Intenionally Mismatching Power Tubes?

[PlayItAll]

Hello,

Just wondering if it might ever be desirable to mismatch power tubes? I'm admittedly just a player - not a technician - but could a power tube mismatch produce good tones? Is it bad for an amp?

Thanks,
Ed

[Angelo Skouras]

Mismatch?!? I think you meen make a push-pull of non identical valves! or not?

OK, let’s put everything on the table!

Push-pull and other circuitry that work with matched tubes like differential amplifier, is based on the idea of adding the output signal while subtracting the shortcomings. As an example we have two valves with their anodes driving both each half of the output transformer. If the one valve is driving a signal to the xformer inverted with respect the other, then any signal that is common between the two anodes will be subtracted. This way, if the two tubes are perfectly matched, and have exactly the same bias, they will also have exactly the same amplitude of 50-60Hz main’s noise, making a secondary have no hum whatsoever. The same will occur with harmonic distortion, making fidelity higher, and with dc current, making a smaller rated X-former appropriate. This is the reason why push-pull amps have smaller transformers than single-ended ones.

Anyway, I think that if we make an amp with two different output tubes, the outcome will be something intermediate between single-ended and push-pull (I speak only about class A operation). A good design like that could compromise benefits of S.E. and P-P, and it will be one of my very next designs.

At class AB operation, or what is next in alphabet, it would be something very against of what a human’s ear prefer, but I don’t bet my head for that…

I hope I didn’t go too far for my medium English…

Thank you very much for the idea. Curry on biasing brother…

[trevorus]

THD Bi-Valve amps are capable of this, IIRC...

[Steve Conner]

I have a homebuilt 50w P-P amp with an independent bias control for each tube, and they have a really wide range, from -25 to -60V. So I can use about any two tubes I want. The strangest combo I ever tried was one EL34 and one EL37.

But I found that it didn't do anything astronomical to the tone (for me anyway) and tended to give less power output than a pair of better matched tubes. So I just use pairs of similar tubes now.

Or maybe you meant mismatching as in using the wrong tap on the output transformer. I do that a lot with this same amp, but that's more out of necessity, because the OT I got for it only has a 16 ohm output. I keep meaning to get a replacement that has 8 ohms too. But in the meantime, it still gets decently loud with an 8 ohm speaker plugged into it, and I've seen no ill effects of this mismatch.

[Amp Kat]

Assuming an AB class which most amps are can sound awesome when using two different mismatched tubes when using power tube distortion but IMO do very little for clean tones mainly for a few reasons. If they are mismatched by a large margin than your introducing hum which is more decernable than when being masked by distortion and noise during power tube distortion. It is also possible to get a more asymetric clipping causing more even order harmonics in the latter order range of the spectrum much like messing with the PI balance to do the same. Not all will sound good as if you get to far apart you can get into crossover distortion of severe hum even when using powertube distortion but if you get the right combo yes it can sound awesome especially if their close enough not to hum and you can also use preamp distortion not going into power tube distortion and still have some even order harmonics going on there. Try a 5881 and a 6L6.

[Enzo]

What do you mean by mis-matched? Do you mean like a 6L6 and an EL34? or do you mean all the same type but not a matched set - for example two 6L6s of different brands?

Mixing tube types is OK as long as reasonable bias levels are there for the tubes. It won't hurt the amp to unbalance things. The results may or may not sound good to you.

[PlayItAll]

What do you mean by mis-matched? Do you mean like a 6L6 and an EL34? or do you mean all the same type but not a matched set - for example two 6L6s of different brands?

Sorry, I should've been more specific. I meant using pairs or quads of power tubes of the same type, but not all matched to each other. As in using 2 x 6v6 tubes with different ratings in a Deluxe.

Good info in the above responses, though!
Ed

[Enzo]

OK, so you are concerned over buying a matched set of two, or just buying two tubes of the same type regardless of "ratings."

Leo Fender sent amps out with random tubes for decades. Only recently are they shipping with Groove Tube matched sets. And in fact those ones they ship with Fenders are not matched as close as the ones they sell on the market. There are only three ratings for Fender.

In any case, most amp makers do not match tubes. It is not necessary. Matching results in reduced hum, and there would be some subtle distortion improvements. But as long as none of the tubes are grossly out of spec or defective, you can stick any two 6V6s in there and nothing will blow up.

[Liam]

I just bought a Marshall Bluesbreaker reissue . It's got a Chinese looking 6L6 and a more recent Marshall branded EL34 side by side in it. I'll bet whoever fitted them didn't even check bias voltage. It sounds fine, only noticed because I took the chassis out for a general look over. Not going to leave it like that, but only because it would feel wrong...

[slidincharlie (Carlo P)]

If I got it right, PlayItAll meant two (or four) tubes of the same type (e.g., two 6V6), of the same brand, but not bought as a matched set. Which means they could draw different bias current, right?
I have read often that some players like a bias mismatch (even up to 8-10 mA) between two power tubes (or the two sides of a push-pull set, generally speaking) compared to a perfectly matched set. And, as Enzo said, amp manufacturers didn't care for matched tubes... but tubes were much better, strong and more reliable in the old days, and they tended to be more homogeneous in specs than today's tubes.
I've never experimented in this field, but for goods I keep my power tubes mismatched by 3-4 mA in my SF Vibrolux, where I have independent bias adjust for the two 6L6's.