Originally Posted by Splattle101
Ok, I’ll give this a go.
First, preamp valves.
You can change one of these at a time. You do not need to change them all at once.
The phase inverter (or ‘driver’) is the one that works hardest and is most likely to need changing first. This is the last valve in the preamp, right before the big valves in the power section.
The little preamp valves are all cathode biased, which means they do not need to be biased when you change them. They are all plug-and-play.
The very first valve in the preamp is the one that has most impact on your tone. This is sometimes called ‘V1’, which is holdover from ancient Fenders. You can try different brands of the same kind of valve (eg. JJ, Tung Sol, Groove Tube, etc), or you can experiment with different kinds of valve (12AX7, 12AT&, 5751, etc). This is a whole subject of its own and I won’t go into it any further here.
Next, the power valves.
In nearly all guitar amps the power valves are in pairs (the old Fender Champ, with one 6V6 is an example of the inevitable exception). If you change one valve in a pair, you should change the other.
Why? Firstly , because if one of them is buggered enough to need changing, whatever buggered it (time, power problems, shorted output tranny etc) has probably buggered its mate, too. Secondly, because the new valve will no longer match the older, worn in one. The amp will be working in an unbalanced way, and it will not sound nice, and it will (probably) shorten the life of your valves and perhaps some other components.
If your amp has four power valves, they should probably all be changed together for the reasons above.
If your amp is configured so the power section is cathode biased, you will not be able to bias your valves when you change them. Examples of this arrangement include the VoxAC30 and the Peavey Classic ranges. This means that the valves are plug and play…sort of.
The reservation is that the new valves you put in have to be the same as the ones that came out if you want the amp to sound the same. I don’t mean the same brand, I mean their electronic properties must be the same. Same gain, same output (current), etc.
Some valve manufacturers and resellers grade their valves. They do this so that you know that if the valve you take out is grade X, then all you have to do is go to the shelf and get yourself another pair of grade X valves of that type and bang ‘em in and you’re good to go. Mesa and Groove Tubes both do this.
However, most amps are manually biased. For these, you should re-bias when you change valves. Again, however, if you liked the way it sounded and the valves in it were graded, simply changing to the same grade and leaving the bias alone should be OK. But if the valves you’re taking out aren’t graded, you’re stuck. It’s got to be re-biased for the new ones.
Unless you know what you’re doing, a tech should to do the biasing. What you’re doing when you bias is you’re setting the voltage on the control grid so that the right amount of current is flowing from the cathode to the anode (or ‘plate’ as the Americans call it) at idle. Sounds simple enough, and it is, except that there’s no easy way – or safe way – to measure current directly. Voltage? Easy peasy. Current? Mmmmnot so easy. What you end up doing is measuring the voltage and then calculating the current.
It is easy to **** it up. **** ups may reduce the life of your valves or transformers.
It is easy to get hurt. You’re playing with the high current end of the amp, and **** ups here are potentially for keeps. Unless you know exactly what you can and cannot touch, and unless you have a good safety drill, I don’t think you should be mucking around in the back of an amp with the power on…or off for that matter. Filter caps hold a big charge for a long time.
Hope that helps.