1П24Б 1p24b

[Murphyv310]

Thanks Peter.
I did toy with the idea of using an interstate transformer, for the quick test I used an ECC83 as a phase splitter. This was certainly not elegant by any means. The result though was very pleasing, sounded nice.
Having the facility of winding your own transformers is great. Something I've never done, small chokes, RF & IF stuff is about as far as I've gone.

[Kalee20]

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.

[Kalee20]

And results of square-wave testing, into resistive load.

This is not a rigorous test of course, stability into open-circuit or 'awkward' load is not assessed, but the absence of ringing on the transitions does indicate that more feedback could be applied.

[Murphyv310]

Very good indeed Peter.
I use a 1J24B in a Pye Black Box as a phono preamp for the Sonotone 9TA ceramic cartridge. Stabilised filament and HT driven from the Amps HT line. Works very nice.

[Amie]

.............

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.

Clever idea.

Thanks for sharing Peter.

So, would a good description for how the NFB works now, be that the NFB signal is common mode and unipotenially "modulates" the whole filament within the circuit, leaving the heater power unaltered?

Thanks,
Amie.

[Kalee20]

Yes absolutely!

It's equivalent to the stage having its very own battery, and just applying the NFB between one end of the battery and 0V.

[Murphyv310]

Of course the conventional way would work with a 1j29b which has a centre tapped filament.

[Kalee20]

Well... No, because the two extreme ends of the filament would have zero feedback signal on it, and only the midpoint would have the full NFB signal on it.

Also, the heating power would increase according to signal level. I've got approx 250mV of NFB. So the filament would see, for heating purposes, total RMS voltage of root-(1.2^2 + 0.25^2) = 1.23V. Not a huge rise above 1.2V, but if signal levels increase, it would become more significant.

[Murphyv310]

I understand the calcs but if the filaments are series fed and isolated, the "cathode" (centre tap) would then return to ground via the "Cathode" bias resistor network and NFB injected as is common practice. If nominal cathode current was less than 1ma I can't see why there would be a swing that would be an issue. Is not the 1j29b filament 25ma in series connection?
With an isolated supply for filaments the AC component of the NFB would be evenly distributed across the full filament or am I missing something?

[Kalee20]

Can you add a sketch of what you have in mind, Trevor? I can't quite visualise it... Thanks!