This amp is a clone of a EF36 + CL33 (or EL33, KT61, 6M6G, 6P25 ) amplifier using equivalent miniature valves. The EF36 is replaced by Mullard EF86 and EL33 by EL821 or 6CH6. Each of these valves has near identical characteristics to the earlier much larger octal versions.
RS have a suitable, and cheap, output transformer.
Question - should I use a valve rectifier, or make the project even simpler by using a silicon rectifier? Using a large series resistor and large capacitors will slow down the initial HT surge a little. What else can be done? A NTC thermistor perhaps, available from RS for less than £1 each.
Since I started thinking about this project I've obtained three vintage RS economy output transformers that are good for single ended or push-pull use with these valves. So now I'm thinking of ECF80 (triode-pentode) for gain and phase-splitter and a pair of EL821 for output.
Using an ECF80 (6BL8) as a phase-splitter isn't a particularly unusual thing to do even though the intended use of these valves was as a frequency changer. See http://hhscott.com/cc/phase_inverter_tubes.htm for one tried and tested circuit and ech21 for another possibility.