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Power supply is onboard on those. Under the two big covers are the power supply and output transformers, the small cover is a smoothing choke or capacitor (or both). Rectifier valve is at the back. The two small valves are the amplification stage and phase splitter for the push-pull. The two big valves are the push-pull output stage. This is just off memory but the voltages are something like 5V for the rectifier heater, 6.3V for the other heaters and the HT is about 450V. It may have a bias tap on the transformer but there’s no 12V line. That’s why I think it might be a solid state preamp / tone control because that would run on low voltage like that.
Jonny69-
Gotcha. The full setup seems to only have one mains cable, going to the preamp, and then two cables come off that, one to each power amp (although this one is mono). Here's a stereo one: https://www.ebay.co.uk/itm/285084893694?var=0&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338268676&toolid=10044&customid=CjwKCAiA76-dBhByEiwAA0_s9aLOqj0JCQXLPXXZYYY81MrLfb3rMJImErmbjtmNFulLLrJC25yLyhoCzo4QAvD_BwE
Would be a waste to stream Spotify through something that lovely, but it's still very tempting.
Sparky
I think the Acoustical is Quad's preamp and supplies power to the power amp - no power circuit in the power amp, I think. But I have no idea what the other boxes are for. Noted that it's mono.