B-52 AT100: No Output

[ZackyG]

Hey guys! This is my first post. It's nice to be with you but I wish it was under better circumstances.

A couple days ago my B-52 AT100 crapped out. I'm pissed because I wanted to sell it to help downsize to a Vox AC15. Oh well.

One day it was working fine. The next day it wasn't. I did nothing that should have caused this - the amp sat in one spot, I did not power it up without a load connected, and I have not used or abused it in any way which would lead me to think I caused it to fail. The amp runs hot but I usually run it for less than a half hour at a time. It sees light use, and I don't gig it. I understand these B-52s are absolutely riddled with problems - destined to fail, as it were. If it wasn't the best-sounding amp I've ever owned (and I own a 1980 Fender 140, which sounds great), I'd have thrown it in the trash already. Or slowly dragged it to the trash. What a backbreaker.

I'm hoping you can help me come up with a solution.

Here is the problem:

There is absolutely no sound output from either speaker jack at 8 ohms. This happened without warning. I was not playing when the problem occurred, I simply went to play through it and when I took it out of standby there was dead silence. I have not tried it at any other impedance because I only own 8 ohm cabinets.

The amplifier powers on. The power indicator and the LED indicators on the front of the amp light up and switch, as do the LED indicators on the footswitch. Therefore, the cause of the problem is not a blown fuse. Whatever the cause is, it is not obvious.

Here are the steps I took to troubleshoot it:

- I tried jumping the FX loop with a 1/4" cable as recommended by a forum user in a 2007 thread. No luck.

- I tried operating the amplifier with the rectifier knob in all three positions (Tube A, Tube AB, Solid State) to no avail. Still no output. No audible pops when the rec knob is switched like some users were reporting.

My next thought was that perhaps I had a blown tube.

- I examined all tubes (4x 12AX7 preamp, 1x 12AX7 reverb, 1x 12AX7 phase inverter, 1x 12AX7 reverb, 1x 12AX7 FX loop, 4x 6L6 power, 1x 5AR4 rectifier) and saw that all the tubes lit up (dimly). Two of the 12AX7 tubes - the two furthest away from the power jack - are shielded (completely, except for the veeery tip poking out) with metal covers, and I could not see whether or not they were glowing.

- I tested each tube with a pencil tap and found none to be microphonic. All of the 12AX7 tubes are Electro-Harmonix - not sure if that's factory stock but I'm guessing it's been fully retubed.

- I examined the tubes again and this time I noticed that the filament in the 5AR4 was NOT actually lighting up, and the tube was not heating. Totally cool to the touch when other tubes were hot. I looked at a spare 5AR4 I have and saw what the filament should look like, and based on this I determined that the rec tube was blown.

- I then replaced the rectifier tube with a Groove Tubes 5AR4. Upon power-up the rec tube quickly heated up nice and bright and the other tubes seemed to be glowing more than before too. Unfortunately, there's still no output. Again, I tried jumping the FX loop and again, I tried the amp in all three rec modes. Still no output.

- I called B-52 customer support and explained my situation. The rep told me that he could only do so much over the phone, and, based on the problem I described, my best bet would be to take it to a tech who could "figure it out within five minutes". All I see is dollar signs.

I understand that the tube glowing does not always indicate that the tube is functional, so my next step was going to be taking my single spare 12AX7 tube and slotting it in for each 12AX7 to find out if any of them are causing a problem.

What steps would y'all take from here? I'm too broke to have an amp tech fix it (do they usually charge you just to take a quick peek and see if there's an obvious problem?), and I'm too broke to do a full re-tube. Either of those options would cost me more than the amp did. I understand how to drain filter caps, and the basic safety principles regarding high-voltage circuitry work (don't put your body in the signal path, hand in pocket, drain caps, have a multimeter on the thing at all times, be f**king careful), so I'm seriously considering popping this puppy open and looking for obvious problems, like busted connections, burns, or browning. I'm a little scared to do that, which tells me I probably shouldn't be doing it, but you know what? I need to sell this damn thing. Please help.

[g1]

The amplifier powers on. The power indicator and the LED indicators on the front of the amp light up and switch, as do the LED indicators on the footswitch. Therefore, the cause of the problem is not a blown fuse.

