive when making the adjustment and am wondering how close to .550
it must be.

Thanks,

ArlanB

it will change with music playing of course, but without an input (or shorted inputs) then try to keep it under 570.
It will also vary with the AC input level, some areas have terrible power regulation but other areas are pretty solid. Safer to be on the low side (like 500) than over 570 which will cut tube life and run hotter.
Another nice thing about the bias on each tube is that you can visually see the position of the bias pots and thus not only judge how well the tubes are matched but if any are requiring more adjustment than any others.

What are your recommendations if running the KT120's?

Bob Latino wrote: You can run the bias up a little higher in the VTA ST-120 with KT120 output tubes but IMHO - the amp won't really sound any different. I wouldn't run it up any higher than .600 VDC per tube - up from the normal .550 VDC that KT88's use. A higher bias point wears the output tubes out quicker and takes a little more effort out of the power transformer. The higher you go with the bias, the warmer that power transformer gets.

Bob

Keep my KT120's at .55VDC, like to know they are just idling and making such sweet music!