Matching el84's

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jcr1234
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Matching el84's

Postby jcr1234 » Fri 10/22/10 8:39 am

When matching el84's what is the most important measurement to match? The plate voltage, the plate current, or the Gm?

Thanks,
Jon
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Postby zaphod_phil » Fri 10/22/10 10:49 am

Matching EL84s (or any other power tubes) is a complete waste of time and money with guitar amps. Power tubes that are slightly out from each other will produce warmer tones, due to the increased presence of even order harmonics. The great famous amps of the past who's sounds we try to copy, all used non-matched tubes.

I've talked to old timers who used to work for those companies, and they say they just had boxes of power tubes and they pulled the next pair or quad out of the box and plugged them into the next amp. I've also A/B tested a pair of closely matched tubes vs a random pair from the same batch, and the tone of the closely matched pair was noticeably sterile compared to the non-matched pair. It seems that Aspen Pitman popularised the idea of using close-matched tubes in guitar amps, presumably because he could make more money that way. :evil:

In hi-fi amps OTOH close matching of power tubes, PI tubes and other components, is highly recommended.
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Postby CurtissRobin » Sat 10/23/10 12:10 pm

In the 60's matching pairs/quads of tubes in audio equipment, which includes guitar and accordion amps, wasn't even discussed among builders and repairmen (they weren't called "techs" at that time; technicians were another stratum entirely). Tube matching was done for certain lab instruments for good reasons and for stereo systems in Beverly Hills houses for ego reasons. Even expensive systems were sold with randomly chosen tubes installed. As long as it sounded good in testing it was boxed and shipped. Repairmen would change both tubes of a P-P pair only if both were bad. No matching, simply pull the next tube or two out of the box on the shelf. The kind of cork sniffing we do today would have been laughable back then, yet we pursue their sound by doing everything they didn't do. Go figure. (I learned electronics in the 60's which were my teenage years and outside of music I didn't participate in the flower people's activities so I do remember).

Z_P's point about even order harmonics is one that would be well taken by all here. Use your ears, people!

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Postby jcr1234 » Sat 10/23/10 7:24 pm

Ok but what if you have a bunch of used tubes? Does it make no difference if One tube has 20 hours of use and the other has 500 hours?
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Postby Smokin-Tone » Sat 10/23/10 9:00 pm

This was true back in the 50,60's when tubes were manufactured well and the quality control was better. Most likely the variation in tubes were not that great. Today's production and quality is a total different story. I would get matched sets and ideally would check them myself. Don't worry about a 5-10 mA difference but to be way off will effect things. I have quite a bit of old glass and most of the time I can throw anything in a cathode biased amp without worries but occasionally I get a pair that are too far mismatched. It does happen.
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Postby zaphod_phil » Sat 10/23/10 10:15 pm

To me it appears that modern production tubes have improved a lot in quality. I do quite a lot of amp building for commercial customers, and don't use any matched tubes in them. Same goes for all amps built by the company I often collaborate with. They work fine, are very noise-free, and the customers rave about their tone, which I'm sure the extra warmth produced by non-matched tubes is in important ingredient of. The power tubes used are from modern production sources with an established good rep, and I've not seen any problems with this approach.
Ok but what if you have a bunch of used tubes? Does it make no difference if One tube has 20 hours of use and the other has 500 hours?
As long as they're roughly somewhere in the ballpark. You don't want them so way out that you lose hum cancellation, or one of the pair starts to red-plate. One trick I've sometimes used to "semi-match" tubes, is to run a pair in an amp and swap one of them with a few different ones to get least hum. HTH
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Postby jcr1234 » Sun 10/24/10 11:18 am

Ok, but if you are looking looking to get them in the same "ballpark", what do I look at to determine if they are in the same ballpark?

Plate current, Idle current, Micromhos?
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Postby zaphod_phil » Sun 10/24/10 12:08 pm

Your ears - that's all. The other stuff can be both misleading, and give you an amp that sounds sterile by comparison. Just pick pairs of tubes that sound great in your amp. Anything else is baloney.
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Postby hotair » Sun 10/24/10 12:34 pm

To me it appears that modern production tubes have improved a lot in quality.

They work fine, are very noise-free, and the customers rave about their tone, which I'm sure the extra warmth produced by non-matched tubes is in important ingredient of.
Maybe if production quality gets too good we will be paying a premium to get an unmatched set. :D
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Postby Brewmaster » Sun 10/24/10 5:14 pm

Maybe if production quality gets too good we will be paying a premium to get an unmatched set. :D
I can see the cork sniffers starting something with this. " Well I find in a M*** amp that when I use tubes about 6 ma off I get the best tone." Then come the flood of requests to various tube sellers for tubes 6ma apart.
Lord have mercy!!! :lol:
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Postby katopan » Sun 10/24/10 8:10 pm

The matching thing is bad enough. And GT could come up with a system for colour coding the level of unmatching to make it 'simple'. :roll: :lol:
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Postby insanecopilot » Sun 10/24/10 9:51 pm

I have a bunch of old pulls of both el84 and 6v6's. There is not a match in the whole lot of them, meaning different brands, types, black plate, smoke glass etc. Is it ok to mix and match that way too?
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Postby zaphod_phil » Mon 10/25/10 7:42 am

You're less likely to find a pair that will work together, but it is possible. Basically you will need to try out various pairs and chose for least hum, also checking in a dark room for any red-plating.

As a further check you can measure the anode (plate) voltage of each tube, which should be roughly within a few volts of each other. If you have a known good or matched pair you can take the anode voltage readings to use as a baseline for comparison with other pairs of tubes. There will always be a few volts difference due to the two sides of the OT primary having slightly different DC resistances.

Also be aware that sometimes tubes labelled with different brand names can be the same inside. I've had a 1970s NOS Brimar 6V6 and a Sovtek which ran together perfectly with only a few mA difference. It turns out that both were actually re-labelled Russian 6pi6 tubes.
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Postby vibroluxious » Mon 10/25/10 5:21 pm

Your ears - that's all. The other stuff can be both misleading, and give you an amp that sounds sterile by comparison. Just pick pairs of tubes that sound great in your amp. Anything else is baloney.
This may be the greatest truth I have ever seen on the Web.
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Postby zaphod_phil » Wed 10/27/10 7:45 pm

Well, the aim of building tube amps is to get something that sounds great to our ears. So let's pick tube pairs that make our amps sound the best.

This new thread is a great example of how you not only don't need the tubes to be matched, you can even use completely different kinds of tubes in a push-pull pair - http://www.18watt.com/modules.php?name= ... ic&t=23669 :D
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