Any information on STC audio amp - possibly PA, possibly by Leak ?

[GrimJosef]

A friend has recently acquired a large mono audio amplifier about which he knows little. I said I'd ask on his behalf if anyone recognised it, or had any information about its capabilities.

The front panel tells us it's a Standard Telephones and Cables type SRA-15-B (or possibly SRA-15-8). My friend understands it was used as a PA amp. The inputs are 'Mic' and 'Spare'.

The valve line-up is 3 6BR7s in the front end (there is a current monitor whose switch suggests V1 and V2 are combined together, perhaps in a 'totem-pole' e.g. cascode arrangement ?), then a 6SN7 (presumably the phase-splitter) then a pair of 807s (presumably push-pull output). The rectifier is a 5U4. There is also a 6F6 positioned between the rectifier and the 807s. It's not clear what this is for. My best guess is that it is the pass valve in an HT regulator, maybe stabilising the 807s' screen grid voltage to allow reliable operation in Class AB2 for maximum power ?

The label on the rear suggests that the output transformer secondaries can be linked to drive either a 650ohm or a 160ohm load

Inside the wiring is neat and tidy and professional looking. Above the chassis the transformers have pressed steel shrouds and insulating top plates. I was struck by the resemblance to a Leak TL/12 Point One amp (underside shown in the 5th picture). In particular the labelling of the transformers is very similar, and the style of the shrouding, and the use of what appears to be a multi-section paper-in-oil HT reservoir/smoother (Leak were very proud of this) in a large black can. In Leak's case the transformers were made by Haddon and the HT cap by TCC.

I read in Stephen Spicer's Firsts In High Fidelity that Leak routinely did sub-contract work for STC immediately after the war, building amps both for PA (e.g. a 1kW unit for Vauxhall's Dunstable car plant) and various other amps for driving STC's record-lathe cutting heads. I'd hazard a guess that my friend's amp was produced by Leak, or the design at least was copied from work by them. But it would be great to know for sure. Has anyone encountered one of these amps before, or come across any documentation on them ?

Cheers,

GJ