Re: Amp Problems :(
One other thing about just changing tubes..... you may need to re-bias them (power tubes) unless your amp was designed with fixed bias. If not they make a handy little tool called a BiasProbe that's relatively cheap and will plug right into a good volt ohm meter.
Anyway, you'll occasionally hear guitarists say things like "we just bought new tubes, stuck 'em in, and started playing", and "We didn't worry about biasing, and you don't need to now", or "Biasing is a myth". In most cases, you don't have to bias your tubes when you change them. You can just plug a new set in and start playing, especially if you aren't too particular about setting up the amp for the absolute best tone. But with the right tools it's easy enough to check.
However, if the new tubes you have plugged in are different enough from the ones that were in there, with respect to current draw for a particular grid voltage, well..... They may end up biased too hot for your amp. In that case, your new tubes will start to glow cherry red on the plates, either at idle or while playing, and they will soon "fry"! In addition, the tube may short out and take out the output transformer in the process, leading to high dollar repairs. The opposite..... not high enough grid voltage if there is other problems and the sound/tone suffers too!
Tubes of the same type from different manufacturers will usually vary greatly in current draw at a particular grid voltage! Even two different tubes of the same type from the same manufacturer can vary widely in their current draw. For these reasons, it is always best to check the bias after installing a set of tubes.
If you're in an emergency situation, such as a blown tube in the middle of a gig, you can go ahead and stick in your spare tubes, but you should turn the amplifier on and look at the plates of the tubes (the large dark grey metal element) in the dark, both at idle and while playing, just to make sure they aren't glowing red.
Often, when a tube fails, it will take out the screen grid resistor, and any new tube you plug in will glow red, or won't work at all. In this case, you have no choice but to repair the amplifier before using it.
So the moral??? Don't assume the manual will tell you all you need to know! Besides, It's best to educate yourself while taking it to a qualified tech! Most will be glad to explain things.
Last edited by jemplayer55; 11-08-2007 at 11:04 AM.
Reason: I need to start using my spell check again.... doh!