31-01-2021, 09:46 PM
Some more developments to the 1p24b UL amplifier: negative feedback has been incorporated.
The scheme I outlined in post #36, of subtracting an (isolated) replica of the output signal in series with the input signal, worked OK with a low impedance source. But with open-circuit input, or high-resistance source, the circuit took off. The reason is fairly obvious, capacitive coupling from other windings in the output transformer to the feedback winding, exacerbated by the grid being a very sensitive, high-impedance point.
By adding screens in the transformer, I could probably tame it, however, a circuit which has windings hanging off a ticklish circuit node point is inherently fickle does not have great appeal.
The traditional point to inject NFB is generally a cathode. Here, with DH filamentary cathodes, it is not quite so trivial to squirt a signal in, without compromising the LT heating current. However, a pair of low-resistance windings on the output transformer can introduce the same feedback signal into both sides of the filament. With this configuration, capacitive coupling from other windings in the OPT is irrelevant because one end of the feedback winding is grounded.
The result is approx. 6db of feedback, with a 2-turn winding. Circuit is as shown.
The scheme I outlined in post #36, of subtracting an (isolated) replica of the output signal in series with the input signal, worked OK with a low impedance source. But with open-circuit input, or high-resistance source, the circuit took off. The reason is fairly obvious, capacitive coupling from other windings in the output transformer to the feedback winding, exacerbated by the grid being a very sensitive, high-impedance point.
By adding screens in the transformer, I could probably tame it, however, a circuit which has windings hanging off a ticklish circuit node point is inherently fickle does not have great appeal.
The traditional point to inject NFB is generally a cathode. Here, with DH filamentary cathodes, it is not quite so trivial to squirt a signal in, without compromising the LT heating current. However, a pair of low-resistance windings on the output transformer can introduce the same feedback signal into both sides of the filament. With this configuration, capacitive coupling from other windings in the OPT is irrelevant because one end of the feedback winding is grounded.
The result is approx. 6db of feedback, with a 2-turn winding. Circuit is as shown.
