Is it safe to mix brands of power tubes?

[BABMusic]

I have 3 sets of extra EL34 tubes that all of them fall into a safe bias range of 31 mv to 37 mv. The brands are JJ EL34IIs, EH EL34, and Tad EL34. Is it safe to mix these brands in a 50 watt head that uses only 2 power tubes?

 

[311boogieman]

Not the expert in the room but generally - yes.

That said, you mentioned 'sets'. How many of each? I mean, obviously you want to match the 2 that are closest in bias and using the EL34II last if possible - as I don't know the physical/technical differences with the IIs. I do have some of those JJ EL34IIs and JJ El34L and like them a lot. Do you have your own probe/tester?


Actually - you should be fine with these too...

"The JJ EL34II (Mark II) is a redesigned version of the standard JJ EL34, featuring a taller, domed bottle and improved, crimped plate assembly rather than spot-welded plates. Technically, the II offers a fuller sound with improved lower-mids and bass response, sitting sonically between the standard JJ EL34 and the more aggressive JJ E34L"

 

[BABMusic]

Not the expert in the room but generally - yes.

That said, you mentioned 'sets'. How many of each? I mean, obviously you want to match the 2 that are closest in bias and using the EL34II last if possible - as I don't know the physical/technical differences with the IIs. I do have some of those JJ EL34IIs and JJ El34L and like them a lot. Do you have your own probe/tester?


Actually - you should be fine with these too...

"The JJ EL34II (Mark II) is a redesigned version of the standard JJ EL34, featuring a taller, domed bottle and improved, crimped plate assembly rather than spot-welded plates. Technically, the II offers a fuller sound with improved lower-mids and bass response, sitting sonically between the standard JJ EL34 and the more aggressive JJ E34L"

I have one matched set of each of those brands I mentioned. The EL34II tubes are slightly larger than the EH EL34, but they are about the same size as the TAD tubes. They are all 25 watt tubes, so I was thinking it might be okay. EH are set to about 35 ma, TAD are about 34, and the JJ is weird because one tube is reading 31 ma and the other reads 37 ma.

 

[FourT6and2]

Yes it's safe. But why would you mix them in a 50W amp when you have sets of tubes?

 

[VonBonfire]

 

[BABMusic]

Yes it's safe. But why would you mix them in a 50W amp when you have sets of tubes?

I guess I was just wondering because one of my matched sets reads 31 and 37 ma on the probe. I've let them sit for a good 15 minutes and it reads these numbers. Sometimes I hear a random "pop" sound when I use this set of JJs in the amp. I wondered if swapping another tube like the Tad that is reads 36 ma would be better than having that kind of 31 and 37 difference.

 

[FourT6and2]

I guess I was just wondering because one of my matched sets reads 31 and 37 ma on the probe. I've let them sit for a good 15 minutes and it reads these numbers. Sometimes I hear a random "pop" sound when I use this set of JJs in the amp. I wondered if swapping another tube like the Tad that is reads 36 ma would be better than having that kind of 31 and 37 difference.

Ah, got it.

Well, 31 and 37 (or 36) isn't an ideal match if you paid for matched tubes. But it won't cause any real problems. And swapping a tube for another brand just to get 1mA closer isn't really going to accomplish anything. But you can try it. Probably better off just putting in a whole new set.

 

[matttornado]

Yes. As long as it causes no problems such as hum or red plating. I use mismatched tubes / brands that have a mismatch up to15 mA apart in 100 watt amp with zero issues & the amp sounds fantastic. Having matched tubes is not really that important. A difference between 31 and 37 is no big deal at all.

 

[tricalibur]

I think any bright tech or builder would generally say no. Matched tube sets are offset with one tube drawing less than the other. This is intentional and by design. Im no engineer, just a hobbyist with a lot of experience rubbing elbows with these guys, but the consensus seems to be you should buy only matched sets to use in your amps. I don’t save used tubes. If one pops out of a matched set I recycle them all and get a brand new set.

 

[Rick Lee]

This is a good reason to buy an old JCM2000 DSL50 or 100. In the 50 the tubes are independently biasable. In the 100 the pairs are.

 

[psychic_driver]

Yes. As long as it causes no problems such as hum or red plating. I use mismatched tubes / brands that have a mismatch up to15 mA apart in 100 watt amp with zero issues & the amp sounds fantastic. Having matched tubes is not really that important. A difference between 31 and 37 is no big deal at all.

Are there tonal reasons for the mismatch or its just what you have on hand?

 

[matttornado]

Are there tonal reasons for the mismatch or its just what you have on hand?

Nope. I have JJ, Ruby, Svetlana, Winged C & Electro-Harmonix all EL-34s and they all sound pretty much the same to me. I can't tell the difference between them. I'm not pushing the power amp hard either.

 

[BABMusic]

Nope. I have JJ, Ruby, Svetlana, Winged C & Electro-Harmonix all EL-34s and they all sound pretty much the same to me. I can't tell the difference between them. I'm not pushing the power amp hard either.

True. The only difference I can tell is a slight increase or decrease in bottom end, but it's inconsequential.

 

[311boogieman]

Are there tonal reasons for the mismatch or its just what you have on hand?

