View Full Version : Red Hot Head Tube !! HELP !!!
02-04-2001, 12:32 PM
I have an Ampeg V-5 and I was playing it today and my sound just faded out to nothing. I smelled something funny and I noticed that a tube was glowing bright red. I had all the tubes replaced about 6 years ago and I play just about every day. Is this what happens when a tube blows ? And if so does this mean that there is a problem somewhere else to cause it to happen ?? Also do different brands of tubes really make much of a tone difference ? any help will be greatly appreciated
ps: should I replace just the blown one or just replace them all ??
02-04-2001, 07:55 PM
Ampegs use their tubes hard, applying high plate votage, and getting long life by paying careful attention to the Plate Dissipation factor. Plate Dissipation is the amount of power that is lost (dissipated) in the plate structure as a result of its resistance, and the current flowing through it. This lost power is given off as heat. A tolerable amount is just part of the cost of using tubes as the active components in a circuit. Too much can lead to what is called 'catastrophic failure'. If you hadn't shut down when you did, the glass bottle might have shattered.
The Plate Dissipation is kept within the design limits by adjusting the negative bias voltage applied to the cathode of the tube. I suspect that since there is no provision for simple adjustment of the cathode bias, that re-tube job six years ago amounted to simply stuffing new tubes into the appropriate sockets and hoping for the best. A fresh set of tubes in an Ampeg amp should last for more than six years. My VT-22 had the original tubes in it when I bought it last year. It was made in 1976!
As a minimum, you will need to replace the tubes in the output section. The fact that only one of them got red-hot indicates a problem in the wiring around just that tube, but while it was dying, the rest of them would have been subjected to stresses that will make it very hard to complete a matched set.
Matching is important in any amp that puts out more than about twenty Watts. The ouput section is usually a push-pull design that depends on having identical tube characteristics on both sides of the circuit. Ampeg amps, in paticular, apply high plate voltages that will destroy lower-quality or mismatched tubes in short order.
High Plate Dissipation (enough to make it glow!) results from insufficient bias voltage at the cathode. You may find a bad solder joint, or a broken resistor on the underside of the socket.
If this is unfamiliar territory for you, PLEASE take your amp to a competant shop for examination by a professional tech. The voltages in tube amps, (especially Ampegs!) are LETHAL!
02-05-2001, 06:59 PM
Thank you very much for the informative reply !!! I love the head so I will not get rid of it. So that means off to the shop she goes do you have any idea about how much this will cost me to fix ?? (New tubes and all) also what brand of tubes do you think I should get ? From what I hear tubes made in Russia are the best but I really do not know. (There are Groove Tubes in there now). I have a PRS guitar and a Fender American Standard that I put Lace Sensors in. The style of music I play is in the mode of bands like Failure,Sunny Day Real Estate,HUM and a touch of Rush. Do different brands of tubes really sound different than others ?? Again thanks for your reply !!
02-05-2001, 08:22 PM
Groove Tubes has built their reputation on the quality of their matching process, but even the best tubes will fail if the bias is not properly set. Here is some of the text of a recent email I got from GT.
"Give us a call here at Groove Tubes and talk with Rick, he can give you all the info you need or will get you to someone who can. We have a toll free number (800) 459-5687. Our hours are 7:30 AM to 4:30 PM Pacific Standard Time."
Rick is knowlegeable about all kinds of amps, including Ampeg. I won't claim that he's totally unbiased; he does work at Groove Tubes, after all, but he seems to be a straight shooter. That's a rare thing in a business that seems more like voodoo than science to most of us.
Good Luck. Let us know how it turns out.
vBulletin® v3.0.17, Copyright ©2000-2013, Jelsoft Enterprises Ltd.