[julie_m]

The Bush RP60 amplifier (UCL82 and UY85) is interesting because it takes its negative feedback directly from the pentode's cathode resistor, on the primary side of the speaker transformer. This means the secondary side is isolated from the live chassis, and allows for the provision of external speaker and tape recorder sockets (the former switched to disconnect the internal speaker). (It also means that any non-linearities in the transformer are not compensated for; however, this is mitigated by the presence of bass and treble controls -- and in any case is no worse than a UL84 running open-loop.)

I won't say too much more about it; the circuit diagram is available from the link "up top" for a very reasonable £1.99, or on Paul's excellent DVD.

For the experimentally-inclined, it should work, albeit with a little less efficiency, with a UL84 speaker transformer -- the hum cancelling winding is not strictly necessary. The UCL82's heater is rated 50V rather than the 45V of the UL84; but the actual error is only 6%, thanks to the 38V heater of the UY85.