Just had one of these lovely old amps across the bench, and was extremely
surprised to see a plate voltage of 730 on the two 6CA7 / EL34 output
pentodes. That is some serious HT, and I'm sure rather higher than I
remember seeing typically on any similar amps. The problem was low distorted
audio, and was down to an O/C screen (G2) feed resistor. Full normal
operation was restored once this had been replaced. G2 volts were around 360
when it was running normally. The bias was about -36v. I actually looked
this configuration up, and it just falls within the maximum specs for those
tubes, which are quoted at 800v on-load plate voltage max. At this level of
plate, G2 and bias, the quoted max output is 100 watts. That's pretty good
going from a single pair. Most amps from that era used two pair to get that
sort of power.

I guess the name "Sixty Five" relates to this amp's claimed output power,
but even that is PDG from a single pair, given that most amps of a similar
configuration are typically only capable of 30 watts or so.

The output tubes that are in it look quite old, but they are delivering the
goods, and are pretty reasonably balanced, so I am reluctant to recommend
replacement as a standard 'service' option whilst it's in, given the dubious
quality and reliability of many tubes these days, even when running at
relatively low levels of plate voltage up to say 500v. Anybody know much
about these amps ? Not terribly common this side of the pond - or at least
I've not had the pleasure of seeing many from this manufacturer over the
years ...

Arfa