05-07-2020, 03:48 PM
Mike Watterson Wrote:Amie Wrote:Just one question though, I've seen diodes used as in Trevor's diagram occasionally, But what is its purpose? Is it acting as a clamp?No, I've never seen a single diode in an official circuit like that, because as you say there is no obvious DC path anywhere on the cathode or anode. Though I can imagine theories to explain why it works, as outlined earlier.
(There is no DC continuity at its anode so is therefore just floating?)
Agree both!
The diode is working as the detector, but doesn't have an obvious DC path.
Mike W's post #28 (quotes are painful with iPad) advocating resistor on diode input to ground, and diode output to ground, would help. Even better, I'd suggest a second DIODE to ground at the diode input (ie anode of D1), rather than a resistor, connected anode to ground, cathode to D1 anode, because then, with the 270pF capacitor, it would form a voltage doubler or charge pump (choose your name!).
Trevor has built it, and it works. So there MUST be a discharge path, and in fact there is, the leakage in D1. But this is essentially bad design as it relies on a loosely-specified, 'nuisance' characteristic of the diode. Moreover, even for a particular specimen for D1, leakage is not constant - it halves for every 10 deg C drop in temperature. So outside in the winter, I'd expect audio distortion as the leakage in the diode falls to such a low level that the grid voltage of the output valve can no longer follow the modulation envelope.
