Matched quad or two matched pairs?

All about tube choices for the JVM series. See workbench for biasing and tube problems you may have.

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ksjprod
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Matched quad or two matched pairs?

Post by ksjprod » Fri Dec 23, 2016 6:47 pm

I know ideally you'd want a matched quad of EL34s in the power section, but how about two matched pairs? I bought my JVM210H used. It still had the original Marshall tubes in it. BUT, it had one pair of the wing C and one pair of the Shuguang. Didn't sound too bad after I biased it. Then I broke one of the Shuguangs when I was removing it to try some Tesla E34Ls. Wasn't happy with the E34Ls so I went back to the Marshall branded wing C and some older wing C that I had on hand.

Should I just bite the bullet and buy a matched quad? Or is this really no big deal.

Karl

aberry9475
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Re: Matched quad or two matched pairs?

Post by aberry9475 » Thu Jan 26, 2017 1:17 am

You really should just buy the quad.

I'm not an expert, but one of the purposes of matching tubes in general is so that the bias (which is global to all power amp tubes) is correct (yes, it's subjective - a little hotter than spec, colder than spec, etc.). If you get two matched pairs, then the pairs themselves should be matched somewhat, such that two are not burning much hotter than the others. There might be other problems, but like I said, not really an expert here..

On the other hand, you can just run the amp with a matched pair with 2 tubes out. The wattage of the amp will be cut in half, and you will also have to cut the impedance in half (if you're running a 16 ohm cab/load, plug into 8; if 8 ohm cab/load, plug into 4). This may be the best option if you're just trying to save money in general.

Edit: If you do this, you must have 1 tube on each side. This means the two inner sockets are empty, the two outer sockets are empty or some variation of this. I've always pulled the two middle tubes out which theoretically allows for greater heat dissipation in between, but this is trivial really. However, if you run two on one side with 1 or none of the other, you're gonna have a bad time mmkay? LOL

Some people actually prefer the 50 watts that way with 2 tubes pulled, although I personally do not (almost seems like less "oomph"). But, inevitably, the differences are pretty minimal, even more so at lower volume. The volume difference as well, if I recall correctly, is only like 3 decibels. Take it as you will.

bordonbert
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Re: Matched quad or two matched pairs?

Post by bordonbert » Sat Jan 28, 2017 6:01 pm

There is a logical argument that says that you get a different sound by deliberately not having a matched quad. As long as each pair is matched there is no question of any problem with the amp. The idea of matching is only to get the same DC current flowing in both sides of the output Tx and cancelling any static magnetisation in the core. Having, say, one pair BOTH running at 30mA and the other pair BOTH running at 35mA because the pairs aren't matched against each other means that is still the case as one of each pair is in each side totalling 65mA each way.

Of course one pair will be running a little hotter than the other but it isn't a huge difference, you must keep their operating conditions within spec of course. The point is that one pair of valves then saturates earlier than the other and the overall distortion characteristic is different and a little more gradual. I'm sure it's another personal preference thing and I bet there are people who will completely rubbish the idea. I wonder how many of us will have tried it? I admit I haven't! I just go with what pairs/quads I have and make sure it's biased as correctly as possible.

aberry9475
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Re: Matched quad or two matched pairs?

Post by aberry9475 » Mon Jan 30, 2017 12:19 am

That's a great thought. With that you will be burning in the hotter pair much faster, but it's an interesting idea with tonal implications I'm not very sure of.

I'm assuming in this though, you will need a somewhat matched "mismatched" quad - if you have two pairs that are quite different, then you will basically roast one pair while the other is colder, right?

bordonbert
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Re: Matched quad or two matched pairs?

Post by bordonbert » Mon Jan 30, 2017 3:05 pm

Technically yes but this idea of "roasting" valves in proper use is a bit overstated. I don't think the difference is likely to be too great or cause too great an issue in real life. We aren't talking about having one pair set to run so high it's out of spec, we would still need to keep the hotter pair within spec and live with a slightly colder pair of course. Lifespan should still be very good. In hifi circles where people generally are, as they should be, much more picky, the usual quote is about 2,500hrs use for power valves and 10,000hrs for preamp tubes. Do the maths and you'll see that's a lot of use. Guitar amps may drive them a bit harder but hifi is a much more demanding area for noise and distortion and the like. The truth is most valves die because of ways they are not treated correctly not because they burn out through use! (You could research cathode poisoning if you're interested).

There is genuinely too much swapping out of many types of component "just in case" because the dark side of the industry has led non-technical people to believe that is necessary. It isn't in well designed equipment. It just generates more money!

Do you see it as "pair A dies earlier" or "pair B lasts longer"? Sort of are you a glass half empty or glass half full person? Others may disagree, I work from the engineering perspective not common knowledge. And I did say openly that I had not done extensive practical testing of this, just never made a point of insisting on matched quads at all stages of my amps' lives. A couple of individually matched pairs has always been fine for me and I've never had any problems with tone.

Green Manalishi
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Re: Matched quad or two matched pairs?

Post by Green Manalishi » Tue Feb 28, 2017 9:04 pm

Two matched pairs should work just fine, doesn't the JVM have bias controls for each pair?
Marshall JVM410H
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Soporific
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Re: Matched quad or two matched pairs?

Post by Soporific » Sun Dec 30, 2018 5:43 pm

Green Manalishi wrote:
Tue Feb 28, 2017 9:04 pm
Two matched pairs should work just fine, doesn't the JVM have bias controls for each pair?
Yes. I learned this while pulling two tubes to sort out an issue and that the amp can run two power tubes without rebias per Santiago.

The bias pots handle a pair at a time.

~S

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