[Arran]

All in all I can't understand the appeal that these 500Cs, and many of these other tubed Fisher tuner-amplifiers seem to have, it sounds like the amplifier circuitry of these 500Cs is of a very poor design, it sort of reminds me of that Croften-White amp circuit that was tried back in the early 30s where you had to have both tubes in the circuit in perfect condition or bad things would happen. I've never really understood why anyone would build and market an amp with tubes driven to the maximum operating limits, other then just for bragging rights in marketing, something goes slightly wrong and it's meltdown time.
Regards
Arran