Output valve matching -- really necessary?

Diapason

Wammer
Wammer
Jan 6, 2012
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Dublin
AKA
Simon
Having recently changed the input valves on my power amps, it seems I now need to change the output valves. They're 6C33C-B valves which not everywhere sells, but they seem reasonably plentiful still in Russia/Ukraine.

I bought the amps needing new output valves, so at that time I went on eBay, bought 8 unmatched specimens, got my dealer to do the biasing, and I got on with my life. They sounded great, but I had nothing to compare them with so who knows if I've unlocked their full potential. Recent reading online has emphasised that for these amps buying matched output valves is a must, because you can only set the bias for a pair of valves rather than for each individual valve. By all accounts the 6C33C-B takes quite a while to settle down from new, and it's tricky to find a valve tester that can accommodate them. That might explain why they are hard to find in matched pairs, and expensive when available.

I have two options:

  1. Buy them unmatched from a well-known Russian site for €84 + delivery; or
  2. Buy them matched from a well-known German site for €300 + delivery (the cheapest I've found for matched pairs).

(Prices are for all 8 valves required, not individual prices.)

Now I know in the grand scheme of hifi this isn't that much money, but I'm a scabby b'std and hate paying over the odds for fooey. Is it really necessary to get matched pairs? Is it likely that any "matched" pairs I buy will actually be properly tested and matched? Will they even stay matched in the longer term if the valves take a while to settle down?

Would be interested in the thoughts of the Wam.

 

Jazid

Wammer
Wammer Plus
Oct 13, 2009
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James
HiFi Trade?
  1. No
My experience of commercial tube matching is along the lines of "if you have it, sell it and claim it's matched". It is rarely worse, but just as rarely better than that. No idea who you would buy off, the only tester that I know of that tests these is the Russian L2 or 3 series (though there may well be others). Anyway that kind of price hike suggests that you ask for at least a decent set of parameters for each tube, and not some single figure Gm and emmissions data without reference to the operating point used.

Maybe, since you will be adjusting the bias and offset voltage (which will help match their various performances anyway) it would be better to get say 10 tubes from the Russian site, then you have a couple of spares to play with in case of early failure or one tube being difficult to set or prone to drift, etc?

If you do get the ones from the Russian site it would probably be worthwhile given it's an OTL to try to get the tubes tested locally for C/H leakage, insulation, and basic serviceabiity in order to lower the risk of catastrophic failure - which might then blow up your amp and/or speakers. They are reportedly pretty reliable valves so its an outside risk, but a nasty result if one or another breaks down...

 

Diapason

Wammer
Wammer
Jan 6, 2012
408
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Simon
Thanks James. My gut at the moment is to get a few extras and take the cheaper route, although I've emailed to see what's involved in the matching process and how long they burn them in for. As you say, given the price hike you'd want to be sure it's done well.

I'd say the chances of finding anyone local to reliably test them is slim, although I'll be passing them on again to my dealer's valve guru so he might have something up his sleeve.

I wish I had the balls to open up these amps and do the biasing myself, but at the moment I don't!

 

rabski

Everything in moderation
Staff member
Dec 2, 2006
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Richard
HiFi Trade?
  1. No
Much like 'auto bias' amps, if the bias setting governs a pair of tubes, then those two tubes need to be as closely matched as possible.

I have never trusted retailers' 'matched pairs', because independent testing has frequently proven they are matched in only one criteria, but not in all the others that matter.

In your shoes, I'd buy three times as many for the same price and find someone to test them so you end up with closely matched pairs. Just stick a post up here. Quite a few wammers have serious tube testers and I'm sure you'll find someone who can help.

 

Diapason

Wammer
Wammer
Jan 6, 2012
408
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Dublin
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Simon
Thanks Richard. I may do exactly that.

This sounds like one of those times when it's a pain being in Dublin rather than somewhere in the UK...

 

Jazid

Wammer
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Oct 13, 2009
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James
HiFi Trade?
  1. No
Tell you what, if you fancy ponying up for a couple of valve bases for these tubes from the old fleabay I'll rig them to my AVO and test them all for you, how about that? It's a valve base issue, nothing more. European tube testers didn't add obscure Russian valve bases to their testers for obvious reasons, but if you have them the parameters to test are straightforward enough. PM me if you want to go down that route :D

 

Diapason

Wammer
Wammer
Jan 6, 2012
408
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Dublin
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Simon
That's a very generous offer, thanks so much. I'll PM when I'm not quite so deep into my gin.

Thanks again!

 

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