This project is a proof of concept for reusing a server PSU PFC stage (340VDC) and various salvaged parts in a (decent) valve amplifier.
Usually people fear server PSU PFC stages because they are not isolated from mains, and this poses a real problem with valve amplifiers as they use the HVDC directly coupled to the input stage... this is overcome by using a low voltage class D amplifier frontend driving an interstage transformer a bit like gate drive transformers in tesla coils, but serving the double purpose of low pass filter along with the inductance of the secondary and the capacitance of the tube gate.
The tube push/pull can then drive a small power toroïdal transformer with something along the 240/12 ratio which would reflect the correct impedance for a 5ohm load (adjust transformer rating for reflected impedance to match your output tubes)
340VDC PFC supply
Class D amplifier
Grid drive transformer
Ratio adjusted to produce significant AC swing to drive your output tube grid
Output power valve (EL19,EL34,etc..)
Output transformer for push/pull operation
A classical power totoïdal transformer with appropriate ratio and VA rating
Slowly progressing as raising children consumes a complete time and more...
Prototype input stage with PAM8304 basic ebay board and homemade 35:5 transformer... experimental results show an unloaded voltage gain of 40 which is nice as a 5V swing will generate a 200V output, perfect for driving any phase splitter as soon as I have the miliamp or so to do it
small news is better than no news, I scored a little 40W push-pull valve amp on a local second hand site for 50€ and will use it as an intro to debugging valve amplifiers with the help of a book from publitronic (modern valve amplifier design) to gain better knowledge of how it works
It looks great! good luck!