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Tube biasing question please

Back into tubes after a few years here is my question hoping someone can help:

My integrated amplifier uses KT-88 tubes, and manufacturer's recommended biasing level is 450 mv, and this is where I maintain the reading on my multimeter. So, my current tubes (let's call them set ''A'') are biased according to man specs.

Now, I came accros a set of spare tubes (set ''B'') I had been keeping for a few years, I had forgotten all about them.

Upon insertion in my amp, NOW the reading says (with no adjustments) 780 to 800 mv each instead of the previous 450 mv on my set ''A'' current tubes.

It was of course a cinch to adjust set ''B'' from 780mv to the recommended 450mv with a turn of the biasing screw.

MY QUESTION IS THIS: What does this say about the condition and life left in both sets? Should I assume that one of set is strong while the other is weak? Which set is ''newer'' from a standpoint of hours used?

I would like to avoid the sonic issues (how they sound) just interested in knowing which tubes has more life left in them.

Both sets are Svetlana winged ''C''.

Thanks if you can help !
soniqmike
jea48
4,609 posts
 
Soniqmike,

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.
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soniqmike OP
140 posts
 
...meaning they most likely have fewer hours on them? Thanks.
soniqmike OP
140 posts
 
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?
jea48
4,609 posts
 
Soniqmike,

You want to set the bias voltage per the manufacture's specs of the amp.

Here is a good explanation by Herman.

05-18-11: 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?
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Jim
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