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Tone Tip: Tube Amp Maintenance

Dave Hunter | 09.10.2008

Like just about anything worth owning, even the best tube amplifiers available need occasional maintenance to continue performing at their peak. In this age of low or no-maintenance consumer goods, where you’re more likely to toss your DVD player in the nearest Dumpster and swing by the local big box retailer to pick up another one for $49.95 than to actually get a small fault repaired (which, no doubt, would cost you considerably more than the new unit), the notion of routine maintenance for electronic goods has largely fallen by the wayside. Genuine all-tube guitar amps, however, even brand new ones, are not like other consumer electronics products; they are the archaic technology of a bygone era, and thanks to that they can sound sweeter than any fancy box of bits that has been conceived to replace them. As such, though, they need a regular check and tune up. Treat them right, and they’ll reward you not only with stunning tone, but flawless performance.

I have known plenty of guitarists who were very much into tube tone, but went from amp to amp with a turnaround rate that found them changing amps every couple of years or so—coincidently, about the amount of time it took for the new tubes the amp came with to grow a little tired sounding, and for a few other minor maintenance items to raise their heads. Re-tubing an amp is something you can almost always do yourself (although some fixed-bias amps will require rebiasing when output tubes are changed, and that’s a job for a professional). If you are gigging or even rehearsing regularly, output tubes are almost certain to need replacement every two years at best, and possibly even every six months or so if you are really playing a lot. Even tubes that are sonically “good” can become noisy or microphonic, and thus require replacement. Preamp tubes generally last a lot longer, but it’s worth swapping in a fresh, high-quality preamp tube in the preamp and phase inverter positions every so often—ideally after you have put in new output tubes—to see if it perks up your amp considerably. If so, you’ve got a tired preamp tube or two on your hands as well. Find the culprit by process of elimination. Be sure to follow your amp manufacturer’s owners manual’s instructions for tube replacement, and use good, properly tested tubes as your replacements.

Every few months, the tube amp user ought to also perform certain items of routine physical maintenance to keep the structure of the amp firm and rattle free. These are no-cost jobs that you can perform yourself with the average home tool kit. Check that all speaker-mount nuts or bolts are tight, and if not, adjust. Don’t wrench them down with all your might, but turn each nut or bolt until it is finger-tight (meaning you can’t turn it any more with your fingers), then give it another 3/4 to 1 1/2 turn or so with the appropriate tool. Tightening the speaker mounts “as much as you possibly can” is not the goal, and can do more harm than good, possibly warping the speaker frame (basket) and/or damaging the wooden baffle that it’s mounted to. Anything else that is bolted or screwed to this vibrating mass of wood and iron called a guitar amp will occasionally need to be tightened too: check the mounting screws or bolts holding your power transformer and output transformer to the chassis, the speaker baffle, the back panels on the cabinet, the handle, the feet/glides, and even the mounts of the chassis itself. Anything that is loose can and will rattle when you get that amp cranked up. I can’t count the times I was convinced that I had a speaker that was on its way out, only to find it was a loose handle or back panel vibrating sympathetically to the music.

One item of more invasive maintenance that will need to be performed occasionally is one that is commonly referred to as a “cap job.” The electrolytic capacitors, also called “filter caps,” that perform filtering duties in your amp’s power stage, to keep troublesome electrical ripple and other noise out of the system, are inherently short-lived components compared to just about everything else in there, other than tubes. Good filter caps should last at least 10 years, and sometimes will go as long as 20 without causing problems. You even stumble upon 40 or 50-year-old vintage amps now and then which appear to be going strong with the original filter caps. By and large, though, these parts will need replacing 15 or 20 years down the road, and if you have acquired an older amp that has never had a cap change, it’s a good piece of preventative maintenance to get one done sooner rather than later. Faulty filter capacitors will lead to an amp that is noisier and flabbier sounding in the low end in particular, and at the extreme will also introduce dissonant harmonics called “following tones” or “ghost notes,” that sound a little like a lower and out-of-tune tone that follows everything you play (kind of like a very sick octave-divider sound). The cap job is one for the professional amp tech, because these parts lurk at points in the circuit where the highest voltages are handled, and they can also store high-voltage charges and release them—into you!—even when the amp is switched off and unplugged. That said, the average cap job shouldn’t be all that expensive, and getting it done can really sweeten and firm up a tired amp.

Resistors will also drift and sometimes fail entirely in older amps, and you will occasionally need to turn to a pro to change a few of these, too. In particular, the 100k ohm carbon comp resistors in the preamp stages of vintage Fender amps (and others) are often the culprits when an amp produces crackling, hissing, and sizzling sounds, particularly while warming up. Replacing these with fresh carbon comp resistors can frequently be an easy cure for preamp noise issues. The larger resistors in the power stage also occasionally wear out from all the heat and high voltages they have to deal with. These aren’t even in the signal chain, so don’t hesitate to have them replaced when necessary. If your amp is in the shop for any of these more invasive procedures, it’s also worth having the repairman check that your tube sockets are all tight, and retension the pins if not, and take a few minutes to squirt some contact cleaner into all the pots and jack contacts too. 

This might sound like a lot of work, but is the kind of thing you’ll want to be prepared to deal with if you want to live in the tone zone. This is fact-of-life stuff for even the best tube amps out there, and you want to pony up and get it done. Chances are, even that mid-’60s amp from the golden days of tube tone that you acquired for top money, and which appears to be in unusually fine condition, will need to go through most, if not every one, of the items listed above. That doesn’t mean you were “ripped off,” and it might still be a great find and even in the “top original condition” it was advertised as being in. But it’s an old tube amp, it needs love and attention. Give it the time and the $150 or $200 it will take you to achieve all of the above, and it will astound you with the gratitude of superlative tone. In the end, that’s usually less trouble—and even less expense—than selling it off in a few years, or months, when it starts to sputter and cough and you begin the hunt for yet another great sounding amp … which in turn, will eventually sputter and cough and sound tired and need to be sold. Fix ’em up before you fire ’em off; in the end, the little time and money spent will pay dividends.