Tube Biasing Questions

[Delta Sigma]

It is funny to read some of the C12 biasing posts I have found on the web. 
People seem to have to label things.  It is partly cathode biased with an added filter to make it work something like fixed bias and there few other things going on if you look close. 
So it should be called C12 biasing if you need to label the biasing

I definitely need to spend some more time with the C12 schematic. To me it the bias looks fixed. The cathode is grounded and isn't the grid biased via pin 4 negative supply and isolated from the rest of the circuit by caps?

 

[Gus]

I definitely need to spend some more time with the C12 schematic. To me it the bias looks fixed. The cathode is grounded and isn't the grid biased via pin 4 negative supply and isolated from the rest of the circuit by caps?

Look closer note the 1k resistor in the B+ power supply negative side that the bias negative voltage comes from. That side of the supply is more negative than the ground node so the 1k is the cathode resistor. What is different is the ground node has been moved from "bottom" of the cathode resistor to the resistor cathode node. Note the B+ supply is from a separate floating winding on the transformer.

 

[Delta Sigma]

Look closer note the 1k resistor in the B+ power supply negative side that the bias negative voltage comes from. That side of the supply is more negative than the ground node so the 1k is the cathode resistor. What is different is the ground node has been moved from "bottom" of the cathode resistor to the resistor cathode node. Note the B+ supply is from a separate floating winding on the transformer.

So a change in plate current causes a change in the current through the 1k resistor (R8) causing the bias to change?

 

[mhelin]

In an old post I showed adding current from the filament supply to a LED in the cathode to ground section, this was shown in the square microphone schematic. I also bypassed the diode with a good low ESR cap.
I used a LED because I wanted to be able to change the heater voltage and keep the same cathode voltage if I did not want this calculating  fixed resistors in a circuit like is shown in the Oliver circuit is what I would do using a filament supply.

IMO you need extra current in the diode. I think the ones with just the diode are a mistake.
However you do this, from B+ or the filament supply make sure the supply is low noise.

Pay attention to what ever diode you use voltage drop change with forward current, some are more constant than others. You can look up specs or do a simple test with a voltmeter, resistor and a variable voltage supply.

I guess the  related posts are
https://groupdiy.com/index.php?topic=1533.msg19816#msg19816

Images are gone.

Other related posts tells about the schematic Zebra posted you and explains it a little bit more:
https://groupdiy.com/index.php?topic=631.msg8780#msg8780

Yeah, the diode biasing is interesting. If you make the current (or voltage through a fixed resistor) through it adjustable you can actually adjust the the tube bias point a little bit. I tried biasing without the feedforward helper current and the mic  was quite sensitive, had huge bass, big, ladger than life sound, but distorted badly on low notes (on acoustic guitar). Now with the current about 0.5 mA current it's more civilized, kind of different mic. I have unused polarization adjustment rotary switch on PSU so I could use it for setting the diode bias current. 

To keep the diode current constant you could take the bias current from CCDA cathode resistor, or use MOSFET like LND150 to mimic CCDA with triode connected pentodes just for controlling the current (not on audio path). Just connect the gate to anode, drain to B+ and source via 110k resistor (in case of U47 type basic tube circuit)  to cathode with the bias LED or two small signal diodes to ground. It is not so simple though as LND150 has max +/-20V gate-to-source voltage requirement, need two zeners there at least to protect the device.