True on the heater wire for the output tubes but not required for the preamp tubes (24 gauge would work!)Structo wrote:20 ga or even 22 gauge wire is fine for most of the preamp as long as it's rated above 400v.
The heaters draw the most current in the amp so that is where you want the 18 ga but 20 is probably fine there as well.
But you don't want that wire to get hot so you can use 18 for insurance.
On 50 watt builds there is usually enough transformer lead wire to do the output tubes, just make sure the same wire in the twist goes to both pin 2's for hum cancellation.