Pleiades bias by light falling on tubes

It was observed on date time code 13031612 on a double triode long tailed pair mic head amp (modified Pleiades V(-2)?) that:


Light falling on the 12AU7 electron tube increases gain.


The grids were floating.


This circuit started off on around 2000 using a UCC85 powered by 48V, 4x12 volt small lead acid batteries. Both heaters and anode circuit was powered by 48V. A rock band recording with a new member, Oratis, had been made with a stereo pair of this amp and 2 AKG C12A, see earlier posts. An unmodified at that time Philips CDR 870 was directly connected to its line in. So the signal path of the nice sounding rock jam was 2 (3 really) electron tubes per channel, 7586 Nuvistor and both sections of a UCC83 per channel. Driving domestic line in, gain was enough to record but of course digital 0VU could not be reached. Unless perhaps musicians played too loud. The mics were a Blumlein pair capturing the whole rock band.  Mics were at the centre so instruments around the Blumlein mic pair in a reverberant room.


Then after years it was converted to ECC82 with 12V operation for both heaters and anode circuits.


The signal path of the amp is as follows:


Mic drives input transformer.


The secondary is connected to both grids.


The primary of the output transformer is connected to both anodes.


The center tap is connected to Vb.


Vb (-) is connected to cathodes.


That's all folks.


It has a loud output of the order of a high output (for reference presure) condenser mic when a moving coil microphone is connected. A microphone preamplifier normally follows after this booster mic amp.


All kind of mics can be connected to it.


When light is falling the gain increases.


It seems logical.


The grids are free to assume their potential.


Because cathode is emitting electrons it becomes positive. So the grids are negative with respect to cathode. See earlier posts on just connected hater voltage to any electron tube.


When light is falling the grids are loosing electrons too possibly because of the photoelectric effect. See previous experiments on connecting just a very high Z voltmeter (1GΩ) across cathode and grid of an electron tube under light.


So under light the electron tube becomes less negative biased, closer to 0 volts and the electron tube or electronic valve can operate at higher gain with the small anode voltage.


Avg or automatic voltage gain could be made this way, in a similar concept to a Urei compressor.


Of course Pleiadss bias can also be supplied by connecting a high Megohm resistor from each anode to each grid.


The operation of such amps with Pleiades bias is high gain and ultra low noise at or because of very low anode voltage.


See also Pleiades V6 schematic, Pleiades bias.