I am using diodes to bias tubes for the input stage of an audio circuit. They will hold the cathode off ground by their forward voltage (Vf). Current will probably fall between 1 and 2 mA, and the voltage will be set by the diode's Vf. I am aiming for .8V-1.6V total, and I can have diodes in series to obtain the right bias voltage (thought multiple junctions tend to raise the noise level). Peak-to-peak AC will be in the 250mV range.
I would like to know if anyone is aware of low noise, low distortion diodes that will have minimal effect on the audio signal passing through them. I've experimented with several types, Schottkeys, 4007, 914, etc. The Schottkey rectifier diodes and the 4007s are best so far. Of course, at 1mA, their rated Vf is quite different from the specs that are usually published. Only the most complete spec sheets show Vf below 10mA or so. The Cree 6010s (rectifiers) came in at .8V and the 4007s came in at .5V.
I realize that most people don't use diodes in this application, so information has been hard to come by.
imagine a triode tube, the anode is fed 155V through a 75K plate resistor, the grid is fed from the output of a microphone capsule and has 200Megs to ground. The cathode has two 4007 diodes in series to ground. Signal is taken from the anode, through a capacitor to the an output transformer. This results in 1V of bias at at the cathode, 75V on the anode, and 1.5mA of current flowing through the tube. The diodes each provide .5V of the total bias voltage and are, themselves, biased by the 1.5mA of current. The diodes never see negative voltage