Sound Advice: You Found a Cool Old Amp – Now What?

Rooting around in an antique shop mostly filled with overpriced dust collectors accumulated since Henry Ford was a baby, I spied something sticking out from under a tablecloth. It looked like the corner of an amplifier. Lifting back the shroud, there it was: An early-1970s Fender Champ. It wasn’t the most vintage of vintage, and it was pretty well covered in dirt. Its grille cloth was a bit chewed up, but otherwise it looked okay, if neglected. They only wanted $45 for it, which made it irresistible. So I bought it, took it home, and cleaned it up. I made sure all the tubes were in place, and plugged in my guitar and turned it on.

The pilot light glowed. Encouraged, I turned up the volume control and hit a chord, and out came a whole lot of nothing. Shutting it off, I wondered what could be wrong. I unplugged the Champ and forgot about it for a few days. Then I thought, maybe the tubes are shot. I went online and ordered replacements. I was now in for another 60 bucks or so, but didn’t feel badly about it, since the amp was nearly 40 years old, and it might have seen a lot of use and abuse before being unceremoniously stashed among heaps of doilies and fancy mismatched dinnerware.

When the new tubes arrived, I wiggled out the old ones, popped in the fresh ones, and got ready to rock (as much as you can with such a tiny amp). This time, it came to life. As I turned up the volume, I heard a lot of buzzing. Examining the amp, I noticed that the speaker cone had a good-sized gash in it. How I didn’t see that before, who knows? Maybe I was blinded by cheapness. Unlike that great “You Really Got Me” Kinks tone that came from an intentionally slashed speaker cone, this just sounded like a crummy, uninspiring rattle with each note. So I ordered a new speaker (I could have had this one fixed, but thought I’d just go crazy). I was in even deeper, to the tune of an additional 40 bucks.

I chucked the old speaker (dumb idea—it might have been repairable) and installed the new one. Power on, and it sounded great. The wee beast was resuscitated.

As I tallied up what I’d spent, I felt a little like the guy in the old parable about stone soup, where he starts with a pebble and some water and keeps adding vegetables, herbs, and some chicken and ends up with…stone soup. But you don’t slurp an amp. I was happy with my little pal.

Re-Cap Conundrum

A few months after getting my antique-store special up and running, I started reading about “re-capping,” the practice of putting new electrolytic capacitors (caps) into old amps, especially in the power section, where they filter line noise out so that your amp doesn’t sound like a beehive or a defective neon “Open” sign. What are these electrolytic capacitors? They look like cylinders, with two leads coming out of them – sometimes one at each end, sometimes both located on the bottom. Why should caps be replaced? Because aged electrolytic caps tend to degrade over time and can cause trouble. At best, leaky or otherwise elderly caps could screw with your amp’s performance. At worst, they could allow dangerous voltages to get to where you don’t want them, including in you. That means both you and your amp could end up equally dead…not an appealing idea.

Photo by Rob Hull

If you are either considering an old amp or already own one, here’s what you should know: You should not even consider re-capping as a do-it-yourself project unless you absolutely, positively know what you’re doing. And even then, it’s usually best to get an experienced pro involved. This is because electrolytic capacitors can hold a deadly charge for a very long time. In this regard, they’re like batteries. Some of the caps can be honking-big and hold similarly formidable charges, some of which can be lethal (hundreds of volts), or at least cause serious burns. Those big fellas in the power supply section can electrocute you. Unless you know how to tell if a capacitor is good or bad, and can read a schematic, perform repairs, and determine if a cap is a symptom or a cause of problems, you aren’t qualified to do the job.

So here’s a special note to do-it-yourselfers who want to replace their own capacitors: While it’s true that capacitors holding a charge eventually discharge by themselves over time, it’s not worth taking a chance. The size, condition, and voltage fed into a capacitor determines how long self-discharging takes (months? years?).You must know how to properly discharge a capacitor, and follow the discharge procedure to the letter in order to ensure your safety.

