Buzz, Ground Loop or Bad Tube?

It sounds like an earthing problem.
Are you playing in a different place? Are you plugged into a different wall socket? If not, it may be that something has come loose in the power cable or mains plug.

Take care, there are high voltages in tube amps, even long after they've been turned off.

Could well be a ground loop.

Troubleshooting buzz in a (tube) amp 101:

First, make sure that all electrical devices around your system are off (really off -- lots of modern electronics don't really turn off, they still draw power, unplug anything you suspect of simply being asleep) be especially concerned about things like florescent lights which tend to cuase odd interference.

Second, test the outlet the amp is plugged into with a Ground Fault Tester No Musician should be without one of these! Never plug any musical gear into an untested outlet!! IF the outlet fails, do not use it for anything and get an electrician in immediately.

Third, plug in the amp and turn it on with nothing but the power cord attached? Is the noise present? If it's not obvious, plug in a cable that is not plugged into anything else, now is it present? Plug in your guitar with no intervening effects pedals, now is it present?

If there is significantly more noise with the chord plugged in than without it, replace the cord.

If there is significantly more noise with the guitar plugged in, you have a problem with the guitar. If it stayed about thes same or even dropped slightly with the guitar plugged in, then the problem is the amp. It is possible that you have more than one problem, but we want to trouble shoot the amp noise first. Some minor amount of noise is present in most inexpensive amps, but it shouldn't be a really bad hum, buzz or rattle.

Let's assume that the noise level stays about the same or drops off after plugging in each thing in turn, then the problem is in the amp . . .

Fourth, identify the type of noise:

A smooth hum in a tube amp with nothing plugged in could be:

1) Inadequately filtered power supply -- this usually happens on old gear and is caused by capacitors dried out or failing. This means a trip to the repair center unless you know how to replace a capacitor on a circuit board and are willing to do it on your amp -- in which case why are you reading this post?

2) "Hum balance circuit" -- tube amps have a circuit in the power supply to supress noise, basically a resistence circuit. On better amps, this is a potentiometer, which you can adjust by turning a small screw. Again -- unless you know what you're doing it's best to take this to a repair guy, as it involves sticking your screw driver into a pot in the power supply, and that can get nasty if you're not an experienced electrical guy.

3) Ground loop -- we've already eliminated most causes of this. One last check is possible though and that's to plug in your amp power cord, and plug your ground fault tester into the other side of the cord -- it may be a small short in the amp power cord. But if that's good, then it's one of the other two problems already mentioned.

Other sound possibility = harsh buzz -- usually only caused by electrical interference or the same causes as a smooth hum. Repair tech time.

Crackling -- serious problems. Most likely repair will cost more than the amp. Seriously, don't even bother with a tech unless the amp is worth more than your car. if you're less than honest use e-bay, otherwise, find a dumpster.

Ringing or squealing sounding hum, noise changes audibly when you 'bump' the amp -- this is usually a microphonic tube. Replace your tubes and you'll be fine. Once one tube goes microphonic, the rest usually follow pretty soon after, so no sense not replacing them all. This is fairly common with combo amps where the tubes hang down near the speaker magnets.

Thanks for your replies.

Could a bad preamp tube actually cause a ground loop? Clearly, it's some kind of loop because when I touch metal on the guitar, the noise goes away COMPLETELY. But my question is could this be caused by a bad tube?

I'm going to try swapping preamp tubes first.

Next I'll try plugging directly into the effects return of the amp.

Then I'll try buying a multimeter to test the power cord...

Thanks again!

Does the sound get worse when the guitar is plugged in from when nothin but the power chord is plugged into the amp?

Yes, the buzz gets worse when the guitar is plugged in. It's not any worse, though, than when an instrument cable is plugged in without the guitar. The sound is a little more of a hum than a buzz when there's a cable but no guitar. With the guitar plugged into the cable, it's more of a buzz...

Any tips on how to check my guitar? Or how to sheild it? I'm using humbuckers and I never had a problem before...