These amplifiers are quite rare today. They did not seem to be as popular as the Leaks and Quads, but were equally as good and very well made. This was mainly due to PYE's set building for the military and war effort.
The sets use ECC33s, ECC35s and KT66s. Reasonably standard configuration circuit wise, but the ECCs are becoming hard to find. Having said that, there are equivalents.
The output transformers have several windings which can be configured to give output impedances from 3.5 to 60 ohms. The operators manual gives instruction as to how to change this and when done, how to change the positive feedback network to give a virtual 0 ohm balance. It's nice that the manufacturer gave the owners a degree of credibility in those days.
Once you have attended to the usual coupling and decoupling capacitors, you have to look at the smoothing. Be careful that one of the main capacitors which sticks it's head above the chassis has all it's insulation in tact. If not, you can catch a 200 volt belt off the can, as the circuit employs two of these 8 mfd cans in series which means one tends to be at half HT potential. I always leave these components in place, with replacement ones wired under chassis. This keeps everything looking original. Take note, the HT is higher than normal at around 425 Vdc. Use components that have at least a 500 Vdc rating with adequate ripple rating.
Once set up, the amps work well and sound very clean. They attract a high price and thanks to the chosen few in the UK, get shipped to far flung places.