Can I fix my guitar amp myself?

January 6, 2004 9:47 AM

I have a Marshall Valvestate 20 (8020) practice guitar amp. The built-in reverb has stopped working, producing just noise when you turn it up and a sort of wobbly sound if you bump the amp. Also the amp itself has a lot of hum, which goes away when I touch the strings. I'm too cheap/broke to get it fixed but know how to use a screwdriver and a sodering iron. Suggestions?
posted by signal to Technology (13 answers total)

If you've got a hum that goes away when you touch the strings, I'd suspect a grounding issue of some sort.
posted by mrbill at 10:23 AM on January 6, 2004

Don't know about the reverb, but about the hum:

Setting aside grounding issues for now, what kind of guitar are you using? If you've got single-coils on there (Strat, Tele, etc), hum is normal. If you've got humbuckers (Les Paul, etc) and it's still humming, then a grounding issue sounds right to me too.
posted by The Michael The at 10:35 AM on January 6, 2004

Response by poster: TMT: I have a tele, 1 lipstick, 1 single coil pickup. I thought grounding, too, but the power cord, plug, etc. are all grounded.
posted by signal at 10:54 AM on January 6, 2004

Then my guess is it's the pickups, which is natural. I have a couple of Strats, and the hum disappeared for one of them when I put in a stacked humbucker. Try out the setup with a friend's humbucker guitar and see if that gets rid of it.
posted by The Michael The at 11:10 AM on January 6, 2004

Both your tele pickups are single coil and single coil pickups tend to hum. Try moving the guitar around (i.e., face different directions, move to a different room etc.) to see if you get less noise. Playing with a lot of gain/distortion will also naturally increase hum relative to signal.

I don't know your amp, but on most amps you can unplug the reverb pan altogether so you can look for broken connections, springs etc.
posted by timeistight at 11:17 AM on January 6, 2004

I've got the same Valvestate. I had similar problems with my reverb: lots of noise when you turn it up/down. Some positions gave a good sound, others only a "underwater-ploink". I just cleaned the potmeter with some contact cleaner. Seemed to work for me... Regarding the hum: any monitors / tube lightning around ?
posted by swordfishtrombones at 11:34 AM on January 6, 2004

that's lighting, not lightning. I guess that would give a hum too.
posted by swordfishtrombones at 11:37 AM on January 6, 2004

Response by poster: swordfish: a desktop PC pretty close. Did you have to remove the pot to clean it? Which part did you actually clean?
posted by signal at 11:38 AM on January 6, 2004

I just located the potmeter, sprayed a lot of the contact spray on/it it. Rotate the pot several times and again after a few minutes. But be aware: I'm a total idiot with respect to electronics (my Hamer guitar is still waiting for its DiMarzio p/u). Probably a Google search for pot meter cleaning will give you a better advice. It did work for me, though. Stupid question: does it help to shut off the monitor ?
posted by swordfishtrombones at 11:46 AM on January 6, 2004

does the amp have a polarity-reverse switch? my old fenders used to, but my new one doesn't and come to think of it everything in the USA now comes with three-prong fixed polarity plugs, so i suppose polarity switches have gone the way of the dinosaur. but i digress...

my new ibanez hums whenever i'm within 4 feet and facing my computer. also, noise is intensified by high gain stomp boxes. in any case, i'd suggest a noise gate in the line just before the amp input, which will silence anything under a settable threshold and keep your rig nice and quiet when you're not playing it.
posted by quonsar at 4:33 PM on January 6, 2004

About the hum; it's probably more likely something to do with the grounding inside the guitar itself, if touching the strings clears it, since you're acting as the ground.

Try the amp with another guitar first. Make sure it's one that you know doesn't hum; preferably with humbucking pickups.

If it's clear with the second guitar, then open the tele up (don't be afraid, just don't break anything). Make sure the ground from the bridge (which is grounding your strings) is going in a similar direction to those from the pickups, pots, and, ultimately, the jack.

Good diagram here.

Buggered if I know about the reverb, though. Take it to a guitar shop, they're probably the best to ask, since, in my experience, modern amps are a lot more technical than modern guitars.
posted by armoured-ant at 4:45 PM on January 6, 2004

Response by poster: Great advice, everybody.

armoured-ant, is the wiring the same in a "lipstick" pickup?
I'm pretty sure the pot on the reverb is dirty cos it makes a lot of noise when I turn it, but I have no idea about how to open it up to be able to clean it properly. I squirted a lot of wd-40 at it, no dice.
The amp itself is quite sealed up. I could probably get it open, but it has these menacing "high voltage, do not open" signs on it.
Is it true you can get shocked even if it's unplugged?
posted by signal at 5:09 PM on January 6, 2004

Is it true you can get shocked even if it's unplugged?

Yes it is, 'though it's a bigger problem with tube amps.

I think it's safe if you leave it unplugged for a few hours so the capacitors discharge.
posted by timeistight at 5:28 PM on January 6, 2004