I'm with Nigel
I made up a 3 gang electrical box with a 12.6v ct transformer wired as an auto transformer. I used one side to install a light switch, and the other to install an outlet.
There is a tab between the set screws that you can cut to separate the top and bottom plugs from each other. That way I have one socket that is 6.3v lower than my line voltage, and the other plug is 12.6v lower.
I could have just as easily used it to raise the voltages by the same amount, just by swapping the secondary leads around.(google transformer polarity test, and you will get info on exactly what's going on)
This way, you never have to modify an amp to run a vintage transformer on today's higher voltages
In the photo, the top socket is line voltage minus 6.3, and the bottom socket is line voltage minus 12.6v.
The secret is to understand that a transformer doesn't care which side you feed it from. You have to be very careful not to put 240 onto the 12v, or secondary side, because you'll get 4500+v on the primary side, and more than likely smoke, flames and a tripped breaker.1547618149376.jpeg