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Originally Posted by Neil Purling
At the moment I believe that I am not driving the existing 6V6f ully. I think that four watts is quite enough for a radio. To fit a EL33 or whatever surely i'd need another o/p transformer. I think the present RS item is only rated up to 4W.
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Neil, you are mixing up slope and power ratings. You won't need more than a couple of watts from a radio like this whatever output valve you use. The power at which the valve operates is determined by the bias, which is set by the cathode resistor - the bigger the resistor, the less power the valve will dissipate and the less audio output will be available if the valve is fully driven.
It is very likely you're not driving the output valve adequately at present. An EL33 is a higher slope valve than a 6V6, meaning it'll produce more output for the same input (simplifying a bit). It's got more gain. Many short superhets of the 40s and 50s didn't have an audio preamp stage and relied on a high slope output valve to produce enough output. A 6V6 type valve wouldn't have produced enough output in these designs because the slope would have been too low.
The same output transformer would be OK with an EL33 so long as the cathode bias resistor was adjusted to limit the valve current.
Best regards, Paul