Matched tubes general questions

fenderslash · Post by **fenderslash** » Sat Jul 14, 2007 8:05 pm

Please excuse what is possibly a couple of very dumb questions from someone who's a little clueless when it comes to the technicalities of amps...

About a year and a half ago I mail-ordered two sets of 6L6 matched pairs for my amp, with the intention of installing one pair in my amp straight away (which I did) and keeping the other pair as a spare set (which I also did). When the valves arrived it turned out that they weren't just matched pairs, but were a matched set of four.

Someday the inevitable will happen and a valve will die. My question is, when that happens is it okay to use just one of my spares from the original matched set of four as a replacement for the dead one (i.e. the amp would then have one with lots of use and the other being brand new - will they still be matched?), or do they deteriorate with use and to retain a matched pair I would have to replace both together so both of the amp's valves have the same amount of playing-time on them?

Another semi-related question, in case the answer to the above question is that it's okay to replace them one at a time...

Is the death of individual valves a random thing, or do they deteriorate at roughly the same rate (assuming matched pairs of the same brand, etc). In other words, if one dies should I replace them both anyway because the other won't be far off dying, or could the second one still possibly have years of life left in it?

admin · Post by **admin** » Mon Jul 16, 2007 2:16 am

Carl: In a perfectly symmetrical universe, equal tubes in equal sockets might be expected to deteriorate similarly. Alas, tubes and amplifiers seldom work that way. Eventually, odds are that they may take a different path of deterioration as they age.

Depending on your amplifier and your tubes and the test equipment available to you, I would simply replace the matched set once one of your power tube goes. By that time, it is likely that the remaining tube will be different from the unused matched set.

At the end of the day, you can experiment with different options and let your ear be the judge. In practical terms, I would simply save the remaining used good tube as a backup. Tubes can and do last for years. The big issue is, do you like the sound.

beatlefreak · Post by **beatlefreak** » Mon Jul 16, 2007 3:58 am

Replace them in pairs. The tube that didn't die will likely be at the end of it's life, and will have deteriorated to the point that it doesn't sound as good as when it was newer.

charlyg · Post by **charlyg** » Mon Jul 16, 2007 4:16 am

Yes, pairs because they work in a push pull amp I ordered a matched pair of 6 and there are numbers written on the side of the box. If one of my tubes went bad, I would order 2 with the numbers 29 and 4.0. I'm not sure anymore what these represent(it's been years since I worked on tube electronics). I think one of them is wattage, or bias voltage, and the other is current draw (4 seems high for that), but I absolutely wouldn't bet on it.

sharkboy · Post by **sharkboy** » Mon Jul 16, 2007 4:41 am

I'd have to go with Kris and Charly's take, but with a little more info.

If you have a cathode biased amplifier (AKA self-bias) you don't need to be as mindful. Also, because the windings in output transformers in most guitar amps are not actually symmetrical, having a truly matched output is impossible (and would actually cancel some even order harmonics.)

If your amp is fixed bias (this includes adjustable, which is the same as fixed, but uses potentiometers for bias resistors instead of regular ones), the safest bet would be to go with replacing with a pair. Usually, tubes drift a bit over time, so the tube in your amp is no longer a perfect match to the extras, but this may still have little audible effect.

Charly, my guesses at the numbers on your tubes are 29 is the bias voltage (would show up as 29 mv in my Flexi's bias circuit when using a multi meter on the probe points.) The 4.0 _may_ be the relation to the pseudo standard for matching ranges for that tube- the numbers that Groove Tubes and others use to make a standard assessment for what tubes can be replaced in an amp without re-biasing.

admin · Post by **admin** » Mon Jul 16, 2007 4:52 am

Nicely done, Mark. The added information is appreciated.

sharkboy · Post by **sharkboy** » Mon Jul 16, 2007 5:31 pm

Kind of you to say, Peter. I should have said that I agreed with you too.

It was an attempt to raise my average information:blather ratio, which I suppose isn't that hard to do.

I'll add these bits:

Despite knowing that one doesn't get a real advantage in most PP guitar circuits with extremely closely matched tubes, I am a proponent of their use, since they would tend to wander from a known, very similar state and that matched tubes (to me) seem to have a more focused, less smeared sound than disparately spec'd tubes from even the same manufacturer.

soundmasterg · Post by **soundmasterg** » Tue Jul 17, 2007 8:22 am

Fender never used matched tubes, and their amps have worked fine through the years. That said, matched tubes can give a more balanced sound and soemtimes more power too. If your amp has only 1 adjustable bias pot like many blackface fender amps, or a bias balance control like many of the silverface fenders, then matched tubes can be a benefit. In my project amps, I put in a adjustable bias pot for each power tube, so I don't need to use matched tubes to get a match. Almost none of the major amp makers out there do this however so matching can be a good idea.

When replacing power tubes, they should always be replaced in pairs or quads as opposed to just putting in 1 tube out of a pair or quad unless the failed tube is brand new and the one you plan to leave in the amp is also brand new. Even then, I'd be suspicious of the tube I was leaving in the amp if it's matched partner failed.

fenderslash · Post by **fenderslash** » Tue Jul 17, 2007 11:41 am

Thanks for all the info guys. You really are a wealth of information, and I appreciate being able to tap into it.

I originally bought the tubes for a Traynor YCV40T, which from memory is supposed to be self-biasing. So from Mark's info it wouldn't be as important to change them in pairs if I was still using that amp. I didn't like that amp though, and replaced it with a Fender Super-Sonic, which I'm happy with and continue to gig with to this day. The Fender is adjustable bias so I'll go with the advice of changing them in pairs if one dies, while keeping the old living one as a spare for emergencies.

The tubes did Traynor-duty for about ten months before I swapped them to the Fender, and have now had a total of one year and nine months use on them between the two amps. I was probably going to play it safe and replace them after three years or so anyway, thinking that it's cheap insurance to avoid the hassle of a failure while working.