|
Post by driventone on Nov 9, 2022 23:40:36 GMT -7
I have read different things about what is acceptable, and I’ve even seen claims that to a certain point, a mismatch in plate current can add dynamics and character to the sound of a guitar amp. But, in a push-pull circuit with two power tubes, what is an acceptable variance?
I would guess that if you had to use a mismatched pair, you would want to set the bias so that the hotter tube is at the max appropriate plate dissipation for push-pull, and just accept whatever the colder tube sits at, assuming it’s not ridiculously low, in which case it wouldn’t make sense to use these two as a pair.
For example, in a two EL34 (50 Watt) push-pull amp, if one tube is set at 40 mA to get to about 70% plate dissipation, but the other tube is at 33.5 or around 60%, is this something to worry about? What kind of effect can that have on the amp?
My guess would be that as for the tubes, it just means that the one that pulls less plate current might outlast the one that pulls more. But, will this cause any problems with the amp’s output transformer or any other components? How about the speaker? I’m picturing the sine wave being higher on the push than the pull side, or vice-versa, since one tube will be putting out a different wattage than the other.
Obviously, it would make more sense to use a matched pair, but in this case I have some really nice vintage tubes I’d like to be able to use if I can.
Thank you for your help!
|
|
|
Post by Seńor Verde on Nov 10, 2022 9:55:57 GMT -7
I'm no expert but have read a 3mA or less difference desireable. In the old days, nobody matched tubes and the bias was set using a signal generator and oscilloscope. I'm guessing most of the time there was a greater discrepancy greater than 3mA, but crossover distortion was avoided by the way the amps were biased with the scope.
|
|
|
Post by frankie on Nov 10, 2022 10:15:56 GMT -7
I have read different things about what is acceptable, and I’ve even seen claims that to a certain point, a mismatch in plate current can add dynamics and character to the sound of a guitar amp. But, in a push-pull circuit with two power tubes, what is an acceptable variance? I would guess that if you had to use a mismatched pair, you would want to set the bias so that the hotter tube is at the max appropriate plate dissipation for push-pull, and just accept whatever the colder tube sits at, assuming it’s not ridiculously low, in which case it wouldn’t make sense to use these two as a pair. For example, in a two EL34 (50 Watt) push-pull amp, if one tube is set at 40 mA to get to about 70% plate dissipation, but the other tube is at 33.5 or around 60%, is this something to worry about? What kind of effect can that have on the amp? My guess would be that as for the tubes, it just means that the one that pulls less plate current might outlast the one that pulls more. But, will this cause any problems with the amp’s output transformer or any other components? How about the speaker? I’m picturing the sine wave being higher on the push than the pull side, or vice-versa, since one tube will be putting out a different wattage than the other. Obviously, it would make more sense to use a matched pair, but in this case I have some really nice vintage tubes I’d like to be able to use if I can. Thank you for your help! You are looking at their current at idle, without input signal. Matching between tubes this way is done mainly for longevity sake. Under 5mA difference is ideal. 7-8mA is not too far off, but one will wear more readily. It will not have any effect on the amp itself, that is, until a tube red plates or fails. In terms of a matched output of a sine wave, this is a AC current and unrelated to the bias. It is determined more by the output transformer (that is unless one tube is extremely weak or dead). A balanced AC output is measured by an oscilloscope.
|
|
|
Post by perryr on Nov 10, 2022 12:27:29 GMT -7
I think I would approach this in terms of % variance rather than absolute mA difference. Probably under 20% would be good. But I’ve definitely heard amps with power tube mismatch @ 20% sound just fine and not humming or exhibiting anything undesirable. And as mentioned, some mismatch could be desirable.
Tubes drift, usually dow, but not always and not at the same pace. Matched sets when NEW has the benefit of lasting longer before that drift works its way into a mismatch that may cause hum or other undesirable operation.
|
|
|
Post by driventone on Nov 10, 2022 16:52:23 GMT -7
Thank you Frankie, Doobiek, and Perry! All great information, and I’m glad to know it won’t hurt the amp.
|
|
|
Post by driventone on Nov 10, 2022 16:55:53 GMT -7
I think I would approach this in terms of % variance rather than absolute mA difference. Probably under 20% would be good. But I’ve definitely heard amps with power tube mismatch @ 20% sound just fine and not humming or exhibiting anything undesirable. How would you calculate the 20% variance? As a percentage of the hotter tube’s current, or the colder tube? Example: 40 mA and 33.5 mA = 6.5 mA variance. Would you divide 6.5 by 40 (16.5%), or divide 6.5 by 33.5 (19.4%)?
|
|
|
Post by perryr on Nov 10, 2022 18:37:39 GMT -7
From the higher number. But again 20% isnt a line in the sand, how it sounds is most important, and/or if you are hearing any humming.
|
|