06-07-2020, 05:04 PM
See, THREE valves.
Basically you are replacing a diode with an EXTRA valve.
It's only an advantage if a valve is as cheap as a diode (which they were once) AND you have no RF gain control or large signals.
The transformer wired as dual winding common mode choke is in series between the filament f+ and f- and the f+ supply AND 0V.
An RF one allows you to use the filament as a signal in (capacitors to f- & f+) in common grid mode.
An RF or audio one allows a cathode follower, for signal out via capacitors from f- and f+. The transformer or common mode choke allows the filament current but "blocks" RF / IF / Audio depending on value of series inductance. Admittedly at Audio, with 11mA, the winding resistances to have a decent inductance might mean a higher LT voltage.
It could have a 3rd winding to step up the voltage.
Really it's practical down to maybe 19kHz for a multiplex decoder and not for audio. Also you always, always need an extra valve, because the valve is replacing a diode.
Actually in Trevor's original circuit, the capacitance of the diode and the massive high input impedance might mean that the valve is biased nearly off by space charge and the positive AC through the diode capacitance is more amplified than negative AC. You might get nearly the same result as an "Infinite impedance" detector, but with real gain, just by a 10M Ohm resistor to -1.4V alkaline button cell or -2.9V lithium coin (depending which has the 1j42a nearly off). The cell will have shelf life. Coupled with the capacitor and no diode at all.
Basically you are replacing a diode with an EXTRA valve.
It's only an advantage if a valve is as cheap as a diode (which they were once) AND you have no RF gain control or large signals.
The transformer wired as dual winding common mode choke is in series between the filament f+ and f- and the f+ supply AND 0V.
An RF one allows you to use the filament as a signal in (capacitors to f- & f+) in common grid mode.
An RF or audio one allows a cathode follower, for signal out via capacitors from f- and f+. The transformer or common mode choke allows the filament current but "blocks" RF / IF / Audio depending on value of series inductance. Admittedly at Audio, with 11mA, the winding resistances to have a decent inductance might mean a higher LT voltage.
It could have a 3rd winding to step up the voltage.
Really it's practical down to maybe 19kHz for a multiplex decoder and not for audio. Also you always, always need an extra valve, because the valve is replacing a diode.
Actually in Trevor's original circuit, the capacitance of the diode and the massive high input impedance might mean that the valve is biased nearly off by space charge and the positive AC through the diode capacitance is more amplified than negative AC. You might get nearly the same result as an "Infinite impedance" detector, but with real gain, just by a 10M Ohm resistor to -1.4V alkaline button cell or -2.9V lithium coin (depending which has the 1j42a nearly off). The cell will have shelf life. Coupled with the capacitor and no diode at all.
