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I would guess that you're here because of one of three reasons:
Numbers one and two are good, laudable reasons to be here. You're a smart, forward-looking, prepared kind of person. Number three is where I'm going to have you being a religious convert next time. If you're part of group three and don't follow at least some of these recommendations, I think you're going to make some amp tech very happy during your life.
There is a lot of things you can do to be ready. These fall into two categories: knowlege you can have already learned for how to proceed, and objects you can have on hand just in case you need them. Let's take the knowlege first.
How does you amp act when the normal channel preamp tube dies?? Easy enough to find out - just pull it out, then listen to the amp. How about the reverb tube? Phase inverter? Shoot, what happens if one output tube dies?
A curiousity about tubes and a real advantage that they have over solid state devices is that a missing or failed tube will usually not cause any harm to the the rest of the amplifier. There are specific exceptions, notably shorted rectifier tubes and shorted output tubes; but you can safely, no harm to the amp, pull any tube out to listen for what it does to the sound.
If you'll spend an hour or so pulling a tube, listening to the results and noting what you can hear and what you can't, which controls work, which don't do anything anymore, etc., you'll already know what happens when tube XYZ dies.
This isn't perfect, of course. Most of the tubes in the preamp section of a guitar amp are dual triodes, and often only one section goes bad. Also, "going bad" doesn't always mean "totally dead". There's all kinds of things that mean only section one or two is bad, one side is noisy, one side is arcing, etc. But you CAN get some ideas.
Here's another idea - your favorite amp tech, who loves to see you walk though his shop door, will think you're crazy, but it makes a great deal of sense to beg for a couple of tubes that are KNOWN BAD. I bet he'll give you a few bad ones free, or at least save you some from repairs, perhaps in return for a six-pack worth of foaming mental lubricant. Just don't ask for shorted rectifier tubes or output tubes. If you can , get a set with a dead section 1, another with a bad section 2, excessive hiss, excessive hum, etc.
The reason an experienced tech can just listen to an amp, twiddle a few controls and make a doggone good guess about what's wrong is that he sees so many faulty amps and finds out what was wrong to cause them to act that way. You can't match your tech's experience in general, but you CAN know a lot about how your particular amp might go bad.
You can spend some time swapping in a tube known to have a bad section 1 and seeing what it does; a bad section 2 to see what controls work and how, a known microphonic tube, a hissy, noisy tube, etc. You'll be amazed at how quickly you learn this stuff, and what you remember when your amp dies on stage. And your bass player will be A-M-A-Z-E-D.
If you get serious about being ready to fix your own, and you have taken the time to learn to do it safely, get:
You catch the drift here - the more you know about how it acts when everything is OK, and about what failures in specific spots sound and act like, the easier fixing it is. You can even just pick your level of comfort. Anyone can swap tubes in and out, and that will catch most of the problems, very quickly.
So - you're going to need replacement tubes someday. Why not get them when you have some time to bargain, shop for a good price, find good ones and interview them, get ones that you know will sound good, not no-name or used leftovers that the local tech happens to have when your amp dies in Western Mudflats, Montana?
Look inside your amp, and make a list of the different KINDS of tubes that you have. Chances are, there aren't all that many. Most even semi-modern amps use only 12AX7's for preamp tubes, most Golden Age amps use 12AX7 plus perhaps 12AT7 and/or 12AU7's in the preamp circuit, very old or rarer amps use the 6EU7 and/or others. It just can't cost that much to get one or two replacements of each kind as cheap insurance.
What's that? your amp uses a rare 7199, 12DW7, or some other esoterica? Well, what WILL you do when our friend the local tech in Western Mudflats tells you "...uh.... those are pretty rare these days. I think I can get some Chinese replacements for that here in three days ... maybe about $50 each, plus FedEx shipping. That OK?"? It's always going to be easier and cheaper to get them ahead of time. Not to mention less stressful.
OK, I see. You think they'll get lost or broken... well, how do you keep up with that guitar, those effects, cords, and amp?
What else, other than spare tubes? If you got this far, you probably have already guessed - anything that you can plug in easily without tools. This is probably just fuses and cords. A really dedicated amp maintainer would have a DMM in his gig bag or the bottom of his ampwith the spare tubes.
Output tubes are special - you MUST rebias the amp whenever a new set of output tubes goes in to be sure you didn't get a "hotter" pair that will run away and melt down on the old bias setting, maybe killing your power and/or output transformer in the process. However, nothing says you can't try a new pair before hand, perhaps with your local tech's assistance, and find a pair that is safe - that is, if the bias is not perfect, it is at least not harmful if you just sub in the new pair.