In order to bias the cathode more positive than the grid (which makes the grid more negative than the cathode), place a resistor in between the cathode and B-. If the grid is being used as an audio amplifier, and is capacitive fed, place a resistor from the grid to B-, from 500K to 5Meg. The resistor will send the negative current to the grid without shunting the capacitor to B-, which would short out its signal. Since the control grid of a tube normally does not draw any current of its own when it is biased negatively, a very high resistance will still deliver the full negative voltage to the grid. Unless you use a vacuum tube volt meter, you will not be able to accurately measure this negative voltage at the grid. Using a standard 20,000 ohms per volt meter will not yield accurate results in this case. THE RESISTOR YOU SELECT WILL AFFECT BASS RESPONSE ALONG WITH THE CONDENSER VALUE. Choose both values accordingly. For maximum sensitivity, use the smallest capacitance possible for good bass response with the highest resistance possible without severe distortion.
Often capacitive fed triodes used for audio purposes other than output can be biased using the automatic bias method, which employs no cathode bias resistor, but instead utilizes a natural phenomenon of electrons collecting on the grid that pass by from the cathode. Excess electrons are leaked off through a special high resistance resistor. For such a circuit, use a resistor from 1 to 10 meg, and feed the grid with a capacitor from .003 to .01 MFD.
For all of the above audio circuits, if the signal is going to be fed to another stage using capacitance, use a resistor from 500K to 1Meg to feed B+ to the plate, and use a capacitor of about .01 MFD. Choose resistance for maximum sensitivity without distortion, and capacitance for maximum bass without undue distortion.
If your tube is being used as an audio amplifier and is being transformer fed, tie one side of the secondary directly to the grid, and connect the other side of the secondary to B-. No resistor is needed. Negative bias will be fed through the transformer coil. Accurate bias measurements can normally be made in this type of circuit with a standard meter.
If your tube is being used as a radio frequency amplifier, and is being transformer fed (radio frequency transformer, such as an antenna coil or RF coil), follow the same procedure as above for transformer coupled audio. If a grid leak is used, the grid leak resistor will supply the tube with the negative bias. Normally in RF circuits, a cathode bias resistor is not used. There are some circuits that call for one. If one is used, making it adjustable will allow adjustment of sensitivity. The more positive the cathode is with respect to the grid, the less sensitive the receiver will be. In a regenerative receiver with a grid leak, the grid automatically biases negative from electrons collecting on it that pass by it from the cathode. Excess electrons are accurately removed by the grid leak, and so an appropriate negative bias is maintained. A cathode bias resistor is normally not necessary in such a circuit.
Thomas