Premier Guitar features affiliate links to help support our content. We may earn a commission on any affiliated purchases.

First-Time Amp Restoration, Pt. 2

Continuing the overhaul of a mid- to late-1950s Supro combo

In last month’s column, John Gilbert wrote in to ask about restoring his mid- to late-1950s Supro combo amplifier, and I began describing different steps he could take to get the amp up and running. We discussed retubing, and also replacing the line cord and grounding capacitor. If you missed that column or just want to refresh your memory, visit premierguitar.com/jan2011 and give it a quick read (the photos are pretty cool, too). Now let’s continue with the overhaul, picking up where we left off.


John, once you’ve added a grounded plug as I outlined last month, a good electrical cleaning comes next. Spray each tube socket one at a time with an electronic contact cleaner that does not contain any lubricant, and then repeatedly cycle the tube in and out of its socket (6-10 insertions and removals are usually sufficient).

Once that’s done, use a similar method to clean the input jacks, but this time use an electronic contact cleaner that does contain lubricant. Again, move the plug in and out of the jack to get rid of any corrosion or dirt.

Now, if there are any openings on the body of the front-panel potentiometers, spray the lubricant cleaner inside the pot and rotate the control numerous times. This is a good time to make sure all the jacks, nuts, and screws are tight. Tight hardware assures the safest and quietest operation.

Filter capacitors are another item to address. Most often, I make the decision whether or not to replace an amp’s filter caps based on the condition of the caps, how the amp sounds, how the amp looks on the bench test gear (various meters and scopes), and whether a customer is concerned about maintaining as much originality as possible.

Because I can’t physically see your Supro or play through it, and your intention is to make it a reliable and safe practice amp, I’d recommend replacing these capacitors. These will be the largest capacitors inside the amp, so they shouldn’t be too hard to find. According to the schematics, the amp should have either three 10 μF 450V or two 10 μF 450V and one 40 μF 450V caps. If these are all discrete components, then any currently available electrolytic filter capacitor will yield more than adequate results for this project. If these are multi-section caps (that is, with more than one capacitor inside the enclosure), then Antique Electronic Supply (tubesandmore. com) will probably be the place to source these.

Incidentally, if you need multi-section caps but can’t seem to find any to purchase, you can also use individual capacitors in place of the multi cap. Connect the positive end of each cap to the appropriate positive connection of the original capacitor, and remember to connect all the negative connections to the same place that the original capacitor’s negative was connected.

Once you’ve done all this, you should be ready to reassemble and check out the amp. If the amp powers up, has a decent output, and is relatively noise free, then I believe a declaration of “job well done” is in order.

If the amp fires up but emits background noise—such as lowlevel popping or a sound like a crackling bowl of Rice Krispies— I’d suggest replacing the 6SL7 preamp tube (if you haven’t already). If the noises still persist with a new tube, it’s time to start replacing some of the only remaining components in this basic amp.

I’d first concentrate on the preamp-tube plate resistors. These resistors might have a value of 100k (brown-blackyellow) or 220k (red-red-yellow) or 270k (red-violet-yellow). Replacing them with any carbon- film or carbon-composition resistor will be fine. I prefer to use 1-watt resistors in tube amps, but traditional 1/2-watt types will work, if necessary.

While replacing these resistors, I’d also suggest replacing the associated signal caps. When working on significantly old equipment, it’s a good idea to minimize the number of times you remove and reattach components on the terminals of the aging tube sockets. Since this is not a “keep everything original” project, let’s do it all at once.

Reviewing the schematics, I see these caps consistently seem to be labeled 0.05 μF. Since a 0.05 μF cap is not a readily available value, feel free to substitute any quality 0.047 μF 600-630V DC capacitor. Again, a Sprague Orange Drop or similar poly-cap will work fine.

If the amp still sounds like breakfast, I’d replace the preamp-tube cathode resistors. These may be older values, such as 2k (red-black-red), which are not as readily available, but it would be fine to substitute a 2.2k or 1.8k in these instances. Again, if there are cathode bypass capacitors associated with these resistors, I suggest replacing them at the same time. They will more than likely be of the 25 μF 25V variety, and any currently available 22 μF or 25 μF 25V cap will suffice.

Well, if you’ve come this far, you have an almost completely rebuilt Supro—and you’ve been a willing participant in the process, not just a spectator. That can yield a lot of satisfaction, so enjoy!


Jeff Bober is one of the godfathers of the low-wattage amp revolution, co-founded and was the principal designer for Budda Amplification. Jeff recently launched EAST Amplification, and he can be reached at pgampman@gmail.com.

Featuring P-90 PRO pickups, CTS potentiometers, and a Custom ’59 Rounded C neck profile.

Read MoreShow less

Wonderful array of weird and thrilling sounds can be instantly conjured. All three core settings are colorful, and simply twisting the time, span, and filter dials yields pleasing, controllable chaos. Low learning curve.

Not for the faint-hearted or unimaginative. Mode II is not as characterful as DBA and EQD settings.

$199

EarthQuaker Devices/Death By Audio Time Shadows
earthquakerdevices.com

5
5
4
4

This joyful noisemaker can quickly make you the ringmaster of your own psychedelic circus, via creative delays, raucous filtering, and easy-to-use, highly responsive controls.

Read MoreShow less

This little pedal offers three voices—analog, tape, and digital—and faithfully replicates the highlights of all three, with minimal drawbacks.

Faithful replications of analog and tape delays. Straightforward design.

Digital voice can feel sterile.

$119

Fishman EchoBack Mini Delay
fishman.com

4
4
4
4.5

As someone who was primarily an acoustic guitarist for the first 16 out of 17 years that I’ve been playing, I’m relatively new to the pedal game. That’s not saying I’m new to effects—I’ve employed a squadron of them generously on acoustic tracks in post-production, but rarely in performance. But I’m discovering that a pedalboard, particularly for my acoustic, offers the amenities and comforts of the hobbit hole I dream of architecting for myself one day in the distant future.

Read MoreShow less

A silicon Fuzz Face-inspired scorcher.

Hot silicon Fuzz Face tones with dimension and character. Sturdy build. Better clean tones than many silicon Fuzz Face clones.

Like all silicon Fuzz Faces, lacks dynamic potential relative to germanium versions.

$229

JAM Fuzz Phrase Si
jampedals.com

4.5
4.5
5
4

Everyone has records and artists they indelibly associate with a specific stompbox. But if the subject is the silicon Fuzz Face, my first thought is always of David Gilmour and the Pink Floyd: Live at Pompeii film. What you hear in Live at Pompeii is probably shaped by a little studio sweetening. Even still, the fuzz you hear in “Echoes” and “Careful With That Axe, Eugene”—well, that is how a fuzz blaring through a wall of WEM cabinets in an ancient amphitheater should sound, like the sky shredded by the wail of banshees. I don’t go for sounds of such epic scale much lately, but the sound of Gilmour shaking those Roman columns remains my gold standard for hugeness.

Read MoreShow less