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When BOTH tubes are conducting then each tube sees a load of Raa/2 but when one tube cuts off the other sees a load of Raa/4

Wow. Thats's news to me. Care to explain why one tube in cutoff halves the load impedance seen by the plates? No sarcasm, this is something I've never heard before and I am an information hound.

One of the benefits of Ultralinear is that it gives a smooth transition from Class A to Class B

Again: news to me. Somehow in all my reading on ultralinear I missed that one too.

Don

well I think I'm going to go with an edcor power transformer 180-0-180@250ma, a triode dynaclone output z565-48@17.5 watts and 8k primary and how about triodes marshall 50 or 100 watt choke@ 5h@120ma dc 115 ohms dc.

Don,
When both tubes are conducting you effectively have the 2 output tubes in series across the full primary winding and so each tube sees half the impedance.
When one tube cuts off it is like you have put a switch in circuit to disconnect half of the primary winding. Its turned off. The other tube is then operating into half as many primary turns - half the turns means 1/4 the impedance since impedance ratio is the square of the turns ratio.

Some guitar players can actually pick the tone / power amp compression change as they play through this Class A to Class B transition and its part of the reason some players (like Angus Young for example) like to use a lower power amp rather than a "Stadium Crusher".

My own amp is Power Scaled (1/2 watt to 25 watts) so that I can set this transition from bedroom levels to louder than I ever play.

Cheers,
Ian

Thanks Ian, you've given me lots to think about.

Don