SzabolcsK

You could have built a tube amp after the bt module, it's not that hard.

You can use a transistor to pull it down until the pwm is ready

No, I just used the chassy of the PSU.Sure you can use a computer Psu, but you will need 12.6V +-5% or 6.3V for the heater(depending on how you wire it), otherwise your tube will suffer. The anode is happy up to 300V if I recall correctly.

We used to do this with a PC fan.

Cool box! Looks like an Orange Guitar amp

You should consider adding a 1:1 isolation transformer for safety.

WTF? cmon, buy some tube sockets they go as cheap as $3!

Rotate the output trannys in 90° to cancel hum from the power transformer! Also twist the 6,3 V filament wires together! Since they carry AC they induce hum.