[murphyv310]

Hi.
It may be quite possible to dispence with using the 1ZH37B and use a 1ZH24B or 1ZH18B. I'd say normal Anode modulation could work with the type of transformer I am using in the existing circuit. The advantage would be lower filament current and possibly lower HT, RF output would also increase.
Perhaps worth a try.
The above two types of valves are also more common and cheaper.