Amp Bias Meters

Chubbs

Well-known member
I'm installing new tubes in my 2203 and have a question regarding using a bias meter. Will I need a meter with four leads for every tube socket or would a tool with two leads work sufficiently?
 
I'm installing new tubes in my 2203 and have a question regarding using a bias meter. Will I need a meter with four leads for every tube socket or would a tool with two leads work sufficiently?
The one with two leads will work, you’ll just need to record the readings on the first two tubes and then install the leads on the other two tubes to test. It’s just more back-and-forth with only two leads.
 
And if I remember correctly, bias both outside tubes as a pair then the inside tubes as a pair.
Good post. Sorry not inside and outside as a pair. Pairs in line. Not only this but match your tubes into two pairs. The Output transformer sees four tubes as two pairs but also sees the combined bias as two, not four, one bias for each pair. In the example below you will want to take your lowest reading tube and your highest and make one pair, and the other two as your second pair.

Tube
1) 35 ma
2) 30 ma
3) 32ma
4) 29 ma

So tubes 1 and 4 have a combined current draw of 64 ma. The other 2 tubes are 62 ma. These two pairs for your quad are seen by the OT as matched within 2ma regardless they are matched individually as 6 ma apart. If one tube is 6ma apart from the others , and you match this way, it is fine. When you have one tube that is too far out it is not optimal to do. But tubes that are five ma apart can be matched tighter with this method.
 
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Good post. Not only this but match your tubes into two pairs. The Output transformer sees four tubes as two pairs but also sees the combined bias as two, not four, one bias for each pair. In the example below you will want to take your lowest reading tube and your highest and make one pair, and the other two as your second pair.

Tube
1) 35 ma
2) 30 ma
3) 32ma
4) 29 ma

So tubes 1 and 4 have a combined current draw of 64 ma. The other 2 tubes are 62 ma. These two pairs for your quad are seen by the OT as matched within 2ma regardless they are matched individually as 6 ma apart. If one tube is 6ma apart from the others , and you match this way, it is fine. When you have one tube that is too far out it is not optimal to do. But tubes that are five ma apart can be matched tighter with this method.
Great info. I never knew to do it that way.
 
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Great info. I never knew to do it that way.
Cool. It's not inside outside tubes as a pair. Its pairs in line. One leg each of OT primarys go to the inside tube sockets so it is one pair left and one pair right of those leads. Sockets 1 and 2 are a pair and 3 and 4 a pair. I only glanced at your post and didn't realize you thought outside/inside as pairs.
 
Cool. It's not inside outside tubes as a pair. Its pairs in line. One leg each of OT primarys go to the inside tube sockets so it is one pair left and one pair right of those leads. Sockets 1 and 2 are a pair and 3 and 4 a pair. I only glanced at your post and didn't realize you thought outside/inside as pairs.
Very cool info. One question though. Diezel outer bias points have the two outer tubes as one pair and the two inner tubes as another. Is there a reason its setup that way instead of down the line or is just because your using a different method?
 
Very cool info. One question though. Diezel outer bias points have the two outer tubes as one pair and the two inner tubes as another. Is there a reason its setup that way instead of down the line or is just because your using a different method?
Which Diezel? The Herbert may be different because it has 6 power tubers. I don't know about the Diezel but generally a pair is in line not inner outer as a pair. . I know when you pull two tubes to run an amp in half power it is usually the outer or inner pairs. This gives you one tube on each side of the output transformer which lends itself to tubes in line(inner to outer) being a pair on one leg of the OT.
 
Which Diezel? The Herbert may be different because it has 6 power tubers. I don't know about the Diezel but generally a pair is in line not inner outer as a pair. . I know when you pull two tubes to run an amp in half power it is usually the outer or inner pairs. This gives you one tube on each side of the output transformer which lends itself to tubes in line(inner to outer) being a pair on one leg of the OT.
This would be on the Einstein.. and I believe on the Dmoll also.
 
This would be on the Einstein.. and I believe on the Dmoll also.
It's the way Diezel does their test points. Diezel doesn't use the method I do. Not wrong or right per say but not giving you the the most optimal matching in the Diezel. Is it really going to affect the amp. No. But I just prefer to know they are matched as tight as possible per each tube.
 
He’s right, i match left two pair and right two pair to be the closest in bias. The positive peak amplitude of a sine wave in phase is amplified by one pair, the negative amplitude that is 180 degrees out of phase is amplified by the remaining pair. The phase inverter splits and inverts the phase of the source sine wave to be amplified in 180 cycles each by one pair of tubes. That is why you have to pull inner pair or outer pair if you want your amp to work for reducing output power while also halving the output impedance.
 
Cool. I always assumed that the pairs were inner and outer so now I know thats not always the case.
 
Bogner Ecstasy. You call pull pair 2 if running half power.

xtc.jpg
 
Bogner Ecstasy. You call pull pair 2 if running half power.

View attachment 79208
Doesn't matter as long as you have 1 tube from each pair on each side of an OT primary. This has nothing to do with matching pairs optimally for bias though so don't confuse running half power with this matching/bias method. You also want to halve your impedance when pulling 2 tubes to run half power. If your cab is 16 ohm, use the 8 ohm primary OT tap to your cab.
 
I prefer to just bias to what sounds good to me. Then I'll check with a meter to make sure I'm not dangerously high or low. After doing it this way for a few years, I've discovered my ears typically love 55% - 60% every damn time. And I like a mismatch. Just adds more flavor. To each their own though.
 
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