Tube and Cap Maintenance on Old Tube Amplifiers (especially Fenders)
Jan 18 '02 (Updated Jan 23 '02)
The Bottom Line If you're not regularly replacing your tubes, you may as well be playing solid state.
Here's some maintenance advice for those of you who (like me) are lugging around old Fender Tube amps.
(Also can be applied to most tube amplifiers).
You probably know that the tubes need to be replaced eventually. They get old, they wear out. You will notice that an older amp with older tubes simply may not sound very good.
Not all tubes are created equal. The consensus for replacing the tubes in an old Twin is to use the Svetlana 6L6 GC.
There really aren't any old 6L6 tubes made in America anymore. These come from Russia. Some folks say they sound as good as the old RCA 6L6s that used to come with Fender gear. Others say they're close.
The only agreement is that for new tubes, these are about the best-sounding, overall.
You hear a lot of folks talking about Groove Tubes. These are basically balanced packages of Sovtek tubes. (Sovtek is another Russian manufacturer.) All Groove does is match the tubes. (And mark them up considerably.)
Sovteks sound good... most OEMs use them. But the Svetlanas sound better (and cost just a little more.)
I believe in the ordinary Twins, we're talking about four(4) 6L6 tubes.
For the 59 Bassman reissue, we're talking about 2 x 6L6.
You also want the tubes to be matched (producing and drawing about the same amount of juice.) Something about "unmatched" tubes causing problems. (I'll leave that for some electronics wiz to explain).
So, when you order your tubes, order all 4 at once, and ask that they are "matched". Most tube re-sellers include "matching" as a service.
When re-tubing the old "super twin" and "super twin reverb" amps, you have more tubes. Ergo, you need to replace not just 4 6L6 tubes, you have to replace 6 of them. Yes, it costs 50% more. Shoot, did you think that 180 watts didn't come at a cost?
With the Supers, these are "fixed bias." This is a confusing term. What it means is that the amp won't automatically adjust the bias.
What is bias? It's basically a setting that tells the amp how much juice to expect from the tubes.
I've never biased an amp myself, and have not heard good reports from other people who have tried. This is probably best left to an amp technician.
It's important to find a technician who you trust because biasing an amp is as much art as science. Too hot a bias and the tubes burn out prematurely. Too cold a bias and your amp sounds worse than solid state. Somewhere in-between is where it needs to be, and that's a matter of taste.
(In the old days, guys just slammed in new tubes and went for it. Then, in the old days, they blew up a lot more amps, too.)
I have heard that due to the design of Super Twins and Super Twin Reverb amps, the 6 tubes do NOT have to be matched, but I've never tried this theory out.
After that...
You may find that your brand-new re-tubed amp still sounds bad. That's when you need to get it re-capped (capacitors need to be replaced.)
Unlike the tubes, this is something that requires some disassembly and a soldering iron. Paying somebody who knows what they're doing is not a bad idea. It's not rocket science, but remember, you really don't want your amp to catch fire. Plus the caps store enough electricity to kill you (literally) if you don't know what you're doing.
New tubes for a matched set of 4 Svetlanas, will set you back around $60-100. (Depends on where you buy them. Remember, you want a place that will pick out 4 tubes that roughly match.)
New caps cost about $50, plus installation.
The preamp requires 7025s. We're talking about $30 per tube.
Aggghhh! If that doesn't scare you off, you are ready to get your Twin!
As a rule of thumb, an old axe-player told me that you need to re-cap about every 25 years. You need to re-tube... shoot, I would guess every 3. Depends on a lot of factors, including whether the tubes were cheapies and how much you played them at what volume.
If you're on tour, you can probably replace them every 6 months. Somebody playing out regularly with regular rehearsal time probably needs to think in terms of once a year or thereabouts.
The folks who turn on their amp about once a week may get 3, 5 whatever.
And as if that wasn't enough: when you replace the tubes, you need to have the bias adjusted.
And the preamp tubes need to be replaced about every 2nd or 3rd time you replace the 6L6 (power) tubes.
Lotsa money, but if you're playing old tubes, you're not getting the benefit of a tube amp: you may as well be playing solid state.
You may find that after caps and tubes what had sounded like a tired old amp really zings.
Other than tubes and caps, the other stuff on your amp only needs to be replaced when it malfunctions and/or breaks. (For instance, you do not need to periodically replace resistors.)
Oh, and one last thing: if you want to get to where you clip (distort) the power tubes, you can consider using a lower output tube for your preamp gain stages. Consult an amp tech for which ones work, but basically, the lower output preamp tubes mean your power tubes have to work harder.
The harder those power tubes work, the sooner you get to "tube distortion."
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