B-52 AT212 suddenly lost volume

[CSlepicka]

I am new at posting here but read entries from time to time to try and learn something new. I have a B-52 AT212 combo which I purchased in 2008. Unlike most people that post things about these amps, I have never had any problems with the amp...until recently. I typically power up the amp for 6 to 8 hours per week. The other day I powered up and heard nothing when plugging in. Usually I'll hear some static, maybe a 60 cycle hum etc. when plugging in but on this day nothing. I first thought I left it on stand-by but this was not the case. I checked each channel and it was the same on each. I then slowly turned up the master volume and each channel's volume and when the master was at 3 o'clock and each channel's volume was at about the same I could start to hear the guitar and static. It was very weak - could barely hear it. So I looked through the metal grate on the back and could see that all the 12AX7's were lit. One was much dimmer than the others but lit. The 6L6's were all lit as well. Two of the 12AX7's could not be seen because they each have a metal sleeve around them. I believe these are related to the phase inverter and effects loop tubes. The speakers were both wired properly. I never have taken the back off of the amp because I never had to and always thought it would void the warranty if I left traces of me being in there. If this turned out to be a tube replacement issue I would do that myself as I believe the odds of something major happening before the warranty expires are slim. Anyhow, has anyone had a similar problem on any amp? My father was an electrical engineer for 50 years before retiring (at 77) and I've learned a lot from him. Mostly solid state stuff. We never discussed tube amps before he passed away so I'm not that informed about them. I emailed B52 and have not yet heard back. I thought I'd ask here before I call to find out where their service centers are located. I do have a schematic if needed.

Thanks for any help!

Chris

[Jazz P Bass]

Can you post the schematic?

[CSlepicka]

Schematic AT212

Here's the schematic
AT212_sch.pdf

[52 Bill]

Just because the tube filaments light up, it doesn't mean that they are working properly.

Isolate the circuits and try to limit the search by sections. I always start at the back of the amp and move forward. Check the speakers, the speaker cable, the impedance switch. Then plug your guitar into the FX return jack and turn on the FX loop. Do you hear any signal?

[CSlepicka]

52 Bill, the only times I get any signal is when the guitar is plugged into any one of the 3 channels and the master volume is cranked all the way as well as that channel's volume. I can barely hear it but it's there. When I plugged into the FX return I again could hear the guitar... but just barely.

[52 Bill]

Do you have access to another amp? If you do, try taking the signal from the FX send jack and see if it's at normal levels there. At least this way you can limit your search to either the pre-amp or the power amp.

Have you tested the speakers? If you can hook them up to another amp you can check them, or take a 9 volt battery and touch it to the speaker plug that you have pulled from the amp jack. You should hear a loud pop through the speakers.

[CSlepicka]

I did check the speakers by hooking them to another amp that I have.

The other amp does not have FX send/return so I couldn't try that. If one of the preamp tubes quit, would I be getting these symptoms? Just curious what type of symptoms I would get if one of the 12AX7's or even one of the 6L6's stopped working. As I mentioned I'm not really knowledgeable when it comes to valve driven components so I'm not sure what can be expected.

[olddawg]

Just run a signal from an iPod, reciever tape out, etc. into the effects return or main amp in. That will verify that the power amp is or isn't working correctly. You can also run the preamplifier or effects send into any line level input like a tape in or aux input on a reciever. You might want to just run a cable between the fx in and put. They are notorious for make/break contact failures and check all of your power supplies.

[52 Bill]

The other amp does not have FX send/return so I couldn't try that.

You can send the FX out signal into the front of the second amp. Keep the volume level down and see if the signal is large enough to be heard.

If one of the preamp tubes quit, would I be getting these symptoms? Just curious what type of symptoms I would get if one of the 12AX7's or even one of the 6L6's stopped working. As I mentioned I'm not really knowledgeable when it comes to valve driven components so I'm not sure what can be expected.

Yes, it is possible that if one of the preamp tubes died it could cause your symptoms, but if one of your power tubes died it probably wouldn't. It really doesn't matter if you know tube circuits or not. If you think of each tube section as a basic amplifier circuit, it would be no different than if the circuit was driven by an op amp or discrete transistor. The purpose of the circuit is to increase the level of a signal, and for the sake of diagnosing your amp, it doesn't really matter how it does it, it only matters if it works or not.

There is a preamp and there is a power amp. If you check each section separately you can limit your search for the problem to a smaller section of the amp. Then once you know that the problem is let's say in the power amp, you can break that down into separate stages to isolate the actual bad component.

Working techs do this exact thing every time they put an amp up on the bench. A lot of the signs that they look for are second nature and do not have to be spelled out, but here on a forum nobody knows what level of experience a poster has, so sometimes we try and ask very specific questions to help eliminate the things that need to be ruled out before a more accurate diagnosis can be made. Sometimes guys will post things that they have run across before in similar situations, but these facts may or may not be relevant to your amp. So the best advice that I can give you, is to divide and conquer.

[rf7]

Are you running the amp on the solid state rectifiers? If not, try it that way.

[CSlepicka]

I did try it when it was in solid state mode. Same results.

The last thing I tried was plugging into the FX return and had the same result as well. I also used a 9 volt batt on the speaker cable and did get the loud "pops" from both speakers. So I'm guessing that means it's not the speakers.