lost the quoted text about 400Hz variacs:(

        to quote from _High Frequency Apparatus_ Thomas Stanley Curtis
reprinted by Lindsay Publications Inc. A chapter describes how to wind you
own 3Kv 500 watt step up transformer for use off lighting mains.

Page 87 (one-half kilowatt transformer outfit) Figure 3-Data for primay
 "Primary data for 110 volts    25~" (~ was used for cycles per second jim)
"463 turns #14 D.C.C    60~ 237 turns # 14 D.C.C   125~ 125 turns #14 D.C.C
  for 220 volts use twice as many of #17 D.C.C"

The core is the same size for all. If we extrapolate: 400~,opps 400Hz, needs
35.6 turns based on the 60Hz turns ratio, or you 400Hz xformer could be run
on 110v * (60hz/400hz) = 16.5 volts.