'Popping sound' on my tube amp?

September 30, 2005 7:56 AM

My tube amp is making 'popping' noises. Help!

So recently my Fender Blues Jr. has started making 'popping' noise after it's been on for about a minute. The sound somewhat resembles what it sounds like when you plug/unplug the cord into a guitar. However, I unplugged the cord completely and the sound stills continues (note: it's not at regular intervals, it's kind of random).

So I guess this is a multi-part question: 1) What could be causing this? Is there something I (not an amp expert) could do to fix this? and 2) If I should get it checked out by someone who is an expert, what would be the best way of going about that (i.e. finding someone who knows what they're doing and not getting ripped off)?

posted by Stauf to technology (12 comments total)

The usual cause is a loose connection, and usually at the jack. I'm not familiar with that amp, but I was able to fix mine with a bit of electrical tape. (Unlike the popping which came from the volume knob brushes being corroded, which I was told would cost about $200 to fix at my local electronics repair shop, at which point I decided I could live with it).
posted by klangklangston at 8:03 AM on September 30, 2005

I had a similar problem with klang's culprit, though I ended up getting a new amp which fixed the problem.
posted by jmd82 at 8:14 AM on September 30, 2005

I don't know what causes it, but the way I find amp technicians is something like this:

First, check out all the places that sell guitars that are not pawn shops or guitar centers. That is, small-business-owned guitar stores. These usually have at least one guy who does repairs full time and it good at his job. Ideally you can bring the amp by, he'll plug it in, and you can show him the sound. All the big stores I've been to just want me to drop off the amp and leave, and I often get an amp back that STILL has problems, because they didn't have me to point out what I was hearing.

Also, I've gotten great recommendations from people in the music industry, who rely on good techs. My guitar teacher usually has a good recommendation for specific types of repairs (and he's a former luthier, so all the better).
posted by RustyBrooks at 8:22 AM on September 30, 2005

If it is a potentiometer like klangklangston's volume control a bit of contact cleaner sprayed in while rotating the knob back and forth can work wonders. I've used it a few times on two way radios with good results.
posted by Mitheral at 8:22 AM on September 30, 2005

I was warned once by an EE that some contact cleaner is electrically conductive, and you need to make sure it's dry before you plug it in. I have no idea if this is true or not, or has changed over time. Just a thought.
posted by unrepentanthippie at 8:40 AM on September 30, 2005

Deoxit D5 is the best contact cleaner I have ever used. Great stuff and much better than regular contact cleaner. I would hit all of the switches and knobs, plus the plugs with a blast of this stuff first. If that does not cure it, pull the tubes and hit the tube sockets with the D5 and swab some on the tube pins. Still popping? Check for loose wires, first with the amp off and then, if you are brave with the amp on. For the latter procedure put one hand in your pocket and with something non-conductive like a plastic rod gently poke at the wires and connections to see if you can provoke the popping sound. You might need to resolder a connection. It could also be a bad tube, but that usually presents as distortion rather than popping. Other culprits, such as a failing capacitor, probably require professional help to track down and repair.
posted by caddis at 9:23 AM on September 30, 2005

Yes, not only conductive but probably explosive too. Sorry should have mentioned that. Please read the label and follow all the instructions printed on it. Make sure your equipment is off and unplugged. It evaporates really fast (it's a design goal) so an hour or so of dry time is probably a suffiecently long time to wait before trying your amp out.
posted by Mitheral at 9:26 AM on September 30, 2005

Deoxit is not conductive.
posted by caddis at 9:53 AM on September 30, 2005

Some further caution if you go poking around in the amp with the power on. A tube amp typically operates at voltages several times higher than the wall voltage 120 v. Also, capacitors can hold a charge for some time after the amp is turned off (minutes in a properly designed amp, hours in an improperly designed one) and then release that high voltage charge through you. Prior to touching stuff inside with your fingers check with a voltmeter. Putting one hand in your pocket helps prevent accidentally touching ground with one hand and a high voltage with the other. Of course if you are standing in water, or on concrete, or even just in bare feet you might still be grounded. Wear rubber soled shoes.
posted by caddis at 9:59 AM on September 30, 2005

First off, avoid plugging things into your amp while it is on. Seriously. No, really. You, your instruments, and your amp will all live longer...

