diytube.com

One channel of a ST-35 requires 6.3VAC @ 1.82 amps filament heating - 0.3 A for the 12DW7 and (2) X 0.76A for the the EL84's. If you were running the three tubes from one dedicated filament transformer, would you choose one rated for 2A, 3A, or 4A? How much would you over rate the current requirement? Would over rating by too much cause an increased voltage? Is there rule of thumb to use?

You can use 2A or 3A transformers if of the E-I type.

Those can have loaded vs. unloaded regulation as bad as 30% (the cheaper, the worse). So chosing close to the rating with a tad bit of headroom will give you your expected 6.3V and not above 6.5V.

Now if you plan on using a toroid, they are tighter - typically 5-8% regulation and you can use as large as you want.
(though I have found the "Avemco" brand toroids horrible - 45% regulation on their smallest ones )

If you plan on rectifying and using DC on tube heaters, take what you calculate for AC current and use a transformer capable of at least double (in your case, 4A) because of the peak currents in rectification can really heat up the core even if the RMS draw is lower.

What would I do in your position? I would use a Triad F-16X (Digikey #237-1923-ND ). It's small, inexpensive, great regulation for an E-I core (15%), has a centre-tap for grounding and dispensing with a hum-balance control and I've had nothing but good luck with them

Cheers!

Thanks Geek