power tube question

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hellaboogie

hellaboogie

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Do you consider a 5 mA difference between a pair of power tubes a big deal?
 
How does it sound? Some people like having different pairs together for some coloration of the notes. If you just want them matched, 5ma is too much.
 
hellaboogie":3319tvjn said:
Do you consider a 5 mA difference between a pair of power tubes a big deal?
Click to expand...
Kinda depends in what context you're asking this. If you got a matched pair and wanted to match it with another pair, effectively making a matched quad - yes, that's too much variance. Depends on the tube too, but EL34s shouldn't go past 35mA typically. So if you've got, say, a quad of EL34s and you're find 2 of 'em are cold to keep the other normal, or 2 of 'em are hot to keep the other normal, not a good thing. If you're asking this as a way to move your sound around a bit, it's fine as you're in control of the floor and ceiling bias rates.

V.
 
ok say I have four 6L6GC power tubes and three of them are biased to 35mA and the forth is at 30mA
would you consider this a big deal
 
hellaboogie":2nky914k said:
ok say I have four 6L6GC power tubes and three of them are biased to 35mA and the forth is at 30mA
would you consider this a big deal
Click to expand...
Not really. Unless I paid for a matched quad.

V.
 
Not really. I usually write down the bias of each tube at a particular bias setting, usually around 35 ma. I then take the highest drawing tube and pair it with the lowest. I can have tubes 5ma apart and match them into pairs that are tighter. The OT sees a quad of tubes as two pairs. For example:
1) 33.8 ma
2) 30.4ma
3) 35.3ma
4) 36.2ma

I bias the hottest tube when biasing (Tube#4) Technically one tube is about 6ma out from the lowest. I would take tube 2 and tube 4 and make a pair, and naturally the other two make a pair.
Pair one) 30.4+36.2= 66.6ma
Pair two) 33.8+35.3= 69.1ma The tubes are now seen by the OT as matched within 2.5ma.
 
Have you tried moving them to different positions to yield a closer match? I find that if I have one that's out I just move it to a different location and it brings it in closer to matching up.
 
I don't really see what you can do about it. There's only one trim pot to adjust. Unless I'm missing something. You are stuck with whatever variation that you've got. Only thing might be to have it repaired, which I don't think is worth it for 5mA. 5mA might be borderline though.
 
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