10-09-2021, 06:40 PM
I haven't had any problems with microphony, or with screening, of 1j24b's.
I did use a turn of 0.002" copper foil with a grounding lead soldered to it, around me 1j24b's in a 2-stage 470kHz IF amplifier (the valves being 2" apart - I just measured it!), but on looking at the sweep response and taking off the foil, there was almost no difference - and the very slight difference there was, could be nullified by a slight tweak to the anode tuned circuit IFT, which I'm therefore putting down to slightly increased capacitance direct from anode to foil.
This is obviously electrostatic screening, rather different to the steady magnetic screening which a mumetal shield would provide, and Amie's findings are not to be ignored - but it does seem unnecessary in my circuit.
I did use a turn of 0.002" copper foil with a grounding lead soldered to it, around me 1j24b's in a 2-stage 470kHz IF amplifier (the valves being 2" apart - I just measured it!), but on looking at the sweep response and taking off the foil, there was almost no difference - and the very slight difference there was, could be nullified by a slight tweak to the anode tuned circuit IFT, which I'm therefore putting down to slightly increased capacitance direct from anode to foil.
This is obviously electrostatic screening, rather different to the steady magnetic screening which a mumetal shield would provide, and Amie's findings are not to be ignored - but it does seem unnecessary in my circuit.
