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
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.