300 Volts is a bit too hot. I explained the math in another post just now.
Each 6.3-Volt heater draws just under 3 Amps. Put six of them in parallel and you need 18 Amps.
Pretty sure this is why Palomar wired them as three series pairs. Cuts the current demand for the heaters down to 9 Amps.
For Antek's transformers, I really prefer the grid view of all their toroid transformers.
[URL]http://www.antekinc.com/grids/[/url]
There is a big gap around the voltage needed for every size over 300 VA. Since John's ratings are for 50 Hz, you can pull another 20% more power from them at 60 Hz. You can comfortably round this off to 400 Watts from his "300VA" part.
Only viable choice I see is this one:
[URL]http://www.antekinc.com/as-3t275-300va-275v-transformer/[/url]
A 40 bucks plus shipping, that's a reasonable as it gets.
This 100 VA 12-Volt transformer:
[URL]http://www.antekinc.com/as-1212-100va-12v-transformer/[/url] should take care of the heater supply. At $21.60 plus shipping, can't see how to go wrong with that.
The two of them together are cheaper than any single transformer would be. But you'll have to fabricate an enclosure and install the cable.
Expensive part, cheap labor to install. Cheap parts take expensive labor to adapt them.
Six of one, half-dozen of the other, pretty much.
Oops. Spoke too soon. Just checked RF Parts and they no longer list the transformer for a six-tube base amplifier. Oops.
They sold that one for years, but not any more.
Oughta look to see if ICA Manufacturing has a cabinet the right size for two Antek toroid parts.
73