Need some help identifying the problem (possibly blown powertube?)

[AndyM]

Just got a B52 AT100 about a month ago. I think the power tubes are blown but I want to make sure before I spend $70 on new tubes.

When I hit the power switch everything lights up; the tubes all heat up normally (heater filaments are still glowing on all tubes). But when I switch from STANDBY to ON, I can't see the voltage jump into either the preamp or power tubes.

I'm still getting some sound out of the speakers, but it's very low volume; with the master and channel volumes on 10, I get a very low-powered overdriven sound from the speakers.

One more question: when I got the amp, I switched out the 12au7 in the PI position with a 12ax7 to brighten it up; could this be harmful to the amp?

[Jazz P Bass]

The 12AX7 in place of the 12AU7 should not have hurt anything.
Without ripping into the amp & taking some (very carefiul) voltage measurements I do not see this amp getting fixed.
Although you could "Easter egg" a known good preamp tube.
One at a time swap the known good tube with the existing preamp tubes. (amp off)
Start at the tube farthest from the power tubes.
If nothing changes, put the old tube back in the socket & move on to the next tube.
If all of this proves to be an excercise in futility (ie: nothing changes) you may still have a bad power tube.
Without the proper test equipment you really are shooting in the dark.

[Amp Kat]

Hate to be the bearer of bad news but this amp is one of the most problematic amps on the market if not #1 in that category. Just do a search on it and read em and weep.

My personal opionion is that you have an open sreen resitor on on side or maybe two. Check them of course be very carefull inside the amp and discharge the filter caps as high voltage could be present. I would ohm out all of those screen resistors and make sure they are ok.

[AndyM]

Checked the preamp tubes. Nothing.

Unfortunately, my multimeter just died (cheap piece of crap I made for school), so looks like I'm gonna end up taking it to someone who actually knows what they're doing. Any suggestions for a good amp tech in the Chicago area?

[AndyM]

Okay, checked the resistors. No opens, they're all reading around where they should. Any more ideas?

[R.G.]

It's a little silly for any owner of a tube amp not to have a spare set of tubes. The first step in debugging any tube amp problem (unless you are so experienced you can go directly to the problem by inspection; there aren't many people like that) is to replace tubes with known good tubes until you prove by substitution that it wasn't a tube.

I say that as a prep for: you need the tubes anyway. Even if you're so disgusted you sell it, you'll have to sell it for scrap prices if it doesn't work. IMHO, you ought to get the tubes. If it wasn't a tube now, it will be sometime, so you'd be ready. If it was a tube, you'd know what "dead power tube" sounds like and could get another set of spares. In both cases, you need the tubes.

That's about the limit of what you can do inside a tube amp unless you already know how to measure voltages inside a live tube amp chassis without endangering yourself and others in the process. If you don't already know how to do this properly, either refer it to a qualified tech, or get a qualified tech to help/teach you how, and solve the problem in the amp as a byproduct of the learning.

If you know how to measure pin voltages on the tubes without endangering yourself, the next step would be to measure all the pin voltages of the amp with the amp operating. If it's a circuit flaw, most of them will show up when you do that.

Again, don't try this unless you already know how to do it safely. It's not worth dying for.

[52 Bill]

Have you checked all of the basic stuff like the speakers, cables, etc.? Low distorted output could be as simple as a shorted speaker or cable, or having the speaker plugged into the wrong output jack. What about an FX loop?

[AndyM]

Have you checked all of the basic stuff like the speakers, cables, etc.? Low distorted output could be as simple as a shorted speaker or cable, or having the speaker plugged into the wrong output jack. What about an FX loop?

Yeah, already checked all the basics. The amp has a footswitchable effects loop on it, but there's no improvement whether it's on or off. I'm pretty sure it's a problem with the power stage, since I can no longer see the voltage jump into the tubes (if you don't know what I'm talking about, watch your power tubes when you switch from "standby" to "on").

I don't have a set of spare power tubes (plenty of spare preamp tubes though) since I just got this amp and money's been kind of tight lately. As soon as I have some spare cash I plan on replacing the power tubes and buying a spare set. I was just looking for other ideas as to what may be wrong.

[R.G.]

The problem with posts that end up "does anyone know what could be wrong?" is that it could be almost anything from a bad solder joint to a leaky capacitor to any of a hundred other things. It's like telling a doctor "Doc, my foot hurts." Could be corns to cancer, with autoimmune and trauma in between.

Tubes are just the most common thing to go bad. The real way to go about this is to swap tubes because that is so easy and so definitive in most cases.

You *could* get out the voltmeter and start measuring operating voltages, as I mentioned, but unless you have the meter and have the skill, this is a positive safety threat. And, as Jazz said, without the proper test equipment, you're going to have real trouble finding what's wrong.

I'm not unsympathetic to your plight; I've been there. But we want you to keep on living to play music.

[AndyM]

I give up. I tried checking the bias and got no reading at all. Taking it to a tech as soon as I can afford it...