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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
atmasphere's avatar
atmasphere
11,778 posts
 
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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uru975
684 posts
 
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?
jea48
4,408 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?
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