A preliminary page of pictures, which will be linked in due course to a page on the JEI O50's design and history. For the time being, some simple observations:

The two main power transistors in the JEI O50 are RCA 2N3054s, giving a power output somewhere in the region of 40 watts, which is the figure given in all the early literature. The "50" of the amp's designation is (and was) a little misleading therefore. In the 1970s, 50 watts appears to have been the more general assumption.

The driver transistors are a pair of RCA 1A04s, and the main gain stage (on the circuit board in the "well" of the chassis) consists of an RCA 40406, 4047, 40408. The topology may have been borrowed from an RCA manual. That needs to be checked, however.

The thing that differentiates the O50 (for organ) from the identical looking B50 (for bass) is the preamp.