First of all, I'm not a tube expert. I feel a 15mA mismatch is too much but that's me. That 'could' put undo stress on the tubes. My personal feeling (on say JJ EL34s) is I get the same brand and match to within 5mA. Some claim, that a mismatch can present some interesting cross-over distortion. I'm not sure I've ever heard it myself. Generally, I like to match as closely as possible.

 

[Kelly]

I think any bright tech or builder would generally say no. Matched tube sets are offset with one tube drawing less than the other. This is intentional and by design. Im no engineer, just a hobbyist with a lot of experience rubbing elbows with these guys, but the consensus seems to be you should buy only matched sets to use in your amps. I don’t save used tubes. If one pops out of a matched set I recycle them all and get a brand new set.

Matched tube sets weren't even a thing for the majority of tube electronics use. When a tube went bad in your radio, tv, juke box, car stereo, etc. you went to the tv repair shop and bought another tube. Whatever tube they had, whatever manufacturer. Fender never sold amps with matched sets of tubes until they bought Groove Tubes.
Matched tubes may get you less distortion at higher volumes, and possibly extended tube life, but it isn't unsafe to run an amp with unmatched tubes as long as said tubes are in proper spec and not overbiased.

 

[FourT6and2]

...it isn't unsafe to run an amp with unmatched tubes as long as said tubes are in proper spec and not overbiased.

Right, matched sets (in terms of testing) have historically not been a thing. But with the caveat that any tube isn't pulling too much current at some bias setting or they are so unmatched that they start to magnetize the transformer. And mismatched (in terms of bias) can cause more hum.

 

[Carol-AnnAmps]

First of all, I'm not a tube expert. I feel a 15mA mismatch is too much but that's me. That 'could' put undo stress on the tubes. My personal feeling (on say JJ EL34s) is I get the same brand and match to within 5mA. Some claim, that a mismatch can present some interesting cross-over distortion. I'm not sure I've ever heard it myself. Generally, I like to match as closely as possible.

Its not cross over distortion that's a positive benefit here, but your advice is spot on. You've gone too far or one of the tubes is way too cold biased if you have cross over distortion. What you want to aim for is an increase in even order harmonic content. If you think about a push-pull amplifier, if both sides are amplifying almost identically a lot of even harmonic content generated in the power tubes is going to get cancelled because of differential cancellation in the output transformer. Two signals that are the same amplitude at the same frequency 180 degrees out of phase obviously cancel each other out. The same cancellation that cancels out the ripple that causes hum. Now granted this doesn't cancel any even order harmonics generated in earlier stages and of course no O/T primary is ever truly symmetrical and no phase inverter is perfect so not everything is cancelled. In fact you don't really need to buy matched triode tubes for the PI. The same theory applies there, though most designs do have some inherent imbalance in the design.
Even order harmonics in the power stage generate width and complexity, which for most styles of music is a real positive. Odd harmonics give you mix cut and focus, even give you width and a more complex tone. The specific balance of these is what gives an amp its unique character.
A good rule of thumb for say a 2xEL34 50W amp is between 2 and 5mA difference as you stated, which will actually sound bigger and wider than a perfectly balanced set and it will not cause any noticeable hum if there's a decent L-C PI filter in the power supply. Go beyond that towards 6+mA and you risk starting to get a bit of noticeable cross-over distortion, which doesn't sound great (in my opinion at least) once non-musical overtones start appearing. Always bias for the hottest tube. Mixing different brands can be interesting if they are reasonably closely mismatched, again 3 to 5mA. All fun things to try and hopefully sheds light on whats going on.

 

[311boogieman]

^ Thanks for the very detailed and well written post to correct and clarify.

Glad you are here.

 

[Carol-AnnAmps]

^ Thanks for the very detailed and well written post to correct and clarify.

Glad you are here.

The one thing I forgot to mention and its often a question is what happens to odd order harmonics when tubes are very closely matched. They actually increase, in fact they are summed.
If you go through the math of the transfer curves as a power series (I'll spare you that) even order components on each side of the O/T primary end up equal and opposite out of phase and will effectively cancel and the odd order components end up in phase, so will sum.

Controlling the generation of these different harmonics through all the different stages of an amp is like a recipe. As a builder you create your own recipe that you like and thats part of what gives a brand a signature tone. People obsess over specific brands of components and materials and there is some validity in that for sure, but generation and manipulation of harmonic content, gain and frequency response of every stage is the key to the front door, especially for an overdriven rock / metal amp. In any simulation software like LTSpice or TINA you can design a stage and do a fourier series to see exactly what the harmonic content of that stage will look like as well as a gain and freq response over its entire operating range. You have full control and analysis versus just tweaking out components and doing everything by ear. That part comes later to make any minor changes. This is what actual 'designing' looks like when done using a more traditional engineering approach. I've wandered off at a tangent.

Back to the question at hand, yes mix those tubes up as long as they are not wildly mismatched. You might be suprised with the results.

 

[311boogieman]

^ Thanks.

Hope BAB doesn't mind but what is your opinion of JJ power tubes and JJ preamp tubes? I'm specifically asking about EL34s, EL34 IIs, EL34Ls and then 12AX7s and any 5751 variant. I'm asking because people dog on them a lot but I've had nothing but great results, including their 6L6s. Again, I'm not a tube expert but anytime I try a different brand - I'm left disappointed (minus NOS of course). So this is more of new production question I suppose.

Am I possibly experiencing confirmation bias and my brain is telling me they (other brands) are not as good?