and dividing it by 0.0047. For example, if the cathode-to-plate voltage is 150 volts and the total idle current draw through the 300B is 105 mA (the cascode's current draw must be added), then the 300B needs to see -12.5 volts, thus Ra must equal 12.5/0.0047 = 2660 ohms. The best move would be to replace the plate resistor Ra with a constant current source, as a current source will help realize much more gain. A tube current source is not possible here, but a FET current source such as the 1N5314 or CRO-470 will work well.
   Since adding a solid-state current source has cracked the hybrid door ajar, let's swing it wide open. Texas Instruments makes a great little three pin adjustable regulator with a 125 volt input-to-output voltage limit, the TL783, which houses a MOSFET pass device. Using this regulator in series with the 300B gives us the best of both technologies, the TL783 is quiet and accurate and the 300B is high-voltage tough. The principle of operation is simple enough. The 3 pin regulator strives to maintain a constant voltage output by increase and decrease its conduction. As the TL783 responds to voltage fluctuations at its output, the 300B sees a varying cathode-to-grid voltage, which allows the 300B to track the IC regulator.

    One problem with this circuit is the input-to-output dropout voltage limit for the IC regulator. In order for TL783 to work, it needs to see at least 10 volts across its leads at 100 mA of current draw. This means that the 300B's grid can only get to within -10 volt of its cathode, which will impose a dropout voltage of 125vdc. Not that much headroom, in other words. Only a 5% drop in the 560 volt power supply can be allowed before the regulator starts to complain. Of course, the lower the total current passed, the smaller the dropout voltage. For example, at 50 mA, the raw power supply can drop 11%, without the regulator falling out of regulation. Still the -10 volt grid voltage sets limit.
    The solution is to pre-bias the grid positively so that a o volt or even a positive grid voltage is possible. The circuit below shows one way to achieve this end.
    Now the raw power supply voltage can drop 20% from its nominal 560vdc value and the regulator will still maintain regulation.

   For greater than 100 mA loads, the 60k resistor that connects to the 300B's grid can be replaced with a 20 volt zener; the .33 capacitor is removed; and the 100k resistor is replaced with a high wattage 1k resistor. This setup delivers the bulk of the output current from the zener and the 1k resistor's path. The 300B handles the difference. Such a only works well when the load current never drops below a threshold value; in this case, it is about 130 mA.

    The zener provides a discharge path for the output capacitor and a means of protecting the IC in the event of an over-voltage from short-circuits and a blown fuse.

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