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Actually no, a 5y3 is an 80 with an octal base. It's a full wave tube. It's also extremely common and cheap.
There isnt a problem with cathodes specifically, but most rectifiers that have them have the cathode hooked to the filament (5z4, 5v4,5ar4,etc), so a sepatate filament winding is needed just as though it didnt have a cathode.
The real reason for using a rectifier with a cathode is voltage drop. All else being equal, there will be less voltage drop across a cathoded rectifier. This leads to less 'sag' under load. This is generally considered a good thing, but in your case you need to lose all the voltage you can.
Either a 5y3 or a 5u4 should work fine, but if I was going in that direction I would use the lossiest tube I could find. 325 volts is WAY too high. You'll have to look in a tube manual, but I'll bet the 5y3 is gonna be the lossiest one. Any extra voltage you dont lose in the tube, you will have to dissipate with a resistor.... so you might as well lose all you can in the tube.
Heres what I think I would do. Leave the 6x5 for the moment. I would hook one of the 6.3/7a windings in parallel with the 6.3v/2.7a in phase. This is 9.7 amps for the 9.1 amp winding. not too bad. Use one of the 5v/3a windings for the 5u4. This leaves you a 6.3/7a and a 5v/3a to play with.
I would then temporarily scab some resistors in the plate lines of the 6x5. 4.7k at about 5 watts oughta do it. Run it up on the variac, and see how far off everything is. If everything is too high, (and It probably is) you can use one or both of the extra windings to "extend" the primary.
If you have any windings left now, you could use one for the 6x5 (or 5u4 or 5y3).
John
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