[morzh]

Ron

For the trimmer cap, mica Dk (or Er), the dielectric constant, is twice the one of plexiglas. Which might affect the resulting capacitance range. Also I am not sure, but many plastics are somewhat hygroscopic and in absorbing water they change their Er and so the capacitance could drift. Also it can exhibit temperature drift.

As for the fotometr, I think it should go darker as the radio tunes in, as it is much easier to tell the darkest from less dark, then the brightest from less bright, as at some point the eye will get saturated and will not discern the changes of brightness well.


As for the principle, I am not sure, but could it employ saturation of the transformer or is it a saturable reactor? When the AVC signal goes down, it lessens the current via the 6J5G tube, which de-saturate the core, increases the inductance and dims the lightbulb which it is in series with.

If the AVC level goes up, it increases the tube curremty, saturate the reactor and so lessens the inductance and the bulb gets brighter.