Since you've eliminated the guitar (and I hope the cord), the problem is clearly internal to the amp. There are a lot of things that can cause a tube amp to do this. The power supplies of tube amps typically produce a few hundred volts of DC power to act as the "plate" supply for the tubes, so if you don't know anything about amps, it's not healthy to poke around inside the amp randomly. That said, here are some common general suggestions:

1) I've seen older tube amps that are just filthy. Coatings of dust a quater inch thick on the chassis and all the tubes make things run even hotter than tube amps normally do, and that ages capacitors and sockets. A clean amp is a happier amp. If yours is in this shape, you can try carefully cleaning it. With the amp off, you can use a small vacuum cleaner in conjunction with a can of compressed air (which you can get at most photo shops) to get rid of a lot of crud. The preferred technique is to keep the vac nozzle near the area being cleaned, and dislodge the crud with short blasts of compressed air, so that the vac catches it on the fly, without the vac touching anything on the amp..

If the dust contains some conductive material, it can even be intermittently shorting voltage to ground, so just cleaning a dirty amp can sometimes solve problems.

2) A tube amp that is moved frequently can develop problems with the cathode heaters in the tubes, that will cause this, at least for a short while before the offending tube blows. Basically, they are like miniature light bulb filaments, and can, like some light bulbs you've probably seen, break physically, in ways that let them momentarily "reconnect." The fix is to replace the tube. But...

That's a job for a tech, because most amps need to have tubes replaced in sets, and to have bias voltages checked and adjusted to make the new tubes work equally. That takes an accurate voltage meter, and ideally, a distortion analyser set up. And if it is time to replace tubes, the tech should be examining the sockets, particularly of the power tubes, and cleaning or even replacing, if the connections are corroded. It's also generally a good idea to check the power supply voltages at the time, and to look for leaking capacitors, overheated transformers, etc.

3) In integrated amps like yours, the speaker is inevitably going to transfer vibrations to the electronics. Over time, it's pretty common that capacitors fail due to this, or that wiring connections can break. You can try carefully tapping the amp, preferably on the electronics chassis, to see if you can "produce" the popping sound in response to your tapping. Be gentle, and don't hit the tubes or any metal tube shields! If you tap and it pops reliably, you probably have a bad connection. If you tap around, and can't find a place where the tapping is reliably producing the pop, it's somewhat more likely you have a capacitor going. Either way, it's a job for a tech.

4) If none of this works, and you have some time and patience, it might be useful if you can see if there is any time pattern to the popping. Sometimes, a capacitor going bad is first evidenced by a popping sound that is fairly regular, but may be variable over a period of a 1 or 2 seconds between intervals of as much as 30 seconds. As it gets worse, the period between pops gets shorter, and the variability gets to be less. At the extreme end of this, the amp "motorboats" (i.e. makes a very low frequency constant rumbling noise, like an outboard engine at idle). If you are in the early phase of a breakdown of a main power supply capacitor, or an input filter capacitor, you might notice a regularity of the popping, with some variability, if you mark the noises against a stop watch. Again, fixing this is a job for a tech.

While you're at this, try to notice if there is some other big appliance, like a refrigerator cycling on and off when the popping happens. Big appliances can produce line spikes that your amp's power supply should be filtering, but if you have a grounding problem, they come through. You can plug something with a motor, like a fan, into the same outlet as the amp, and switch the fan of and on to see if there is any coincidence with the popping.

Hope this gives you some things to try at least.
posted by paulsc at 10:05 AM on September 30, 2005

Hey guys, thanks for the advice on cleaning things. I've never tried to do it myself, mostly because of the wicked retarded internal design (like, I'd have to take everything out from the back to get to the volume controls, though the input jacks are really easy to get to). Maybe I'll order up some of that contact cleaning solution and give it a go though...
(Best part about the tube amp that I've got? It'll keep playing for about 15 seconds after the power is shut off...)
posted by klangklangston at 1:44 PM on September 30, 2005

How long have you had it? How often is it on? Have you ever changed tubes?

If the noise only occured when turning a knob it would be a potentiometer issue. But if it does it when you aren't turning knobs it's something else. Sorry that isn't too helpful!
posted by 6550 at 10:30 PM on September 30, 2005