guentherj2 wrote:peterh - that sounds right but could you please elaborate? I noticed that three of my Sovtek KT88s have black lebels and one has a red label. It is that red labeled KT88 that results in the bias pot being at a different setting than the other three. I would think it doesn't matter since I am adjusting the bias pots to get same readings (voltages) on each of the tubes but might there be an issue with having an "odd man out" tube in there?
It seems to me that you have tubes that is not matched. That is ok, but you will most likely have different bias on them.
When you speak about voltages, is it the voltage measured across the cathode resistor ( 10 ohm in vta amps) that you mean ? In that case you should measure equal voltage for each of the two tubes on each side.
Voltage measured on the grid is not interesting, that is more an issue of the tubes properties. The mere fact that tubes are differing ( unless one has selected tubes to create matching set's) is the reason to have individual bias pots.
There is one more "property" of the tubes that needs matching, it's the amplification aka transconduktance. bias puts will do nothing here, if the tubes differ you will have to resort to your feedback to reduce it's impact. Normally the differences are small and the feedback will reduce the unbalance to low levels. ( some amps has an ac-balance that might compensate )
In an vta you need 2 matched pairs, although a matched quad is fine since you among the route might combine them to be at least one pair even if 2 of them goes bust.
That would be my guess, too, that maybe you've got an un-matched tube. Jim McShane's your guy for ensuring you get matched pairs or quads. They work in push-pull fashion in VTA amps (lotsa research there on that topic elsewhere ) but basically if they're not well-matched, one tube works harder than the other in the push-pull pair and regardless of bias setting, you're compromising sound quality and tube life. To over-simplify, imagine a teeter-totter with a 200 lb person on one end and a 50-pound person on the other. Average weight is 125-lbs a side,which is what your bias pot is telling you across two tubes, and you can still go up and down but one side's working a lot harder and the other. And you won't get an even swing.