topology (a highly desirable feature, as the signal amplification will not alter the amount of current being sourced from the power supply and consequently not perturb the power supply, thus greatly simplifying the design consideration of the power supply).

5. The Twist
So far the circuit looks like a dozen others that have appeared in the tube-audio press albeit with the difference that the resistors were chosen to insure constant current draw from the circuit as a whole. Still, a Grounded Cathode amplifier cascaded into a Cathode Follower...what could be more obvious? Here is where the twist comes in: the Cathode Follower's output is not taken directly at its cathode, but rather at the junction of resistors R4 and R5. The cathode resistor R4 increases the output impedance of the circuit and effectively decreases the tube's transconductance, much in the same way an unbypassed cathode resistor decreases a tube's transconductance in a Grounded Cathode amplifier. The addition of this resistor will go a long way to overcome one of the most commonly voiced criticisms of the Cathode Follower: it don't sound as good as does the Grounded Cathode amplifier working with same tube and power supply voltage and idle current.

To understand what is going on here, let us look at the Grounded Cathode amplifier first. When the cathode resistor is left unbypassed, the input overload voltage is increased by the percentage of decrease of the tube's transconductance. This is so because the input overload voltage is largely defined by the quiescent or idle current of the tube divided by the tube's transconductance. Thus, the greater the idle current or the lower the transconductance, the greater the overload voltage. Here is one reason why tubes usually sound better than transistors: tubes have substantially less transconductance than transistors; which means, given the same idle current, the tube will have a substantially greater input overload voltage than the transistor. Furthermore, the unbypassed cathode resistor serves to increase linearity of the circuit by straightening the tube's transfer function.

In this circuit, both the Cathode Follower's input overload voltage and linearity are increased by the additional unbypassed cathode resistor, but at the cost of a worse, i.e. higher, output impedance.  One way to look at it is to imagine that at the tube factory someone had soldered a small fixed resistor in between the cathode and the external base pin. This "new" tube would trace a set of plate curves on a curve tracer that would exhibit much less transconductance, much greater linearity, and a higher rp than the normal tube.

A further advantage to the extra cathode resistor in the Cathode Follower leg is that it will  counterbalance the effect of the load impedance shunting the normal cathode resistor, which will  restore a constant current draw to the circuit as a whole.

6. What does the diode do in this circuit?
This diode does not do anything during the normal operation of the circuit. It can't; as the diode is so placed that the cathode would have to be at some lower voltage than is the grid, which under normal operation does not happen. The tube (being a depletion mode device) conducts current in spite of the grid being negative relative to the cathode, which is the basis for cathode biasing, or as it is sometimes called "auto biasing." If the grid were to become positive relative to the cathode, however, the diode would conduct and the greatest voltage difference between the grid and cathode would equal the voltage drop across the diode, which is usually between 0.3 to 1.2 volts. A situation that could happen if the grid were driven with an excessively large input voltage or if the B+ voltage were established and the tube remained too cold to emit electrons. The latter situation is what usually happens every time a fast solid-state power supply comes up to full voltage before the tube's cathode has had time to heat up sufficiently to conduct current. In this circuit at startup, without the diode, the