19-09-2021, 01:45 PM
(18-09-2021, 05:47 PM)Kalee20 Wrote:(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!
Obviously possible using a separate supply, battery or mains. As I implied on the o/p valve bias system on battery portables. I wasn't explicit enough. Another trick is a choke (1:1 transformer) with the DC currents cancelling feeding the filaments. Easy at RF & IF, larger at AF. Allows cathode follower or grounded grid.
VFDs often use a centre tapped filament supply to reduce brightness gradient across a panel.
