I collect old test gear as a hobby, and like to have it working too. I
recently bought a 110v valve sig gen in very nice condition and it works
fine off a 110 supply.

Looking inside - as you do - I noticed that the transformer primary was
two windings in parallel. This, of course, is common these days - parallel
operation for 115 volts and series for 230.

So I removed the transformer, and series the windings. On UK mains, under
500mA load, the heater volts are 7 volts rather than 6.3 which makes sense
if the windings are 110 volt. Will this matter?

--
*All men are idiots, and I married their King.

Dave Plowman London SW 12
RIP Acorn