(20-10-2023, 03:47 PM)Jez Wrote: Nice! Is that a short-wave communications receiver?
No - a LW/MW high-quality home radio.
I should also mention another project, actually a joint one with a friend (she did most of the concept work and built the bread-board amplifier; I designed and made an output transformer, and did a bit of optimisation). This basically was a low-power amplifier, just under 0.5W, using two 1j29b's in a self-splitting output stage and dubbed the Amīcamp: one valve's grid is driven; the other valve's grid tied to ground, and a single transistor constant-current source feeding the cathodes.
This was preceded by a 1j24b as voltage amplifier.
Being directed heated, the 1j29b's needed a floating filament battery. But, rather elegantly, they are biased to consume 11mA HT current which is exactly what the first-stage needs to heat its filament - so still only one LT battery is needed.
I did introduce NFB to the first stage cathode with twin tertiary windings on the O/P transformer, passing the heating current to its directly-heated filament while applying common-mode signal feedback. This worked well, and sound quality is good enough for serious listening.
Eventually, separate batteries were dumped and the whole thing powered from a little push-pull switching supply... trying AC, albeit high-frequency AC, on the output valve filaments (from a 0.6 - 0 - 0.6V winding on the power transformer was successful! We included a nulling system in case the HF appeared on the output, but (based on a sample of one!) it wasn't necessary.
Photo shows the amplifier module (the diecast box houses the first-stage 1j24b) and the power supply module, mounted on a baseboard with the battery. But sorry, I don't have the circuit diagram on my phone!
