[Amps] amp conversion filament quandry

[gdaught6 at stanford.edu]

Hi,

I am converting an old Alpha-70V to use a 3CX1200Z7 instead of 
the teapot type 3CX1500V.  The original filament transformer 
provides 5 volts at 30 amps.  The 1200Z7 wants 6.3 volts at 25 
amps.  I'm thinking of rewinding the secondary of a 50 va size 
transformer to make a 1.3 volt, 25 amp secondary, and putting it in 
series with the original Alpha filament transformer.

What I'm not certain about is feeding the bias to the new 5 + 1.3 volt 
combination.  I can bypass the transformer secondaries heavily, so 
RF shouldn't be a problem.  The cathode current will have to go in 
an unbalanced manner through the transformer combination.  Is this 
going to be a problem, or is the 1 amp of DC cathode current going 
to be trivial when compared to the 25 amps RMS already flowing?

Otherwise, I can bite the bullet and buy (gasp!) a 6.3 vac 25 amp 
CT transformer... but I sure would like to use the existing 5v 
winding.  Of course, if it isn't used, then the (contiguous) plate xfmr 
should be able to supply more current!

Any suggestions?  Any 6.3 volt transformers?

73,