Power tubes, matched quad or 2 matched pairs ok?

  • Thread starter Thread starter MadAsAHatter
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MadAsAHatter
MadAsAHatter
Well-known member
Trying to confirm this... In an amp with 4 power tubes and only one bias adjust knob do you absolutely need to have a matched quad? Or will using two matched pairs be fine as long as it's balanced properly?
 
You need a matched quad. And all within 5mA of each other. Two matched pairs most likely will not be matched closely enough when used together.
 
You need a matched quad unless all the tubes just happen to test the same. Or you install another bias pot.
 
IMO, all power tubes should be biased within 5-7 mA. That said, you can mix and match brands provided they meet the previous criteria. I don't mention it much, but in my tinkering, I've mixed brands of tubes to achieve different tonality. Like KT77's and EL34's, etc.
 
I’ve had matched quads, pairs, and some quads that start at say 30ma and end at around 40. At the end of the day my mismatched quads sound just as good as my matched. As long as you’re not too hot or too cold you should be fine.
Ideally techs would tell you matched is best but I’ve never had any issues with slightly mismatched power tubes.
 
A matched quad will give you the lowest noise. If you have to do with 2 matched pairs, which I had to recently due to the tube shortage, you want to make sure you stagger the two pairs, so you don't want the colder pair amplifying the top half of the waveform and the warmer pair amplifying the bottom half of the waveform.

So I checked this recently on a friend's amp, where he was not able to get a matched quad. I was able to notice less hum when I had a cold and warm pair on each side.
 
To make sure I'm understanding it correctly relating to 4 tubes in an amp with only one bias adjust knob....
-The obvious ideal situation is to have a matched quad with all 4 tubes within about 5 mA of each other.
-A slight mismatch between 2 matched pairs is tolerable provided the pairs are no too far apart from each other within say 10 mA.
-If your in a bind and only option is having to use 2 matched pairs that are further apart from each other you can compensate by placing one tube from each pair (1-"cold" and 1-"hot") on either side of the waveform so it's balanced.
-4 tubes that all read different and you're SOL until you can at least find their match.
 
I've just always stuck to the rule that I mentioned and have had no problems with my amps. I guess you'll have to do your own experimenting to decide what works best for you and your amps. Best of luck to you.
 
Thanks for the info. That cleared it up for me much more than the crap info floating out there.

Fret-Shredder said:
I've just always stuck to the rule that I mentioned and have had no problems with my amps. I guess you'll have to do your own experimenting to decide what works best for you and your amps. Best of luck to you.
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I pretty much just get matched quads and not planning on doing any experiments with mismatches. I unexpectedly needed to replace power tubes in 2 amps at once recently. Fortunately I was able to buy matched quads for them. Just thinking since I have another few amps with older tubes and things like to happen in threes to get one set to have on hand. With shortages, if I couldn't find another matched quad could 2 matched pairs be an option.
 
Lol Panhead My friend had a Marshalled Princeton with Bassman transformers and one 6L6. It ripped.
 
The best one I got, played with a bass player for 2 years played solid state preamp power amp at practice, first gig shows up with a svt and a direct box. We had large pa side fills etc the 80`s, i said where `s your cabinet? He says dont need one im running direct and im like you need to have speaker cab hooked up to that svt for load. He says nope done it like this before. He did and he did it for the whole 2 years he played with us not one hiccup.
 
Too funny. Dumb luck I guess.
I ran one EL34 and one 6L6GC by mistake for a long time. The bottle shape and Mesa label were exactly the same.
 
My quad is mis matched by about 12 mA and it sounds fantastic. I pair the highest with the lowest and hope both sides balance out within reason.

I think having matched tubes is not that necessary and is over hyped. As long as that they're not too far mis-matched, the amp works properly and sounds good, you're ready to rock! I have no problem with a 5-12 mA mis-match.

If I blow a tube, I replace it with whatever brand tube bias' up within reason and I'm done. Some people blow a tube and buy a whole new quad for one bad tube! CRAZY.
 
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