Not so fast.
The external mains fuse may be fine but there are fuses inside for the high voltage section which could be blown. If so, the heaters and lights will still work.
So that would be the first thing to check. They are called F2 and F3 and are shown on the attached schematic in the B3 sector.
If they are not visibly blown, remove them and check their resistance with your multi-meter.
Welcome to the forum.

[The Dude]

Also check F4, the B+1 fuse (HT fuse).

[Enzo]

Hi, welcome.

Everything works until it doesn't, don;t stress over why. B-52s may not be the best made amps in the world, but there are thousands of them out there NOT breaking down.

So...

Therefore, the cause of the problem is not a blown fuse.

Actually all we know is the mains fuse is OK, and the low voltage supply fuse, but we don't know if the heater fuse is OK or the high voltage fuse. I assume it has a HV fuse.

If your recrifier tube didn't heat, that would cause absence of high voltage. if you replaced it and hte new one lit up, then it seems like the old tube was bad. But that doesn;t mean the high voltage circuit has no problems, I expect some circuit failure caused the death of that tube.

Many if not most technicians will give an amp a quick look-over - the sorts of things you can check from the outside - for nothing. He wants to know what he is up against too. But if I have to take it apart, then I am "working on it", and I have to charge you. If it takes me an hour to figure out what we need to do, shouldn;t I get paid for that work? I think for most techs, though, the complaint of no sound at all, it generally not hard to diagnose. Difficult problems are things like "occasionally distorts".

[The Dude]

I've been inside these things more than a few times. I don't see them as built any better or worse than most other amps on the market these days.

[ZackyG]

Not so fast.
The external mains fuse may be fine but there are fuses inside for the high voltage section which could be blown. If so, the heaters and lights will still work.
So that would be the first thing to check. They are called F2 and F3 and are shown on the attached schematic in the B3 sector.
If they are not visibly blown, remove them and check their resistance with your multi-meter.
Welcome to the forum.

Thank you! I'll watch some more videos on chassis disassembly and cap draining and take a peek under the hood. Will post results in this thread. Lots of other people had similar issues and very few posted details on the outcome. Hopefully this post can help others in the future more comprehensively than other, similar threads.

Also check F4, the B+1 fuse (HT fuse).

Will do, Dude. Appreciate your comment on durability as well. Based on similar threads my understanding is that the wiring job may be sub-par in these models, but you know what? Don't judge a book by its cover. I'll take photos when I get inside and we'll see how well this thing is constructed. God knows the enclosure is built like a Panzer.

Everything works until it doesn't, don't stress over why. B-52s may not be the best made amps in the world, but there are thousands of them out there NOT breaking down.
All we know is the mains fuse is OK, and the low voltage supply fuse, but we don't know if the heater fuse is OK or the high voltage fuse. I assume it has a HV fuse.

If your rectifier tube didn't heat, that would cause absence of high voltage. If you replaced it and the new one lit up, then it seems like the old tube was bad. But that doesn't mean the high voltage circuit has no problems, I expect some circuit failure caused the death of that tube.

Many if not most technicians will give an amp a quick look-over - the sorts of things you can check from the outside - for nothing. He wants to know what he is up against too. But if I have to take it apart, then I am "working on it", and I have to charge you. If it takes me an hour to figure out what we need to do, shouldn't I get paid for that work?

Point by point:

Firstly, Enzo, thank you for your thorough reply. I saw your comments in other B52 threads and I appreciate your continued input.

- don't stress over why
I stressed a lot but I'm over it now. Ready to get my hands dirty.

- there are thousands not breaking down
very good point

- circuit failure in the high voltage tube
based on troubleshooting the internal fuses seem like a likely failure point. I wouldn't be surprised either. Again, I will reply to this thread with photos when I get inside.

- amp techs and the value of work
I agree with you, and I will happily pay anyone for time they spend fixing my problems. I'm a self-employed creative type and I'm not the type to say things like "I don't have money but the exposure will be great for you". I'm just anxious. Never had amp work done, very stressed about money right now, my first thought was "oh god, they're probably going to charge me $200 just to diagnose it". The work at that point would simply not be worth it. I'd be better off saving the $200 for a Vox AC30.

