The BTH SUPA Projector
One of my grammar school friends in the mid-1960s had a father (Mr Gilbert) who worked for AEI Sound Equipment. He used to take charge of the PA equipment for the school, especially the summer fete, and had 'acquired' several ex-SUPA amplifier racks which were obviously a slightly newer version of what is described in your article.
By the mid-1950s, the power ampifiers had been upgraded from what is described in your article, the power outputs being doubled by substituting KT66s (1 pair push-pull for 45W and 2 pairs for 90W) for the earlier Pen44s. The preamplifiers remained unchanged.
In the early 1960s the whole system (3 pull-out 'trays') was progressively replaced by a single tray incorporating pre- and power-amps and power supply, with more 'modern' valves. From memory the valves included EF86 inputs from each projector, ECC83s for interstage amplification and phase splitting, and 1 or 2 pairs of EL34s, with twin GZ34 recifiers. Voltage stabilisation in the power supply was abandoned.
As an aside, Mr Gilbert also sold my father two B.T-H. / AEI 5-20 (EL34s) power amplifiers and a Leak Varislope Stereo for his HiFi system. I still have them - in working order....