The chassis is very large and there are many unused holes, this is because the same chassis was used for the DX Plus Seven, a complete receiver with power supply and push-pull triode (PX4) output stage. It may have been used for other sets.
Here's an example of the same chassis in a high quality Sound Sales radiogram, most likely with paraphase driver (6SN7GT) and PX4 push-pull output.
(picture courtesy HiFi Works)
Valves. This set uses the Magic Three valves -
Power. The DX Plus One requires an external power supply. The power socket is a type B5 valve socket, which takes either a B5 or B4 plug - pin 5 is used for audio when the external supply is from an amplifier. The power lead that came with my DX Plus One, and which is most likely original, has a B5 plug at one end and an International Octal at the amplifier end.
Power requirements are 1.5A at 6.3V (including dial lamps) and 10mA at approx 300V. I don't have a manual, so the HT supply was what I had at the time. I used a 410V supply with 10k series resistor. The failed 7200 ohm wirewound resistor was replaced with 3W 6k8 ohm wirewound. There didn't appear to be any other fault (except both dial lamps had failed) - all three valves tested OK, as did the electrolytic smoothing capacitor and the Sprauge paper output capacitor. The set worked well and checking the HT voltage gave 324V before the 6k8 resistor and 257V after.