Thoughts, Facts, Myths, and Opinions on Cap Jobs in Vintage Tube Amps

We have all been there. Your amp starts to lose its luster, maybe the bass has gotten way too loose and flabby sounding, maybe there’s excessive hum, maybe there are ghost notes when you play in the higher register. You’re thinking it’s probably just bad tubes. So you take it in, and the tech tells you that you need a “cap job.” 

Then come the questions. “CAP JOB?!?” What is THAT? Why do I even need it done? Is my tech just trying to scam me?

The direct answer is “No.” We are not perpetuating a scam. Cap jobs are not fun, quick, easy, or (in most cases), enjoyable. Living as I do on the other side of those questions, I’d like to delve into this topic a bit and hopefully provide some answers to the above questions for you. I’ll also explain the science behind my answers, and I will try to keep it simple and eliminate as much technical jargon as I can for the non-techs out there. That said, a basic understanding of what’s going on with these components is necessary to fully grasp what’s happening and to make an informed decision about what you want us to do. 

What They Are:

When a tech tells you that you need your capacitors replaced, he or she is usually referring to a special TYPE of cap in your amp: the electrolytic caps. 

Caps come in all shapes, sizes, and methods of construction. Some can last (theoretically) indefinitely, and some have a much shorter lifespan. They are almost always referred to by type, meaning the material used in its construction.

In essence, a capacitor is nothing more than two electrical conductors separated by a non-conducting insulator in the middle. This insulator is what gives a particular capacitor its name. “Polypropylene” caps use a polypropylene insulator. “Mica” caps use mica insulators. Ceramic caps? You guessed it. 

Electrolytic capacitors are a bit different. They use a liquid paste (typically applied to a thin layer of paper) as the insulator. Note that the insulation material is liquid. That’s extremely important. Electrolytic capacitors are NOT hermetically sealed; they aren’t air-tight. Thus, over time, that paste dries out, and when that happens, all sorts of bad things can happen: it can drift off spec, it can physically leak its innards all over the circuit board (or chassis), and, in some positions, it can explode. Yes. EXPLODE. I’ve had to clean up the mess left by an exploded cap many, many times, and it’s never pretty. Or fun. Or cheap. 

The obvious question arises: Why even use this type of capacitor when it will dry up and fail over time (and there are other cap types that won’t)? The answer is relatively simple: electrolytic capacitors can pack more capacitance and much higher voltage handling capability into a smaller package than any other cap type. If your filter caps were to be made of, say, polypropylene, they’d be as big as your fist...at least!!!

So how often should your caps be replaced? That the million-dollar question, and every tech seems to have slightly different metrics to answer it, but I usually advise a cap job on any amp over 15-20 years old. 

What They Do:

Most of the capacitors in your amp will be doing one of three basic jobs (and sometimes a single cap can be doing a combination of these jobs). I’ll break them down by job description, and tell you what happens when they drift, leak, or fail outright. 

RESERVOIR (FILTER) CAPS:

These VERY important caps take the rectified A.C. (alternating current) from your amp’s power transformer and store it, releasing it gradually as true D.C., (direct current), which various parts of your amp need to operate. They also provide the backup reserves your amp needs to tighten up those low notes and chugging power chords (more capacitance here = a “stiffer” power section - to a point). There are filter caps on both the high voltage and low voltage lines. These caps see a LOT of current and action; they work very hard for you.  When they dry out and begin to fail, a few things can begin to happen: number one, since they’re no longer fully functional, they’re not completely removing the A.C. from the D.C. line anymore. Now that there’s A.C. where it shouldn’t be, the amp hums. It’s gradual at first, but as the caps slowly wear out the hum becomes more and more pronounced. Secondly, as the caps dry out, their capacitance destabilizes, and this can lead to loose, flabby, or even non-existent bass response, as there’s no longer any power left in the amp to support these low notes. Finally, they can physically leak or explode, causing a huge mess (more on that shortly).

It should also be pointed out that due to the extreme voltages and current levels at play with these caps (not to mention the extreme heat that tube amps generate), they are the capacitors that tend to wear out the fastest, and they can literally explode when dried out enough to short internally. This leads to a big, audible KABOOM and usually a foul smell. In extreme cases, these epic failures can also irreparably damage your power or output transformers, thus causing hundreds of dollars in damage. I don’t mean to sound like an alarmist; explosions don’t happen all the time and not all filter caps fail this way, but I’ve had to clean up their messes way more than once. 

D.C. BLOCKING (COUPLING CAPS):

All healthy capacitors block D.C. whilst allowing A.C. to pass through. In all tube amps, there are dedicated caps that remove the D.C. from your signal in between amplification stages. In tube amps, these are usually polypropylene or polyester caps that we generally don’t need to worry much about. In solid state amps, these are often electrolytics that do need to be replaced over time. These caps can cause problems when they fail by allowing D.C. to flood the next gain stage, which can mess with the operation of the next tube (or transistor) down the line. Unlike filter caps, however, they rarely explode or cause excessive damage to other components. They should simply be checked routinely and replaced when necessary. 