Note: I read this detailed description of the process of draining electrolytic caps. After my heart slowed down, I decided I’d never, ever attempt it – and I’m pretty comfortable working on electronic devices.

The coupling capacitors between stages of an amp might also be candidates for replacement (if you can’t read a schematic, skip all of this, but mention it to the tech you take your amplifier to). These don’t usually look like the electrolytic capacitors in the power supply section. In old, old, old amps, these film-type capacitors come in a variety of colors. Common colors are orange (Sprague “orange drops”), yellow (Astron “yellows”), brown (“chocolate drops”), blue (ITW “Paktron” and “blue drops”), red (Astron “reds”), white (Mallory 150), black (Sprague “Black Beauties”), etc.  The two style options often encountered have either the two connection wires exiting the components via opposite ends (axial leads, such as the Mallory 150′s) or from the same end (radial leads such as the Sprague “orange drops”). Luckily, suitable replacements for all of these caps are available from a variety of online sources (for example, Mojo Musical Supply, Antique Electronics Supply, TubeDepot.com, Triode Electronics, The Tube Store, etc.).

Here’s something you might not have considered, but should: If your amp is old, it may be collectible. If so, replacing any parts may put a dent in its salesworthiness, especially if the replacements are done in a less-than-perfect way. If you’ve ever watched American Pickers on TV, you’ll see plenty of cases where they’re excited to find something like a vintage Harley, pay a bushel of bucks, and then find that some of the parts are newer replacements…dragging the price into the abyss.

Bottom line: If the amp is more than 20 years old, it probably needs new caps, but have it checked out by an amp tech. Mere age doesn’t always mean that all of the caps require replacement, but safety’s a good thing. And like old sports cars, amps need a bit of TLC and maintenance to keep them running in their old age. Chances are, your amp would be, at best, dangerous and, at worst, a non-functional trophy without new electrolytic capacitors. So if you intend to play through an old amp, get the work done. If you have an old Fender from the 1950s, or an earlier amp like a Gibson from the 1930s or 1940s (or something otherwise rare, vintage, and/or worth a heap of cash), spend the money to have a pro do any work inside of it. And keep documentation of the work and save all the original replaced components in a zip-lock bag for the next owner, should you ever decide to part with the amp. These actions will improve, or at least, preserve the value of the amp.

Professionals know this stuff, but here are some guidelines when replacing electrolytic capacitors (and for that matter other components):

Know how to solder and desolder. Seriously. Monkeying around inside an otherwise good amp is not the way to learn how to solder. A bad solder joint can mess with the performance of your amp, and perhaps be dangerous. Also, when you desolder an old part, you can cause damage by overheating a component or, on newer amps with printed circuit boards, breaking a trace (the coppery lines on the PC board), causing it to lift off with the part.

Avoid buying surplus. Electronics surplus places often have extraordinary deals. The problem is, you don’t know how old those parts are or why they’re surplus (and in many cases, the people working there don’t, either). If you buy unused electrolytic capacitors that have been sitting on a shelf for years and years, they may be degraded by age. Here’s why: They have liquid inside that permeates paper, and because most aren’t airtight, eventually the liquid evaporates, even if no voltage is applied, rendering them worthless. Spend the extra bucks and get good capacitors from a reputable electronics supply house.

Match the voltage rating. Always buy capacitors with the same voltage rating as the original, or higher. If a capacitor is rated at 120 volts (it’s marked on the side of the cap), then the new one must have a 120V rating. It’s okay to go higher, but never lower, as you can endanger your amp and yourself.

Match the polarity. When replacing an old electrolytic capacitor, pay attention to the polarity when you insert the new one. Make sure the lead labeled “+” on the new one goes where the “+” where the old one was. In addition, if you replace an electrolytic cap and get its polarity wrong, that cap can explode when voltage is applied.

