insurance.  In other words, the simple Cathode Follower functions just like the more complex versions and differs in having a smaller valued cathode resistor and the DC ground level coupling at the input.  The only aspect of this design likely to raise an eyebrow is the seemingly high value of the cathode resistor, compared to what it would be in a Grounded Cathode amplifier with the same tube. This higher value is explained by the omission of the plate resistor which, if present, would have lowered the plate voltage by the voltage drop across itself and when absent, the higher plate voltage requires a greater negative grid-to-cathode voltage to bias the tube to the same idle current. It is this fairly high valued cathode resistor that allows the simple Cathode Follower to swing a few volts before hitting the ground. This means that the Cathode Follower will exhibit a very asymmetrical potential voltage swing. Its output can only swing negatively the amount of voltage that is developed across the cathode resistor at idle. On the positive swing, the upper limit would be the onset of positive grid bias, which probably would yield a positive voltage swing twice that of the negative. 

Easy Pentode Circuit Description
The pentode version differs from the triode by the necessity of supplying grid 2 a reference voltage.  A resistor and a capacitor define the easy solution; an additional Cathode Follower and supporting components, the complex solution. Whatever the approach, the better filtered the voltage at grid 2, the

lower the noise at the output. This is one advantage a pentode enjoys: grid 2 only draws a fraction of the current that the plate does, yet it effectively overwhelms the plate in current controlling ability, which means that regulating the voltage to grid 2 yields the same benefit as regulating the whole B+ power supply. As the Cathode Follower already enjoys a very good PSRR, the need to optimize further for low noise might be questioned. When it comes to noise, however, less is always better. And

Pentode with triode configuration

as tubes with both a triode and a pentode are readily available (6AN8, 6AL8, 6AU8, 6CM8, 6CU8, 6EM8, 6GH8, 7199), adding the additional Cathode Follower is not that daunting a task, as it requires only two extra resistors.

Of course, for those who cannot abide pentodes, grid 2 can be attached to the plate, which would turn the pentode into a triode. Usually, a 1K resistor is used to bridge grid 2 to

the plate as a safety measure, as this resistor will current limit the flow through grid 2, which can survive only a fraction of the current that the plate can handle.

Suitable Tubes
Triodes:  6BL7, 6X7, 6J5, 6SN7, 12B4, 12BH7, 5687, 7119.
Pentodes: 6BQ5, 6AC7, 6CW5, 6V6, 12BY7, EL86.

The best choice of 9-pin triodes, if the heater current demands can be met, is the 5687. This tube, with a 200 volt power supply and a 1k cathode resistor, will draw about 10 mA of current. Here the 5687's