How close is close enough when it comes to power tube matching?

War_in_D

War_in_D

Well-known member
Recently picked up a used original Maximatcher and I've been going through a bunch of my power tube stash to see what's up. For consistency, I've been running the Maximatcher through my AmpRX Brown Box and testing all tubes at 120v-121v. I read that the Maximatcher can read differently based on how many tubes you test at any given time, so I've been testing my like branded tubes in pairs, even if I have a full quad. I've discovered that I have a lot more pairs of like branded tubes than I have quads, so I figured by testing them as pairs I could conceivably mix brands if needed as long as the plate current was pretty closely matched.

I know that ideally, we'd all love to have a perfectly matched pair/quad but I'm finding that most of mine are pretty closely matched but not perfect matches. So, my question is "How close is close enough" when it comes to matching a pair (or quad) of power tubes? I'm thinking as a percentage (3%, 5%, 10%, etc.) Also, does transconductance figure into the equation, or is power tube matching based purely on plate current?
 
psychodave said:
I like to keep the quad within 8mA or less of each other or you can get some hum. I have so many tubes that I’m usually within 5 mA.
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Thanks for the feedback! An 8mA spread is way more than any of the matched pairs that I have are showing, most are actually matched within 1mA of each other. These were a bunch of old tubes that I had pulled from previous amps for spares, partials left over from quads were one was bad, etc.. I tested the quad of EL34's that's in the Fusionbear since I had it out of the head shell and they're around a 3mA spread when testing all four at once, with two of each being matched to each other (like two matched pairs at 35mA and 37mA). So, I moved them in the amp to match up with each other as inner/outer pairs. I've only tested my EL34's so far, but I'm going to start on the 6L6's as soon as I have some time. The only 6L6 amp I have left is the Mesa so those aren't a priority.
 
GuitarGuy08 said:
Lucky! Used Maxi Matchers seem to be hard to find, as people tend to keep them. Good find. Happy rolling!!
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Indeed they are hard to find! I've been looking for a used one for several years. There was a seller on Ebay that listed two of them (one at a time) here a few weeks back. I made an offer on the first one they listed and they took it, and then to my surprise they listed a second one!! To see two of them pop up is pretty rare. I'm still looking for a used Maxi Preamp though.
 
I find that most of my ‘matched’ pairs stay with 3mA of each other with the amp idling against a load, volume on 0 for about 30-40 min. After that, they start to drift a bit wider - one continues to rise more than the other. It takes a long time for them to really ‘settle.’ And I’m sure that with the volume up and the power section working, it’s even more dramatic. But so far, no catastrophes and the amps all sound fine.
 
FourT6and2 said:
If I'm trying for a match, I shoot for 3mA spread. But as @Nigel just mentioned... I actually like a mismatch in some amps because it does yield more harmonics. You can go pretty wild with how big the mismatch is. "Back in the day," nobody matched tubes btw.
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So how would you mismatch? For example you got two matched pairs but one pair runs 10mA cooler than the other. How would you install them and bias them in a 100w Marshall? You could pair one ‘hot’ tube with one ‘cold’ tube on each side of the OT or you could keep the matched pairs together. And you could bias the hot pair to 70% and then just live with whatever the cold pair yields.
 
My post above regarding 5-8mA is solely based on my experiences. Greater than 10mA can introduce some hum or other random noises. Working with a TON of NOS power tubes I’ve found 5-8mA to be fine. Since tubes are individual, there is no guarantee that a matched quad will be matched while under use. They also drift after they break in. I’ve had amps that sound glorious and during a tube tune up (when I pull all of the tubes and check them, not necessarily looking to change them) they are off a bit more than when originally put in the amp. This is why I prefer NOS tubes, as they tend to drift a lot less.

This is why I bought the maxi matcher testers a decade ago. I check every tube before it goes into an amp and periodically check them for performance. For example, I put a fresh set of tubes in my Strategy 400 back in 2015. Those tubes are still working, sounding and testing great.
 
SpiderWars said:
So how would you mismatch? For example you got two matched pairs but one pair runs 10mA cooler than the other. How would you install them and bias them in a 100w Marshall? You could pair one ‘hot’ tube with one ‘cold’ tube on each side of the OT or you could keep the matched pairs together. And you could bias the hot pair to 70% and then just live with whatever the cold pair yields.
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You can do it in any number of ways, including what you just mentioned. There are no rules, only what you thinks sounds good. If you want more of a balanced mis-match, you can pair closer tubes together on either side of the OT. If you want more of an unbalance, you can put the matched tubes on one side, so their counterparts are unbalanced across the push/pull.

And yes, as mentioned, a large enough mismatch can cause hum.

All my amps I build have individual bias per tube, so matched tubes isn't a thing for me. I don't need to buy matched tubes. Saves me a lot of headache and money in the end.
 
psychodave said:
My post above regarding 5-8mA is solely based on my experiences. Greater than 10mA can introduce some hum or other random noises. Working with a TON of NOS power tubes I’ve found 5-8mA to be fine. Since tubes are individual, there is no guarantee that a matched quad will be matched while under use. They also drift after they break in. I’ve had amps that sound glorious and during a tube tune up (when I pull all of the tubes and check them, not necessarily looking to change them) they are off a bit more than when originally put in the amp. This is why I prefer NOS tubes, as they tend to drift a lot less.

This is why I bought the maxi matcher testers a decade ago. I check every tube before it goes into an amp and periodically check them for performance. For example, I put a fresh set of tubes in my Strategy 400 back in 2015. Those tubes are still working, sounding and testing great.
Click to expand...

Or make sure your amps have individual bias control per tube.
 
I had a set of EH EL34’s that were way out of spec, like 25% maybe more mismatch. Second best sounding set of tubes I ever had.
If they’re going in an amp with irreplaceable transformers I look for a 3(ish) Ma variance. If it’s a clone or something I can get trannies for I like some degree of mismatch.
 
I'm ok with up to 10mA spread. I put the highest and the lowest mA measurements together as one of the pairs with a quad. Always worked well for me with great results & tone.
 
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This is an excellent thread..I have pulled power tubes out and checked them and had several quads that as tested there were pairs within 3mA..and the other 2 were 8-10mA apart and was wondering why the were installed like that. .I still have them boxed up.
 
I get them as close as I can with transconductance and current draw. Sometimes the way they act in the tester isn't necessarily the way they act in the amp so we can obsess too much about getting them perfect and then once installed they are actually a lot farther off than expected. As long as nothing is double digits off it's fine. I scrutinize my personal stuff a lot less than my customer's amps. I try to get theirs perfect. Mine, shoot, as long as they glow they're good.
 
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