Check and recheck your work. Every part on a commercial plane is typically working perfectly and is well maintained. This doesn’t mean a pilot doesn’t go through a regimented checklist before every flight. Murphy’s Law never takes a vacation, so when you’re done replacing parts, make sure you check everything you did – and not just the capacitor replacement. If you removed a nut or screw, moved another wire for access, etc., be certain you put them back where they belong. And, as stupid as this may sound, make sure you didn’t inadvertently put back an old capacitor that you thought was a new one. (Never work inside an amp when you’re tired, distracted, or in a hurry.)

Be a good citizen. Dispose of the old capacitors and any other electronic parts with e-waste. Don’t toss them into landfill garbage. Many communities have e-waste disposal sites, or you may have a bin at work. Plus, in some countries it is illegal to throw electronic components into the trash.

But Wait – That’s Not All!

Capacitors are important, but when you’re sizing up an old amp – and I hope you do it before you buy it, and especially before you turn it on – inspect it carefully. If there are no big problems, the small problems (usually cosmetic) can be dealt with. Hopefully, they won’t cost you an arm and a leg to remedy. And even if you aren’t up to the task of changing electronic components, there’s still plenty you can do.

A Good Cleaning

Old often equals dusty and dirty. Get out the vacuum cleaner, put on a soft brush, and suck as much dirt and debris as you can out of any nooks and crannies. If it’s an open combo amp, where you can see the speaker, be extremely careful not to touch the speaker’s cone. It might be brittle enough to be damaged without much effort. Avoid using canned, compressed air inside an amp, as you can sometimes force debris deeper into places. Wearing a dust mask is a good idea, especially if you have asthma or allergies and if the amp is old or has been sitting in a basement, attic, or garage for a while. Besides dust, there can be mold spores or other allergens.

Photo by Rob Hull

Where There’s Smoke…

Sniff around and check for any burned smell before you plug the amp into a power source and turn on the amp. Then, if you see or smell smoke or something seems very, very wrong, turn off the amp immediately, and then unplug it. Don’t mess around. Open a window, let the amp cool, get away from the fumes, and allow the room to de-smellify. Then take the amp to someone who works on amplifiers. In fact, set your bar low for any amp-related problems. If you’re a guitarist or bassist, leave the weird electronic voodoo to someone who knows how to properly troubleshoot and remedy problems, and spend a little more time practicing.

If the amp has a fuse, and it blows when you turn it on or soon afterwards, replace the fuse and try again. Never replace a blown fuse with a fuse that has a higher current rating.  Also, never assume that the fuse value installed in the amp is the correct value (the last owner may have thrown in a wrong-value substitute).  Always install the value specified by the amp (usually marked near the fuse holder) and/or amp manufacturer. If the replacement fuse blows, quit. When a fuse blows, it’s telling you something’s wrong in the amp. One time can be a fluke, but twice tells you to get the amp to someone who knows how to troubleshoot and repair amps.

Cabinet

Amps are typically housed in a wooden enclosure, covered with tweed, Tolex, carpeting, or other material. Some are bare wood with a finish. In all cases, you should check that the cabinet is undamaged. If the joints or bracing are loose, they may need regluing. That’s pretty simple and requires some carpenter’s glue and perhaps a clamp or two. If a joint or brace is extremely loose and a part is warped, then you’ve got other problems. Check to see if there are signs of water damage. Telltale streaking or other discoloration from the bottom up are a good tip-off, as are rusty corners, screws, etc. Like so-called “Hurricane Katrina cars,” the amp might have been in a flood at one point. If you suspect such a thing, it’s probably worth looking inside the chassis (if it’s a combo) or inside the cabinet (if it’s a sealed cabinet), just to look for signs of corrosion, rust, and even mold.

If an amp is covered in Tolex (the rubber-like faux leather material that’s been used since the 1960s, often in black), you can use water and a rag to clean the surface. You can even use a mild dishwashing liquid like Dawn, which has a minute portion of alcohol that helps to cut through grime. I don’t have to tell you to make sure the amp is unplugged. Oh. I just did. Be careful not to get any water on the grille cloth, which can cause weird shrinkage or stretching.

