Volume dealer

So I'm thinking of adding a switch to cut out the tone circuit, and a switch to cut out the feedback loop. I should also remember to add a "master volume" control.

What's a master volume control? I'm glad you asked.

Part of the appeal of tube amps is the euphonic distortion they produce when they are turned up. The tubes get pushed a little too far, they begin to distort, and that creates the classic loud rock-n-roll guitar sound. But to get this distortion, you need to crank up the volume. High. And even a five watt amp (like the Champ) is very, very loud when turned up.

(Playing in a noisy bar, with a loud drummer, it's not too loud, so there's a reason you might want more power.)

But, you might say, five watts? My car stereo is more than that! My clock radio is probably more than that!

Maybe. But (human perceived) volume is logarithmic -- a 50 watt amp only sounds twice as loud as a 5 watt amp; a 500 watt amp only twice as loud as a 50 watt amp. So if a 50 watt amp is enough to overpower a noisy bar (and drummer) (and it is), a Champ's 5 watts can put out 1/2 that volume when turned up. So enough to drown out a quiet bar, or a restrained drummer. Either way, it's too loud for practicing at home. So we can't really crank it all the way up and get the distortion we want.

The trick is to put a second volume control in. You turn up the first volume control (which is usually called a gain control in this configuration) to get the tubes to distort, and turn down the second volume control (usually called the master volume) to cut back on the overall sound level. Voila! Distortion without too much sound.

But where to put the master volume in the circuit?

There are two kinds of tube distortion that people look for from this kind of amp. You can get distortion from the preamp (small volume gain) stages, or from the power amp (large volume gain) stages.

(There is a third kind of tube distortion, "sag", when the short-term demand on the power supply is too great, as happens when hitting the strings really hard, but that is more under the control of the player. And apparently it doesn't come into play with low-wattage amps, since the amps aren't drawing that much power anyway.)

(And there is the distortion you get when you push the speakers too hard, "breakup". But you don't get that until you're making a lot of noise.)

To get preamp distortion without too much volume you can put a master volume control (a "pot", a potentiometer, a variable resistor) between the preamp stage and the power amp stage. The signal there is small enough that a simple pot can do the job.

If you want to get power amp distortion at low volume you need to use some sort of a "power soak", a device that goes between the amplifier output and the speaker, to cut back on the signal sent to the speaker. An example of such a device is the Weber MASS; it's basically a fake speaker that soaks up some of the power without making noise. Doing this is more complicated (you're soaking up big power-amp power instead of small preamp power) and more expensive. But a power soak is easy to add on between the amp and the speaker, and it lets you really push all of the stages of the amp to their limit, so it's a popular add-on to an existing amplifier.

But if you're hacking on the guts of an amp (like I am), it's easy to add a master volume between pre and power amp.

In the case of the Champ the factory-supplied volume (a.k.a. gain) pot is between the first (12ax7) preamp stage and the second (12ax7) preamp stage. (The 12ax7 tube is really two amps in one tube, hence the schematic being labeled "12ax7 (1/2)".) The place to add a master volume pot is between the 2nd preamp stage and the power amp (6v6gt). Easy enough to do, I'll add it to my list.