18-09-2021, 05:47 PM
(18-09-2021, 02:25 PM)Mike Watterson Wrote: Naturally direct filaments can't use a cathode resistor...
But whyever not? I have an Ever Ready model 5033 which uses a directly heated triode (S30D) as output valve. It has its own filament winding on the mains transformer with the 'cathode' taken as centre-tap of this - with a resistor and bypass capacitor to chassis!
A friend has been experimenting this week using 1j29b's fed from AC using a few turns thrown onto a toroidal mains transformer, with encouraging results. This allows the 'cathode' connection to be elevated easily.
I've wondered about how the 1p24b would cope. With its thicker filament, temperature variation with AC will probably be a comparatively minor matter. There's still the voltage variation along the filament, but if 1.2V operation with the two halves in parallel is used, no part of the filament will be more than 1.2V x root-2 (= 1.68V) away from the mid point, at any part of the cycle. And as the 1p24b does not have a very sensitive grid, I'd expect this to make little difference. There's only the magnetic field due to filament heating current, within the valve, left to consider. And, putting as many filament windings on a toroid, each at just 1.2V, is the work of a few minutes each!