A tweed-covered amp provides many challenges. Some tweed is glued on, some is lacquered, and some is unlacquered. Manufacturing techniques with tweed depend on the manufacturer and when the amp was made. Before you decide to clean your amp’s tweed, remember that old tweed has its own funk that comes from age, so don’t think of it so much as grime as patina, unless it looks as if it had been pulled from a swamp. Among suggestions I’ve heard for cleaning tweed are to use Woolite that’s extremely diluted (4 to 1 or greater). Dunk a rag in it, wring out most of the moisture, and then gently wipe the tweed. This kind of activity should make you nervous, especially if it’s a collectible amp. So always test on an inconspicuous place, such as on the back somewhere. After you wipe with the wet cloth, dab with a dry one, and then let the amp air-dry. If you’re just dealing with a spot (like a mud splash), don’t clean the whole thing. The big picture here is to just attack what really needs it. And don’t use solvents such as naphtha or aerosol carpet cleaner. Most aren’t good for you, and some can eat the finish or labeling right off the front panel, should any get on it.

Tubes

Tubes aren’t cheap, but they’re expendable and sooner or later need to be replaced. One obvious problem that crops up with old or damaged tubes is that they become microphonic. As the name implies, microphonic means they bring in a sound. Warm up your amp, crank the volume, and then tap each tube with a pencil eraser. Be very careful, because when an amp is on, that’s when it’s the most dangerous: electricity is flowing, sometimes through barely protected areas. It is normal for a preamp tubes to amplify a short, high-frequency “thump” when tapped.  However, if the tapping causes feedback, sustained ringing, or crackling sounds, the tube is microphonic, and it should be replaced. If you have some thick cloth gloves (vinyl isn’t a good idea: it melts), try gently wiggling the tubes. Be careful; they can be really hot. Again, if you hear ringing or feedback, yep, they’re microphonic. It’s most likely that a microphonic tube will be a preamp tube (one of the smaller ones). Out with the old, in with the new. Just make sure you let the amp cool down before you remove a tube.

With the amp unplugged and cooled off, examine the tubes to make sure they’re in tight and straight. Never twist them! Gently grasp the tube and wiggle it into place. Because some of the tubes are inside of metal sleeves, you will have to remove the sleeves to check them for microphonics. Grasp the sleeve with your fingers and depress it (it’s spring-loaded) and turn to the left (counterclockwise). Now pull it off; this may require a little wiggling action.

Problems with power tubes often show up when you turn on the amp and the fuse blows or if one of the big output tubes glows cherry red. In either case, turn off the amp. Allow it to cool and replace an obviously bad tube – the one glowing cherry red.

Make sure that a tube is oriented correctly when you install it. If you look at the end of the preamp tube and the socket, you’ll notice that the pins are arranged in an incomplete circle. Always make sure the pins are aligned correctly. Never force a tube into its socket or twist it. Never install a power tube with a broken guide pin. And if a tube had a metal cover sleeve, remember to put it back on after you check or replace a tube.

While you’re checking the tubes, check that the sockets aren’t damaged or corroded. Sometimes tube sockets and tube pins get dirty and need a little cleaning. Isopropyl alcohol, acetone, and non-residue spray type contact cleaners are all good choices. It is important that the cleaner used is a non-residue type, and that you always use them in a well-ventilated place. “Contact enhancement”-type cleaners (such as Deoxit) are discouraged. And be sure that chemical cleaners are not allowed to splatter on neighboring surfaces. With the amp turned off and unplugged, give the inside of the socket a spritz, and allow it to dry completely before you slip the tube back in. Do the same for the tube pins. If the places where the tubes’ pins mate are bent, straighten them out if you can. And if the socket is really in dismal condition, it might be time to replace the socket. Remember, going into the chassis you risking lethal voltages. There tends to be a lot of wires connected to each socket, and overheating with a soldering iron can sometimes wreak havoc on the insulating part of the socket or to other components soldered to the sockets. Yeah, this is another job for the pros.

