True, a floating power supply is awkward to implement, usually requiring a separate transformer winding and rectifier. However, since the effective mu of a pentode or beam tetrode is very high, usually several thousand, both the line regulation and output impedance are very good.

  In this case, assuming that R1 + R2 >> RL, the DC regulation and output impedance is:

Kr = [(rp + Rs) / RL] + [mu * R1 / (R1 + R2)] + 1
Zo = [(R1 + R2)(Rs + rp)] / [R1(mu + 1) + R2 + Rs + rp]

If C is used, then the AC behavior is the same as for fig. 2.3.

   If good behavior at both AC and DC is desired, then a stable DC reference voltage can be used to supply the grid. This can be from some external source or can be from a voltage reference dedicated to the regulator. Common references are constant-voltage shunt devices, described in more detail in the shunt regulator section of this series. For tube regulators, either zener diodes or gas reference tubes can be used. Zeners are more compact, but have a noticeable voltage drift with temperature and tend to be unreliable in tube circuits (they are susceptible to short-out from high-voltage transients). Gas tubes, especially the reference types such as the 5651 or 0G3/85A2, are actually more stable with temperature and are quite reliable. In fig. 2.5, resistor R is chosen so that Iref is within the proper operating range for the gas tube (1.5mA to 3.5mA for the 5651) over the full range of expected input voltages. Vout is the Vref plus the bias voltage for the tube. Using the example with the 5687 above and using an 85V 5651 reference tube, the output voltage would be 85 + 6 or 91V.

  A compromise that improves only the AC behavior is to capacitively bypass the screen grid to the cathode, as shown in fig. 2.7. The time constant of capacitor C and R in parallel with the equivalent screen resistance (Vg2 / Ig2) must be well below the audio band. Vin must be high enough to support a reasonable screen voltage, so the advantage of a low drop-out voltage possible with a pentode is lost, unless R is sourced from a separate, higher voltage than Vin. Even so, some of the input voltage noise gets passed to the output through the bypass capacitor, C.


In the next installment: Pass Tube Performance


References:
Greenwood, Holdam, Macrae, Electronic Instruments, McGraw-Hill, 1948, pp. 531-532. Part of the MIT Rad-Lab series, there is an excellent section on power supply design.

Landee, Davis, Albrecht, Electronic Designer' Handbook, McGraw-Hill, 1957, pp.15-32 - 15-34. A good handbook with a bit more theoretical treatment of regulators.

Tung-Sol Corp., Tung-Sol Premium Tubes, 1956(?). A small tube manual that has a detailed 5687 data sheet.

  This type of regulator is somewhat limited because the output voltage is tied to the reference tube voltage (although several tubes can be cascaded in series). A more flexible, higher performance series regulator will be described in the next installment.

  One improvement, though, which also is applicable to the more complex regulator types, is to use a pentode or beam tetrode as the pass tube. Of course a triode-connected pentode can be used, but it behaves just like a triode. To utilize the higher effective mu and low minimum plate voltage of a pentode, the screen needs its own floating power supply, as shown in fig. 2.6.

pg. 6

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