Hi Jeff, Wire the output of the variac to the primary of the
transformer, and the secondary to the 220V power input of the Amp. You
may want to remove the tube(s) or disable the filaments to reduce the
burden on the step up transformer.
If the power transformer in the amp has a 110v tap, connect it up and
then just use the variac. Be careful when you are doing this, there
are all sorts of lethal voltages present. Good luck.
Gary N4AST

Thanks, Gary, but the devil is in the details.

The 220 out from the step-up transformer is from one end of the
secondary coil to the other.

The amp is essentially asking for two 110 V lines, working against a
common "ground".

So, somehow I must go from this:

0 -----220----- 0

to this:

0 0

0

and therein lies my confusion.

Here are the connectors labeled:


0 0
A B

to this:

0 0
C D
0
E

.... all I need to do is to connect the dots!