Scratchy Pots, Switches and Jacks

The pots (short for potentiometers) on old amps often have open cases, which means they have probably allowed in some dust, grime, and cigarette smoke – some of the stickiest stuff ever, which binds to dust to make a real mess to get inside. Even if the pot looks pretty well sealed, over time some crud can get in and settle between the wiper and resistive element, giving rise to crackly, scratchy sound when you turn the knob up or down.

Bear with me here. Sometimes guitars and amps are authenticated by checking info stamped into the pots (check out zeus.lunarpages.com for some info). Sometimes age can be determined, as well as the original manufacturer of the component. This is esoterica here, folks. Fender used codes stamped inside the amps (check out www.unclesamsjamms.com and do a Google search for other sites that cover amp dating via pots); other manufacturers have used other schemes. You shouldn’t be too hasty to replace a pot unless it’s really a scratch fest every time you even breathe on it and the control cannot be properly cleaned.

It’s possible that just turning the knob quickly back and forth a little more aggressively than usual might clear it up. Then again, it might not.  Scratchy switches, on the other hand, often need replacement if they cannot be cleaned. Try toggling a troublesome switch several times. But this may do no good, especially if the switch was a cheap one to begin with, or if the amp had been stored in a humid environment such as a basement.

You can use lubricated cleaners such as Deoxit or other high-quality contact “enhancements” for pots and switches. Never, ever use oil, grease, or graphite powder (the stuff that’s sometimes used in locks) in a pot or switch. In a pinch, you could try alcohol or WD-40, but it’s best to use something designed specially for pots.

There are also cases where a pot can be opened, cleaned, and rebuilt. With some old CTS pots, if the resistive element hasn’t deteriorated and is still within specs, then some newer CTS pots’ components can be substituted. Esoteric enough fer ya? If you have an amp worth a crazy amount of money, this might be the way to go. Otherwise, an equivalent, modern replacement may do just fine.

The input jack on some amps can also be a source of trouble. For instance, some jacks have a small, exposed switch contact that can oxidize and become intermittent. Cleaning the switch contacts with extremely fine emery paper or thin cardboard soaked with lubricated cleaner can remove the oxidation, but sometimes it’s best to just replace the jack.

Inexplicable Noises

Sometimes, an amp just makes weird noises for no reason. If that amp is very old and has spent a lot of its life by the seashore, it’s possible that the fiber board on which the components live has absorbed moisture and/or minerals or salts. This is tough to check, but because it has gone from non-conductive to partially conductive, it can allow some current leakage where you don’t want it. This calls for the pros to figure out, because it requires specialized equipment such as a megohm meter (one that applies power and checks resistance). In the very worst of cases, it might require taking all the parts off the board, replacing the board, and then putting them all back. Sound unsavory? Luckily, it doesn’t happen very often.

That Big Ol’ Output Transformer

The output transformer is the last stop between the amp’s output stage and the speaker. So you might say it’s critical. A dead transformer is just that: Dead. Unless something drastic happened (running the amp without a proper speaker load, for instance), and if it was reasonably well designed, it shouldn’t cause you concern or any trouble. Some amp manufacturers used to cut corners (and cost and weight) by using cheaper, less robust transformers. These tend to be inefficient, run hot, and in some cases muck with your tone. If you’re concerned that the transformer is in poor condition, take your amp to a pro. The output transformer is an expensive part, among the costliest in the whole amp, and doing anything with it requires a tremendous amount of care. Sometimes old transformers can be rebuilt, which might be an option (a really expensive one) that you want to pursue if you want to keep an old amp as close to vintage as possible. Here’s a good article on the topic from Premier Guitar: Why and How to Replace Your Amplifier’s Output Transformer

A final word about output transformers: If a filter capacitor or power tube in the power section shorts out, it’s possible that it will cause the transformer to be hit with excessive amounts of current (which is one of the reasons to use only properly rated fuses). This high over-current condition translates to a bunch of heat, which can ruin the transformer’s insulation, causing short-circuits and when this happens, you may smell something pretty evil. Immediately turn off the amp and get it checked.

