Russian rod pentode circuits

[Jez]

 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!

And there was me whinging in another thread a few days ago that diy, homebrewing etc were dying  Great work there Kalee20!

Your solution to the heaters issue is ingenious. It reminds me of some all valve integrated amps and receivers from Japan in the 60's which used the output stage cathode current to give DC heater supply to the phono stage valves.