Update - I recently met a guy in town who's a bit of a tube amp guru, he's repaired tons and even built several from scratch. So I took this amp over for him to take a look. We pulled it apart and nearly shit our pants at what we found inside. There was a third preamp tube just dangling inside. The chassis (i.e. the metal box containing the circuitry) had one end chopped off to fit in the head (i.e. the wooden 'Traynor' box), the circuit board was shifted over, and (I guess) the third tube socket was in the cut-off portion. The components were all British. The circuit board was a very primitive PCB, probably the first generation post point-to-point wiring. The amp guru pulled out his schematic book and ta da - it matched an early 1970's 100W no master volume model Marshall!! I have no idea why someone did such a cosmetic hack job on such a rare amp (possibly disguising a stolen amp?), but the insides were all in tact. The plate voltage and current were running super high - the reason the tubes were blowing, and the transformer would go into a self-protect shutdown state (and likely the reason it was in the trash!). A few new capacitors, re-soldered joints and a bias adjustment later and the amp was running perfectly. Suffice to say it was a fascinating experience for me to trouble-shoot this amp with someone with this kind of experience. Marshall plexi's go for big bucks on ebay. I'd have to refurbish it if I ever wanted to get what it's worth, but for now I'll just enjoy playing it.