[Amps] What's a better rheostat placement ?

[w2cqm@juno.com]

Due to a high 250VAC  primary voltage, the dedicated filament voltage
transformer (15VCT @ 22 amps)  for a series wired retrofitted  pair of
3cx1200A7's in a Henry 3KA  is about 1 volt too high. The  220VAC Variac
I have is a bit too cumbersome to locate  into the lower section of the
amp.  A small footprint  rheostat (22 ohms @ 50W)  appears to be the way
to go.  Is it preferrable  to wire it into one leg of the 220VAC  primary
or  to wire it into  the low voltage  output side of the transformer? Any
comments would certainly save me a great deal of trial and error. Thanks,
Ron W2CQM/3