(old) amps are built for.
Well, there is a free cure that does not involve external transformers or
The price to pay is to switch to solid state rectifer.
The unused 5V filament is well isolated, in fact it is designed to isolate 500V from the
What is needed is to connect this winding in series with the primary winding. It could be
connected in two ways, to add 5V or to subtract 5V, the only way is to test either way and
measure on the 6.3V winding if it increases or decreases. The one that gives a decreased
voltage is the "right way".