Since the grid draws very low current, a high value resistor and capacitor can be used to provide a very slow turn-on. This technique (using power MOSFETs) was used by John Nunes in the design of some recent Dynaco and MFA amplifiers. He called this a power supply "buffer". When a tube is used, the Thevenin equivalent resistance of R1||R2 should be less than the fixed-bias maximum grid resistor spec, otherwise the output voltage can drift if the tube has any residual grid current.
At frequencies several times above the time constant of R1||R2 and C, the grid of the tube is essentially stable for AC. For AC, the line regulation (AC input noise / AC output noise) is:
kr = [(rp + Rs) / RL] + mu + 1
rp = plate resistance of tube (at operating point used)
Rs = source impedance upstream from regulator
RL = equivalent load resistance = Vout / Iout
mu = mu (amplification factor) of tube
The output impedance (AC output voltage variation / AC load current variation) is:
Zo = (rp + Rs) / (mu + 1)
If Rs is much less than rp, Zo is approximately:
Zo = rp / ( mu + 1) or approximately 1/gm.
The voltage divider resistors R1 and R2 should be chosen so that the grid of the tube is less than the desired Vout by the expected bias voltage for the tube. This can be obtained from the curve of the tube. As an example, if Vin is 300V, Vout is 150V, and the load current is 15 mA, then for 1/2 of a 5687, the bias voltage would be about -6V. At this operating point, the gm is 7000 umhos, the rp is 2.5K and the mu is 17. If Rs = 0, the output impedance would then be 1/gm or 145 ohms. AC noise on the input is reduced by a factor of 18.5 or 25 dB.
The capacitor C in fig. 2.3 isolates the voltage reference from the input for AC, but the circuit has no immunity from slowly changing DC input voltage variations. A way to improve the DC and very low frequency performance of the cathode-follower regulator is to take the reference voltage divider from the output instead of the input as shown in fig. 2.4.