I bought a PA amplifier at a garage sale on the weekend for $10, it has two 12AX7s on the front end, a 6AQ8 as phase splitter and a quartet of 6BQ5s on the output (35 W). I have never heard of the brand but it says "Made in Japan" on it. I fired it up and it worked, but the tone controls are crap (more effect on the volume than the tone). I have been tracing out the schematic over the past couple of days, it has a biasing scheme for the 6BQ5s that I have never seen before, the AC 6v to the filament of the 6AQ8 is applied to the ends of a rheostat with the wiper connected to the cathode of the 6BQ5s, I'll have to scope the voltages to understand this! Other than that it all seems very conventional. I will scan and post the schematic when I get it, if anybody wants.
The question I have is: why the 6AQ8 (ECC85)? The data sheets I have found indicate this tube is mostly for RF work.