Old Wires

Several decades ago, wires were insulated with cloth, usually braided and wrapped around the conductors. Today, wires are insulated with plastic-like material that’s flame-resistant and designed to handle heat and resist deterioration. Cloth is a different story. Make sure insulation on wires in an old amp aren’t deteriorating. If the amp was stashed in an attic or garage for a long time, bugs could have munched the cloth, or some might have deteriorated on its own, leaving wires exposed and able to short out if they were to contact other wires or the chassis.

Don’t forget to check the power cord. It’s easily overlooked, but if it’s damaged, it can be dangerous. If the prongs are damaged, or if the ground prong has been broken off (or if the cord never had a ground plug), absolutely replace the power cord.  This is a safety matter you shouldn’t ignore.

Speakers

A damaged speaker usually sounds damaged, or at least a bit off. My little ol’ Champ was just pathetic when it had that demolished speaker. I’m betting someone tossed a bunch of tools or pedals or something sharp into the back of it, equating that open space with “storage.” If you have an open-back amp, grab a flashlight and look for signs of shredding or cracks. If the grille comes off the front, carefully remove it and take a look at the speaker’s front. Some speakers made in the 1970s had a chrome dome in the middle of the speaker, which looked cool but often came loose over time, creating a weird whistly rattle. I have a Sunn cabinet from 1972 that needed its speakers replaced years ago because of that very thing. Look for signs of deterioration around the rim of the speaker cone where it meets the frame. Cracks are your best indicator of deterioration.

Test your amp at moderate volume by playing notes in different ranges and letting them sustain. Smack some chords too. I’ve found that playing an open low E or A string and detuning it slowly as the note sustains is a good way to ferret out rattling or buzz. Just listen carefully for any rattling, fizziness, or jingling.

And before you go yanking out your speaker, do a couple of things first. Check that the wires are tightly connected. Sometimes manufacturers use those spade lugs on the ends of the wires, which just slip onto the speaker’s lugs. These can become loose, or their wires can rattle around. Also, if the wires are soldered to the speaker’s connectors, check that the wire isn’t broken (I’ve seen this – the wire appears to be soldered, but somehow the wire got snapped, even though it seems to run to the speaker). Check the nuts around the speaker’s perimeter too. If they’re loose, tighten them – carefully and not too much. Tighten them like lug nuts on a car’s wheels: Tighten one, then the one opposite it, and then another followed by the one opposite it, etc. The key is to go easy. If you tighten one nut too tight, you can distort the physical shape of the speaker frame and shorten the life of the speaker. And be careful with any tools you use around the speaker. As you’re trying to do something good, one slip of your wrench or screwdriver can zip right through the speaker’s cone.

If you do replace a speaker, make sure the impedance is the same (8-ohm for 8-ohm, etc.). Never install a speaker with a lower impedance, because it’s an easy way to fry the amp. As for maintaining the amp’s vintageness, you can have the speaker rebuilt or repaired. If you do replace the speaker with a new one, keep the old one around. Some collector (maybe the one buying your amp someday) will want it.

Wrap It Up

Don’t be afraid of old amps, despite all the things that can need repairs, replacement, etc. You won’t find a new 1950s Fender Bassman or a 1965 Marshall, but you can find old ones. Vintage amps are vintage for a reason. Some are rare, some are just desirable, and some are the sonic heart and soul of classic music. What you do to or for an amp will largely be based on whether you want the amp to be playable or just a collector’s item. Either way, do it safely – especially if you intend to play it. Otherwise, clear a place for your funky old prize, and tell your awestruck friends they can look…but not touch.

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Special thanks to Paul Rivera, Sr., and Rob Hull for their invaluable assistance in this article.