Thank you all for your input. Eager to get to work on this.

[olddawg]

Just make sure you know how to safely troubleshoot an amp. If you have absolutely no experience or training that repair cost is cheap compared to your life.

[52 Bill]

... my first thought was "oh god, they're probably going to charge me $200 just to diagnose it". The work at that point would simply not be worth it.

Rather than assume things that may or may not be true, why not call your local tech and ask him or her what it would cost to diagnose your amp. Most shops are quite reasonable in giving estimates or quotes for little or no money, and you can opt to not take the amp into the ones that do charge $200.

[ZackyG]

Rather than assume things that may or may not be true, why not call your local tech and ask him or her what it would cost to diagnose your amp. Most shops are quite reasonable in giving estimates or quotes for little or no money, and you can opt to not take the amp into the ones that do charge $200.

Indeed! I dropped by a music store today and found out that their minimum to open an amp is $80. Not bad at all, but I paid $110 for the head (in worse condition than it is in now). After fixing the issues it had before, I was hoping to sell it for at least $150. If I'm going to drop more than the amp's value on repairs I might as well just keep the damn thing. It's a great-sounding amp.

I also visited a hardware store today and picked up an extension cord to use to keep the amp connected to earth while I work (I yanked the hot and neutral pins but left the ground pin) and some alligator clips and resistors to bleed out power from the caps.

The more information I absorb and the more I watch and re-watch videos from Premier Guitar and BillM Audio on amp safety practices the more confident I feel in working on it myself. I am very fastidious and safety-minded. I will sit on it for a few more days, collect more information, speak to more people, and see if I can find someone willing to let me watch while THEY work - now that would be worth paying for. If that doesn't pan out I'll crack it open myself.

[52 Bill]

Indeed! I dropped by a music store today and found out that their minimum to open an amp is $80.

I don't know where you are located, but you might try a few other shops to see what they charge.

Building a power cable that you describe is nice, but is not really a necessary thing. The only time that you need the chassis grounded is when it is plugged into the wall and you are trying to take readings on live circuits, which you can't do with the ac power disconnected. Once the filter caps are drained of any residual voltage the entire amp is pretty much a boat anchor and can't create any harmful voltages.

Grounded chassis' are important in computer work where static discharge can do harm to certain components, but in 90% of instrument amplifiers, there is no reason for it.

[olddawg]

Indeed! I dropped by a music store today and found out that their minimum to open an amp is $80. Not bad at all, but I paid $110 for the head (in worse condition than it is in now). After fixing the issues it had before, I was hoping to sell it for at least $150. If I'm going to drop more than the amp's value on repairs I might as well just keep the damn thing. It's a great-sounding amp.

I also visited a hardware store today and picked up an extension cord to use to keep the amp connected to earth while I work (I yanked the hot and neutral pins but left the ground pin) and some alligator clips and resistors to bleed out power from the caps.

The more information I absorb and the more I watch and re-watch videos from Premier Guitar and BillM Audio on amp safety practices the more confident I feel in working on it myself. I am very fastidious and safety-minded. I will sit on it for a few more days, collect more information, speak to more people, and see if I can find someone willing to let me watch while THEY work - now that would be worth paying for. If that doesn't pan out I'll crack it open myself.

In most cases the last place you want to bring an amp for repair is a music store. Generally they farm them out and inflate the bill so they can make a little themselves. Honestly.... if you understand safety, own a multimeter, and keep one hand in your back pocket while wearing rubber soles shoes, I don't see why you can't take some voltage measurements. I would bet it's something simple if your filaments are lighting up. Can you read a schematic. Is your high voltage there?

[Enzo]

olddawg:
That is not a fair blanket statement. I ran the service department in a large independent music store for eight years, and I tried mightily to treat people fairly and ethically. We did all repairs in house in my shop. In fact we took in some repairs from smaller stores scattered around the middle of the state, when their level was not up to a repair. I left there to start my own shop, and located inside another music store. I had a cooperative agreement with them, but I was a separate business. I kept the same ethical standards. Like any other enterprise, some are great at it, some are total dogs, and many are soso. Some have their own techs, some don't, some have a shop in store, others serve as a drop off and pick up for a local tech based elsewhere. I kinda don't like being painted with the same brush as the kid who can;t put a new end on a guitar cord right.

