I'm kinda kicking around the idea of building an amp. Partly because it'd be a cool thing to do, and partly because it'd (hopefully) be cheaper than buying the kind of amp I really want. Don't worry, I have a mate with experience of this to help.
My question is though, if I take a design based on one type of tube (6L6 or EL34) and want to run something else (KT88/6550's) is it just as simple as plugging in the different tube type and setting the bias, or are there some behind the scenes components that need to be changed too? Obviously the output transformer would need to be mathced to the valve output, but what about resistors and capacitors?
Basically, I'm wondering about building a 3 channel amp with Fender cleans and SLO100/5150 type distortion, but running KT88's
Hehe! Yay, a thread I can butt in into...
EL34/6L6 --> KT88/6550: You need to change the ~220k bias feed resistors to 82 or 100k, and make sure the bias voltage is wide ranging enough to deliver the correct voltage. Also, make sure your filament windings are up to scratch, they draw more current!
My advice to you would be to build a SLO100 as it's a pretty proven buildable design, and modify it for 3 channel operation: I've done 3 channel SLO's before and it's a pretty easy task... if you're interested I can put together some schematics and details of what you need to change. Modify the cathode resistor on the clipping stage (the one with the 39k cathode resistor; it needs to be 15 to 10k, lower = more grind) and swap the first cathode bypass cap (1uF) to .68uF or so to "5150-ise" it if that's what you're into.
Basically - 2 or 3 independent tone stacks/gain knobs/channel volumes, all switched via optos, and a couple of mods to the crunch channel to make it generally more creditable is all you need to get an amp with 3 channels clean/crunch/lead. Very minimal changes to the actual amplifier circuit, it's more a switching and adding more EQs thing.
If you're interested in parts, I have a half finished 100w rackmount 3 channel high gainer which could easily be built into a SLO kind of thing knocking about somewhere... you'd be interested in the case, case mounted components, transformers, valve sockets, all that good stuff.
If anyone's interested in me finishing it up for them incidentally, I will do it for a deposit + final payment kind of deal as I don't have the parts to finish it right now and would need to buy it... but if anyone want a custom amp, I might be offering that if James doesn't want it for parts.
IN FACT... I HAVE ABOUT 9 MONTHS FREE AND MIGHT BE UP FOR DOING SOME CUSTOM AMP WORK FOR PEOPLE IF YOU ARE INTERESTED IN GETTING HOLD OF SOMETHING VERY METAL AND UNIQUE! (Pass that round, hehe.)