Ah yes of course. Drawing a line horizontally at 20mA on the 12B4 plate characteristics chart I see what people say about the great linearity the 12B4 exhibits.
Is the voltage drop across the CCS constant? The designs aims for 120V B+ and the plate voltage is 90V, does the CCS always drop 30V. Could I make my B+ 150V and the plate voltage would end up 120V?
What role does the plate voltage play in the bias? I would think the plate voltage would determine how far the signal can swing.
Any 2 of plate current, plate voltage, and grid voltage define the 3rd parameter. The CCS "eats" extra B+ volts, as is necessary. A problem occurs, when the B+ supply is not "tall" enough to operate the CCS and yield the necessary anode voltage.
In my version of a 12B4 preamp, the 20 mA. CCS and 500 ohm cathode resistor force the grid to be at -10 V., with respect to the cathode. Therefore, the potential on the plate is defined by the tube's characteristics. You want a comfortable margin of B+ volts, to ensure correct operation, without dissipating excessive amounts of power in the CCS.
In my version of a 12B4 preamp, the 20 mA. CCS and 500 ohm cathode resistor force the grid to be at -10 V., with respect to the cathode. Therefore, the potential on the plate is defined by the tube's characteristics. You want a comfortable margin of B+ volts, to ensure correct operation, without dissipating excessive amounts of power in the CCS.
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