Up for sale here is this vintage Ducon 500k / 200k ohm dual concentric potentiometer. This is a linear taper pot. The  500k has a loudness tap. This is a dual concentric pot, so it has one shaft inside the other that can be adjusted separately. One section is 500k ohms (measures 0 to 792k ohms) with a loudness tap and the other is 200k ohms (measures from 0 to 230.3k ohms). This has a power switch attached to the back, so when you turn the pot clock-wise initially it will operate two switches. Double Pole (DP)

The power switch is marked 2A 250V and would be fine for most tube amplifiers. This is new old stock.

The overall length of the shaft is 6 cm.

This can be cut down to suit your application.

See the photos