[ZackyG]

Building a power cable that you describe is nice, but is not really a necessary thing. The only time that you need the chassis grounded is when it is plugged into the wall and you are trying to take readings on live circuits, which you can't do with the AC power disconnected.

I was curious about this, thanks so much! I made the ground cable because I figured it was probably best to have a route to ground available at all times (just in case!) but I understand that having that is essentially useless if there is no current in the circuit anyway.

What I'm not sure I understand is this (from another forum, on taking voltage readings from the filter caps):

"
Anyway, set your multimeter to a high voltage DC range. Black lead to ground (chassis) then touch the red lead to each of the lugs on the cap can in turn.
If you have a cheap meter (10k ohms / volt) you can watch the cap drain through the meter.
Hint: I use an old crimp on alligator clip on the ground probe. I can slide the probe into it when I need it, pull it off quickly when I don't. I clip my ground lead to the chassis and probe away.
Other hint: Don't short anything with your red lead while you're in there.
Goes without saying warning: Amp should be off and unplugged.

"

If the amp is off and unplugged, and the current is allowed to drain through the 10k resistor into the ground (which is now a floating chassis, as the amp is disconnected from earth) what happens to that current? Is the chassis electrified? Presumably if I touched the black lead as the current was draining I would get a nasty shock. Does this mean if I touch the chassis while the current is draining I will get a shock? Or is the electricity quickly dissipated as heat energy through the chassis and wood blocks?

Despite all the material on this subject I have consumed, nothing has quite managed to make this clear to me. Some clarification would be great! I want to make sure I fully understand the forces at play here before I get to work.

I would bet it's something simple if your filaments are lighting up. Can you read a schematic. Is your high voltage there?

I can't read schematics fluently, but I am also not useless. I believe I understand enough for the purpose of this repair.

[Enzo]

There is ground as in the earth connection, then there is ground as in circuit common. We should call them earth and common instead of lumping everything under ground.

Energy is stored in capacitors. I can charge up a cap and hand it to you, not wired to anything, and there will be a voltage difference from one end to the other. Touch both wires and you get a shock. Think of it like a battery. To discharge the cap, we provide a path for current from one end to the other, usually just a wire. No "ground" is needed.

Put the cap in an amp, and it is connected to other things. The one end is usually wired to common, our ground. The other end is wired to something else. To discharge the cap, we connect a wire from one end to another. Since one end is connected to ground, it is just as easy to connect our drain wire to ground as directly to the cap itself. It is convenient to clip our wire to the chassis, and probably less so to connect it directly to the cap.

Discharge current? It flows from one end of the cap to the other. it is not like a swarm of bees hiding in the chassis. The discharge voltage is not flowing into the chassis, it is flowing to the grounded end of the cap

[52 Bill]

If the amp is off and unplugged, and the current is allowed to drain through the 10k resistor into the ground (which is now a floating chassis, as the amp is disconnected from earth) what happens to that current? Is the chassis electrified? Presumably if I touched the black lead as the current was draining I would get a nasty shock. Does this mean if I touch the chassis while the current is draining I will get a shock? Or is the electricity quickly dissipated as heat energy through the chassis and wood blocks?

Well, when the stored charge on the filter caps is "drained" to ground, that doesn't mean that it has traveled into the chassis to be stored there, it means that the charge has been removed and dissipated mainly as heat through the resistors inside the meter or in the case of your amp through the two 220K bleeder resistors that are connected across the two main filter caps.

When the amp is powered up the circuitry acts as a closed loop if you will, the energy that enters from the wall outlet is used to run the circuits. When the energy source is cut off, the circuitry continues to use the stored energy until it can no longer operate, just like if your car runs out of gas. Some circuit designs can leave a charge on large value capacitors when they are turned off. Some amps like yours have resistors that are designed to drain off any residual charge when the amp is turned off. Even without the bleeder resistors, the caps will eventually lose their charge through leakage.

Edit: I see Enzo has explained it better than I could. Either way, I hope you understand it better now.