No! Don't use it for a Mains dimmer!!
If you look at the pots used for a dimmer, they have plastic shafts to insulate the user from the "big bities".
An ordinary old tube era 500k volume control does not have this feature, as it is intended to be used with low level audio signals.
These sort of pots often had a switch which was operated by the same mechanism as the volume control, but not electrically connected to it.(If you look at such a pot, the actual potentiometer connections come out of the side of the device, whereas those of the switch protrude from the rear.
Most radios were switched on by turning the volume control from minimum, when you would hear a click,
(If the switch mechanism has failed, you may not hear the click), then after the tubes had warmed up, you could advance the volume control to your required sound level.
More rarely, the switch was operated by pushing the shaft in, or rarer still, by pulling it out.
These switches generally look cruddy, but they were safe & quite reliable, even when used at 240v.
Not the pots, though!! I must reiterate that the guts of these were quite separate from the switches & not insulated for Mains voltages.