The higher initial reading of the set you just installed indicates that they have more transconductance, which is a good thing. It can be an indication of hours but that can't be counted on due to the variance in the tubes themselves.
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Atmaspere is definitely more knowledgeable on this than me but I have read consistently that when putting new tubes in one should check them every ten minutes for about the first half hour to see if there is any drift or change in the reading and then adjust them accordingly. It is not unusual for tubes to read higher initially and then go down or change from first being put into the system. My only question for you is did you recheck them after they had been in awhile?
Just guessing tubes "A" probably were in the amp a few years and bias was reset a few times.
Tubes "B" low hours and probably would test like new.
It is a good idea when ever replacing power tubes, that have a fair amount of hours on them, before installing new power tubes/(tubes that test like new) the bias pots should be turned down somewhat first.
I am surprised Atmasphere didn't comment on the subject.
Thanks everyone. I have owned many tube amps in the past and used to biasing, but I've never actually questioned what was going on regarding tube usage.
Does this mean that, for example, a tube that is set at 450mv and used frequently, supposing it starts to degrade, it may read 380 (or whatever less value) down the road, and will need an upward bias turn of the screw back up to 450 and so on?
Also, for the exact same setting, with exact tubes (Svetlana KT-88), the one showing 800mv versus the other one showing 450mv the 800mv is a lot newer correct?
You want to set the bias voltage per the manufacture's specs of the amp.
Here is a good explanation by Herman.
"The bias sets the point where the tube idles. When you apply a music signal the amount of current increases and decreases. Lets say for max power the current needs to swing plus and minus 40 mA from the bias point. If the bias current is set too low (say 30 mA) then it won't be able to make a full swing down since it wants to go down 40 but will hit zero and clip before it gets there. Even if you don't hit the zero point the tube operates in a non linear manner as it approaches zero so anytime you get near there distortion goes up.
If you set the bias current too high you might hit the limit on the plus 40 swing and clip, but even if you don't the tube will run hotter than it needs to at idle and shorten its life.
So you want to set it where it can make a full swing in both directions without running the tube too hard or swinging down into the nonlinear area near zero."
What the heck is bias anyway?