Up for sale here is this NOS vintage Ducon 1 meg ohm potentiometer.  This has a loudness tap so I'm assuming it's an audio taper pot.  It has a long 1/4" shaft with a flattened edge, so any knob with a 1/4" hole and a set screw will work great with it.  This has a power switch attached to the back, so when you turn the pot initially it will turn on the power switch.

The power switch is rated for 2A 250V so it should be good for at least 4A or 6A at 125V and will be fine for most tube amplifiers. This is guaranteed to work great for you, though it may need some Deoxit. This is a pretty rare potentiometer and would be great on your guitar amp. You can always cut down the shaft if you want.  Please see my pictures for details.

US shipping and handling is $5, and international is $18, no matter what quantity you buy. All my small parts are set up for combined shipping, so just use the "add to cart" feature and then check out all at once, and it will combine shipping. If you have any questions, please ask.

I have more pots and other vintage electronics listed--feel free to take a look!