My friend just got an old amp from his dad, it is definitly a champ copy from the late 50's early 60's. It has no drive knob, and he wants to put one in. How would you add one to the existing wiring with tubes and such?
You would have to just use the regular volume knob as one. As most people know, when you crank a tube amp, gain becomes added since the tubes are pushed. What you have to do is add a Master Volume knob so you can adjust the volume with that, and use the regular volume as a gain type knob. Im pretty sure this is what you want to do, but check with Pink Queen since he knows a lot about modding amps.
im pretty sure the drive knob only controls the drive on the preamp, so if you drive say a blues junior with the drive knob way down you wont get a whole lot of distortion in the preamp, and if you drive it hard you will get distortion, however it doesnt control how hard you push the power tube(s) and thats where alot of tube sound comes from, and part of what made the old fender tube amps great was that they had to be loud in order to sound great,

now if you really want one talk to greenbox, thatpinkqueen or wait here, they are prettymuch the tube amp gurus (specially greenbox) and even if it means tearing out half of your amp they can make it happen for you.
Just to clarify..."gain" is simply the amplification power of the tubes. To get some dirt, just crank the volume and the tubes will overdrive into distortion.

You can't really add much of a master volume/volume to a champ type amp. Mainly because you can only have two gain stages with one 12a*7, so you can't really drive the preamp tubes. You can of course drive the power amp, but that's what you do when you turn up the volume.
I'm not very active here on UG currently.
I'm a retired Supermod off to the greener pastures of the real world.