Internal Wire Gauge?

18 gauge for the power wiring to and from the mains xfmr and 20 gauge for the signal wiring to and from the output transformer. The wiring should be UL1015 listed (600 volts and 13 amps for #20 and 18 amps for #18 at 40 degrees C)

I don't know about other folks, but I try to avoid accidentally buying 300V wiring for tube amp builds. The wire rating I cited above is used by transformer manufacturers in their mains and output transformers. UL 1015 is not shielded wire; it is a PVC jacketed hookup wire with a rating of 600V and is specifically designed to work in appliances, including tube amplifiers.

And don't forget to check the voltage ratings of the resistors. Some of those boutique brands (including Vishay RN60) are rated for 200 volts, and you can easily drop that voltage in tube amps.