Shunt 300B Regulators
     All the advantages that the 300B brought to the series regulator it also brings to the shunt regulator. The shunt regulator works best with load currents that remain mostly fixed, such as single-ended amplifiers and push-pull Class-A amplifiers. The problem the shunt pass device faces is that it must see the whole voltage differential that the load sees. This will greatly decrease the maximum cathode current the 300B can draw. Given the 40 watt dissipation and a 400 volt cathode-to-plate voltage, the maximum current is 100 mA. This 100 mA limit is not maximum idle current. 50 mA is closer to the real maximum idle current, as the shunt pass device must vary its conduction through the series resistor is what makes the shunt regulator to work.

    Taking the hybrid route is tougher to accomplish than it was in the series regulator. Because of the high idle current, the TL431, the obvious choice, is not useable.
    (One trick might be to use the TL783 in an wildly different configuration. I have not actually built this shunt regulator, so no guarantees are offered. But the premise seems reasonable enough: the TL783 is controlling the current through the 300B. If the output voltage climbs higher than 400 volts, the TL783's adjustment pin sees a voltage closer to its output pin's voltage. As far is the TL783 is concerned, the output voltage is collapsing so it increases its conduction to compensate for the dropping output voltage. This increased conduction will increase the voltage drop across the shunt resistor Rs and bring down the voltage.) 

    The circuit above is an example of an inverted shunt regulator. The zener works as the voltage reference. The 300B's transconductance provides the feedback mechanism to maintain the output voltage. Unfortunately, the regulator is poor due to the 300B limited transconductance.
    Improving the regulation requires more gain, which the 417A provides plentifully, in the following circuit. The 100 µF capacitor relays deviations at the output to the cathode of the 417A, which is then is in phase amplified at its plate. The plate's connection to the 300B's grid via the coupling capacitor is what drives this regulator.

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