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Matched Power Tube, but different usable/age between the two


jjang1993

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Hi Everyone,

 

A while back, I bought a quartet of GT-6L6GEs. I put two in my Fender Vibrolux clone, and kept the other two as backups. I noticed a couple weeks ago that one of the backups had a loose getter ring. Today, I noticed that one of the 6L6s developed a rattling noise, so I took it out of the amp and the guide pin got separated. I put in the backup tube to replace it, everything sounds fine, and I checked the bias on both of them and they're within 5ma of each other, which isn't surprising seeing that they were both from the same quartet. My question is, is there any danger to running an 6-month old 6L6 and a brand new 6L6 if they're technically matched?

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It is like putting a used tire on a car that has new.

Its fine for now but its not something I'd normally recommend for the long term, The weaker of the two is likely to wear out before the other.

 

Bias alone is important in making the amp safe to operate but if the tubes come from two different manufacturers, the two halves of the sine wave can wind up being Asymmetrical.

 

Its like using two different spark plugs. If one produces a bigger spark and ignites the fuel better, your engine wont run as smooth as having matched plugs producing the same sparks.

 

The other thing is, tubes are good for so many hours use. Unless you are the original owner and know how many hours are on it, you simply wont know how long the used one will last. The tubes in a combo usually die from vibrations, so its likely they will exhibit microphonics before the tube become weak.

 

In wouldn't sweat it unless you hear a tonal difference. Seeing your one tube became microphonic I suspect the other will follow in short order. Its all a matter of manufacturing tolerances and quality control. Normal fatigue vs Mechanical durability can be very different however.

 

If your tubes died because they're maximum hours of use have been reached, I would definitely advise replacing them as a pair. A mechanical/microphonic failure is a matter of metal fatigue. Unless you know the welding tolerances failures can be highly unpredictable between one manufacturer and another.

 

Many of the tubes being made today are pretty good when new but the material sources and factories aren't original. GE's were never a favorite tube of mine simply because I saw so many fail. They were OK for Hi Fi where there were low vibration but they were not a musical amplifier tube. Out of the vintage tubes back in the day, they rated at the bottom of my list mainly because of microphonics. Many new tubes being made today highly superior when it comes to durability.

 

If they were RCA, Mullard, or one of the other most favored audio tube makers they had more than double the lifespan and a much better audio quality during that lifespan. Of course you only learn this stuff by being a tech and doing tons of repairs. In comparison an amp owner will only replace tubes so many times during the life of an amp .

 

When you see a couple of dozen tube amps per week needing repair you quickly learn which tubes last and which don't. Customers tell you too. When you quiz them for information, ask how long its been since the last replacement, how many hours they use the gear, then install a different brand and get good reports back from them over a similar period of time its pretty easy to tell who made the best components. I say made as in past tense because there are no new vintage audio tubes around any more. The best you may find are a few singles being sold by reputable sellers at ridiculous prices. Even then, you cant determine the hours on a tube and pirating tubes with fake labels and boxes in rampant because of those high prices.

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6L6's generally have a service life of about 1000 hours (http://andrewsamplab.com/blog/?p=129). With, say, 6 hours of use per week that's about three years, with 12 hours per week about a year and a half, and with 18 or 19 hours per week about a year. Depending on how many hours you used your old tube it may be half shot already or it may be fine. Only you can answer that.

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Thanks for the input guys. Normally I would have stuck the other new 6L6 pair in there, but one of the tubes has a loose getter ring rolling around freely inside. This was a warranty replacement set I got from Groove Tubes when two of their 6L6s were microphonic and sounded like a bongo drum through the speaker if you tapped on them. Despite my frustation with Groove Tubes, they're really good with their warranty. I've replaced the Groove Tube 6L6GEs with Tung-Sol 6L6GC-STRs and there is no rattle or microphonics. I have another set of Tung-Sols coming in the mail. I also really like their 12AX7, has the brightness and clarity minus the microphonics and rattling/ringing you get with Groove Tube 12AX7Cs (basically Sino/Shuguang 12AX7As).

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