TUBE AMP TROUBLE HELP!!!

[yjmevh1]

I have a B-52 At-100 head. Its a high gain 100 watt head with 4 5881 power tubes and 7 12ax7b tubes. I bought it used over a year ago and its been working fine until two weeks ago. The sound is incredibly soft, the tones awful, and there is loud popping. Also, my power tubes are glowing blue which I dont think was occuring before. I think also the preamp tubes are glowing more than usual. My worst fear is having to spend $300 to get all the tubes replaced. This is the first tube amp Ive ever owned so I dont know what to do. Please help me out thanks.

[TUBEDUDE]

The blue glow is ionized gas in the tubes. The output tubes might be a problem but not necessarily. Time for some measurements. Do you have a multimeter, and any technical experience?

[Diablo1]

It's most likely to be a tube that died, that's why tubes are installed on sockets for easy replacement. Tubes, in general, last a very long time - years or decades. But, they can and do fail. It may only cost you around $10 to replace a single bad preamp tube. I would buy a couple of 12AX7 tubes - Chinese Shugaung would be good enough and they're inexpensive. Then remove one 12AX7 at a time and replace with the new tube and test the amp each time. If it doesn't fix the problem, replace the original tube and repeat the process on the rest of the tubes until the amp tests OK. Preamp tubes do not need a technician to replace as there are no bias adjustments. Simply plug and play. Make sure the amp is off and unplugged from the wall before you replace a tube. Let the tube cool first, and make sure the tube pins are aligned correctly in the socket so you don't bend or break 'em.
Let us know if this fixes your amp.

[xtian]

+1 on Diablo's post. Good advice.

It's OK for your power tubes to have a blue glow. You don't want the black plates inside to start glowing red ("red plating"). A little red glow in all tubes is perfect (it's the heaters doing their job).

[M Fowler]

Chinese Shugaung tubes are of excellent quality and I am glad they are coming out more types available.

Check the tubes one at a time.

[Cliff Schecht]

I have fixed a few of these amps. Every single one I've worked on needed the first filter caps replaced. They put an effective 110uF after the tube/SS rectifier and when you use the tube rectifier, it quickly eats the first stage electrolytics because they're cheap as shit and can't take the ripple current for very long (I'm surprised it doesn't cause rectifiers to arc more often too). I replace them usually with the same value cap (it is part of the B52 sound if that's what you're after) but use a much higher quality part(s). It's unfortunate but these caps go bad pretty quickly IME, like within a year of people buying the amp typically.

There is also a PCB mounted sand-cast resistor (don't remember the value but it's a 10-15W job) that tends to crack easily, especially in the super heavy combos when people set them down hard right after playing (the heat + stress causes this resistor to crack).

I charge about $60-$80 to fix this depending on if this is the only issue but a professional tech at a music store might charge you closer to $120-$200 because it's so tricky working on these PCB amps if you aren't equipped to do so. If I have to start moving wires and working on the preamp boards (there are LOTS of wires in these amps) then the cost will go up quickly. I sure hope your problem is just the electrolytics.

[yjmevh1]

Thanks a lot for all the responses. I went out and bought myself some tubes. I checked for the bad one and replaced it. Done! Good as new. Thanks for helping me out!

[niftynev]

yjmevh1, I noticed that in the post before yours, szlash280z had a kinda similar problem as yourself - as far as the popping problem goes anyway!

His problem was arcing between the pcb & one of the valve pins. I was just thinking you may want to check in that area as well, to try & prevent problems down the track.