Amp issues

[exalted]

Hey guys!

I'm having some trouble with my amp. When I was playing at practice, it sounded like the speaker was blown out, but only when I played specific notes. I was using my huge pedal board, but the only active effect was distortion.

When I went straight into my amp and cranked it WAY up, I couldn't get the sound, but the amp was making random popping noises, even when my guitar was turned all the way down.

Does this sound like a bad tube, a blown speaker, or both?

 

[bonepickerx]

Could be several things, Transformer, Tube(s) etc. Filter caps.

Go to the Amplifier Debugging page and check it out:

If there is no sound at all and the power indicator glows, there is AC power reaching the indicator. This indicates that although there is power in the box, something in the signal path is not letting any sound through.

Once you eliminate the silly stuff, like speakers not plugged in, etc, you can start looking at:

tubes:
do the filaments on the rectifier tube and output tubes light up? replace them if they don't
failing rectifier tube
failed phase inverter/driver tube (sometimes the output tubes are quiet enough to not make a detectable hiss or hum without drive)
power tube bad, although it's rare that they would both fail
power transformer faulty
open choke
output transformer faulty
blown speaker(s)
bad wiring or cord to speakers
Open/bad speaker cord or wires leading to the speakers.
First dropping resistor after the choke is open (it is rare that you would not hear hiss or hum in this case, but it could happen).
Parasitic oscillation

I have been building and repairing Tube Amps since 1988 What you have described is rarely easy to fix and rarely inexpensive.

 

[exalted]

Dang. Sounds more serious than I was anticipating. I guess I'm glad I've decided not to play any shows until 2011.

 

[Superlizard]

For t00b amp:

If the sound is like an odd overtone when you play certain notes, chances are the filter caps (big ones) need to be replaced.

 

[exalted]

It sounds like a nasty distortion that sort of overlays the normal tone. So, I can hear the normal note faintly, but it mostly sounds like a broken speaker. Is that what you mean?

 

[Patrick from Davis]

When the filter caps go, there is leakage of noise that normally goes to ground into the signal path.  It sounds like a lot of things, from a twelve string in a hurricane, a ring modulator-esque/bitcruncher distortion overlayed on the normal signal, to a sluggish distortion very similar to normal with more background hum.  It depends a lot on how much leakage and which cap.  It basically turns out to be some form of a ground leak into your signal.

For the other possibilities I would suggest busting out the multimeter.  If you don't have a multimeter, get one.  Then check the speaker to be sure that the resistance is right.  It should be about 6-6.5 ohms for an 8 ohm speaker.  Move the cone in and out gently, you will feel it if it rubs.  Check the surround for tears.  Try another speaker if you can, that is the quickest way to eliminate that as the culprit.  

Good luck
Patrick

 

[exalted]

I didn't have a multimeter when I posted this, but I ordered one from Amazon and it should by on my doorstep by the time I get home.

Do you know any good amp techs in the Sacramento area, Patrick?

Oh and thank all three of you for helping out!

 

[Patrick from Davis]

Sorry, I don't really know of anyone, I generally tear them apart myself with the help of the AX84 forum if I get in too far over my head.  Most things are not that hard to fix, once you diagnose the problem correctly.  First thing is first, see if Schematic Heaven has the schematic for your amp.  This will help with all kinds of questions if you can get a copy of one.  It is always easier to make a parts list when you have the schematic.  Also, you can fix things that others have "helped" out with.

Sorry I don't know an amp tech, good luck
Patrick