Dinky little 2-valve Chinese pre-amp

[Nick]

Is that not a voltage tripler behind the power in socket?
4 diodes plus 4 capacitors so 110V DC.

Don't think so - its a straight bridge with several small smoothing caps rather than one large one to:

a) lower the effective ESR
b) to keep the size down to fit the enclosure.

Thes valves will happily work at +12VDC so this one running at +28VDC (or whatever) B+ is well within bounds - its just the quality that suffers - their Ia/Va plot is not at all linear around that B+ and you need a Vg of close to 0V (which is an advantage here as you don't need a -ve C- supply.