SETTING THE FREQUENCY RESPONSE:

Another unique trait of capacitors is that they can also block out bass frequencies depending on their capacitance rating (and the surrounding circuitry). The rule of thumb is that the more capacitance they have, the more bass they let through. Thus, for example, they can be used in tandem with other components to tailor the corner frequencies of your tone controls. They’re also frequently used on a tube’s cathode to provide a hefty boost in signal, and at the same time emphasize certain frequencies IN that boost depending on their value. In tube amps, these frequency shaping caps are often the coupling caps mentioned above and the cathode bypass caps (which are almost always electrolytics). When it comes to tone shaping, as these caps fail they tend to go UP in capacitance, letting more bass through than the amp was designed for. Since the amp is now amplifying more lows than it was designed to handle, it can lead to a perceived loss of power and a “muddy” tone. As with the D.C. blocking caps mentioned above, when these caps fail they’re rarely dangerous or harmful to any other components, but they will (in most instances) make your amp sound less than its best.

The way I view it, replacing the filter caps makes the amp more “alive” and punchy again whilst lowering the noise floor which is like restoring the amp’s heart. Replacing any faulty D.C. blocking caps and the cathode bypass caps throughout the preamp is what gives it back its tone and soul. 

WHY GO HALFWAY???

I’ve also seen some techs replace ONLY the filter caps that leak physically or test badly, leaving all of the remaining filter caps, cathode bypass caps, etc. in the amp. This has NEVER made sense to me. The remaining vintage caps are just as dried out and old, and they’ll also fail eventually just like their siblings did. When I refer to a cap job, I mean ALL electrolytic caps in the amplifier. Bar none. Even if they “test well,” they’re old, dried out and not at peak performance. It’s YOUR amp, so it’s YOUR decision. My two cents? Leave them in at your own risk. 

THE REFORMATION:

You’ll also hear some techs talk of “reforming” old caps. This is a process wherein old capacitors (and/or caps that haven’t been used for a while) are brought up to working voltage over a matter of hours using a Variac. As the caps are gradually brought up, they’re allowed to slowly re-acclimate to their in-circuit conditions. This is the safest way to “tune up” old caps in general if you MUST do it. In my opinion, however, this is a temporary fix at best because it can’t re-hydrate a dried out old cap; that simply can’t be fixed.

SO...SHOULD YOU OR SHOULDN’T YOU?

After all of that explanation, what do the pros and cons of a cap job boil down to? The cons are that some collectors of older and vintage amps want NOTHING changed; they want their museum pieces to remain exactly as they left the factory. They don’t even want the dust blown off of it.  As far as that goes, I understand and respect that ... if you’re just going to stare at it. 

The pros of a cap job are better stability and the restoration of the amplifier’s original sound. It’s also a safety issue (for the amp). When filter caps short or explode, they can do a LOT of damage internally. 

If I’m going to be totally honest, I must point out that some original caps seem to fair far better over time than others, but again that depends a lot on environment and the quality of the caps’ original construction. There are some old Mallory can caps in Fender Champs, Vibro-Champs, Princetons and the like that are still going strong after 40-50 years. Still, they ARE drying out and I advocate replacing them whenever I work on them, ESPECIALLY for touring clients who don’t have their own touring techs! 

When you are presented with a cap job, you really have to ask yourself what you want from the amp. Do you want a museum piece that should remain EXACTLY as it was when it left the factory? Is it a work of art that you plan to preserve and cherish? Okay then. A cap job most definitely is NOT for you. However, if you’re going to actually play, gig, record, and enjoy the amp as intended, then a cap job every 15-20 years is mandatory maintenance. It’s like changing your car’s spark plugs or tires, but far less often than those need changing. 

In point of fact, whenever I work on an older amp and the client refuses a cap job, they have to sign a waiver stating that I’m not responsible for any damages the old caps might cause down the line. Those old caps really make it hard for me to guarantee my work as I would normally do. To that end, when presented with the facts (and the waiver), ALL of my clients have opted for the cap job (I also bag up the old, dead vintage caps and return them with the repaired amplifier to the client. Collectors can still say they have the original parts if and when they eventually sell the amp - but most have told me to just me toss the parts in the trash where they belong). Almost all of these clients have called, emailed, or texted me later to tell me how much better their amps sounded after this routine maintenance. 

Here’s my parting thought: when you consider the cost of the cap job - factoring in the restored tone and the almost two decades those caps should last you - it really isn’t that much of a financial investment, and you’re not permanently altering anything in the vast majority of cases. So why not? 

Are you a collector or a player? The choice is yours‼!

Lane Sparber is an amp / guitar / bass / pedal tech who lives and works in Fresh Meadows, Queens, New York. He started repairing gear in his mid-teens, and he’s 46 now, so you do the math. When not causing mayhem in his workshop, he can be found desperately trying to hide his latest orchid purchases from the rest of his family. You can find him on Instagram at @amptech74 or on Facebook at www.facebook.com/lane.sparber. Come say “Hello!”

Related Posts: