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I suspect that the weirdness is intentional. Weird designs have existed from? the earliest day of DIY radio receivers, before 'electronics'? was in the dictionary. I recall a report of a circuit that had the 2 V lead-acid cell apparently in series with the antenna circuit. Objective performance measurements are typically not to be applied to such designs. DO a web search for 'Bravo Audio reviews'.

Le 20/02/2025 à 15:27, Carlo a écrit?:

On Thu, Feb 20, 2025 at 05:40 AM, Andy I wrote:

There is some feedback from the audio signal into the heater voltage. Was that intentional?? Or just an undesirable side-effect?? I don't expect it would have very much effect on the heater's temperature (and from there to the triode's characteristics), but it looks undesirable to me. Should there be filtering?

Sorry, are you asking whether the audio signal feedback into the heater voltage comes from a design intentional choice ? Actually I don't know since I took it from the schematic of a commercial audio amplifier (Bravo Ocean).

It's extremely unlikely.
Heater temperature varies extremely slowly compared to audio signals.
It could result in distortion at very very low frequencies, definitely out of the audio band.
Now this design is weird from the start. Choosing to power a tube circuit from 24VDC is a major flaw, unless the goal is to create distortion.