why the 6BX7 is much more linear than the 6BL7. The ratio of the  distance of the grid to the cathode over the space between grid wires would probably yield the linearity factor of the triode.)
   The remaining weak link in our shunt regulator design is the voltage reference. What is needed is a stable, precise, low noise, temperature-independent voltage reference. Precision IC voltage references come immediately to mind, but they are seldom available in high voltages. Zeners do come in higher voltages, but are neither precise nor low noise. Of course, their noise can lessened by bypassing them with a large value capacitor. Furthermore, if the absolute value of the output voltage is not critical, then the zener might fill the need. A possible circuit using a zener voltage reference is shown below.

Hybrid Shunt Regulator

Hybrid shunt regulator using a triode with a
TI-431 at its cathode.

The two resistor voltage divider network has been replaced with a three resistor string so as to protect the resistors and minimize the voltage distortion from the resistors (there is a voltage induced nonlinearity in resistors that is ignored in most electronic design because it really only betrays itself at very high voltages). The capacitor bypasses the two high value resistors to give the TI-431 more AC feedback and to shunt the noise from the resistors. The zener is there to protect the IC at turn-on, as the capacitor must be charged up from 0 volts across its leads. The grid of the triode connects directly to ground, but if need be, could be connected to a higher or lower voltage. All in all, this is not a bad regulator.
   What is missing from this regulator is more of the triode's virtues, a softer, more musical sound. In this circuit, the IC is king: it is in charge of the current flow. Giving back more of the workload to the triode is what is needed.
   
Improved Hybrid Shunt Regulator
     The management of current through the regulator can be divided into AC and DC tasks. The triode should be in charge of AC currents; the IC, DC idle current. Bypassing the IC with a capacitor prevents it from reacting to AC signals from the output, while still allowing it to adjust the quiescent current flow to maintain an accurate output voltage; bypassing the 12.6k resistor with a capacitor also serves to shunt any signal from the output to ground. Capacitor coupling the grid to the output, allows the triode to react to unwanted AC signals at the output, while it ignores slow changes in output voltage. 
Bipolar Shunt Regulator 

Shunt regulator using a triode with a zener as a voltage reference at its cathode to stabilize its  output voltage.

   Now even if the zener were replaced with a string of precise, IC voltage references, the output voltage stability of this regulator would still fall short of acceptable for many users. A change in heater voltage, tube aging effects, and even a current of air across the triode could shift the output voltage. Locking the output voltage will require more than the triode alone can furnish. Texas Instruments makes a great little shunt regulator, the TI-43. It is a small three-pin design that can source an amazing amount of current (100 mA) for its size. Of course, it cannot survive the high voltages that the triode can, but then in a cascoded arrangement with the triode buffering the IC from the B+ voltage, there would be no need. It comes with a precise, internal voltage reference of 2.5 volts, which it uses to compare with the output voltage after this voltage has been reduced to 2.5 volts by a two resistor, voltage divider. If the output voltage drifts upward, it conducts more current until the voltage developed across the series resistor pushes the voltage down to its previous value. Conversely, if the output voltage drifts downward, it conducts less current until the collapsing voltage developed across the series resistor pulls the voltage up to its previous value.
   Placing a triode on top of this solid-state shunt regulator does not alter its functionality, only its maximum voltage (30v) and dissipation limits (775mw). (The current limit remains unaltered, of course.)
   The circuit above defines a high voltage shunt regulator.

Hybrid shunt regulator with the  triode in charge of the AC  and a
Ti-431 in charge of DC.

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