In fact, one way of looking at this topology is to see it as a means of making a pentode act more like a triode and making the triode act even more triode like. Let's review the basics. Normally, a pentode is extremely sensitive to variations in grid and screen voltage, but almost completely insensitive to variations in plate voltage, thus the pentode's high output impedance. The triode, on the other hand, is sensitive to plate voltage variations, as those variations will result in real variations in plate current. Put mathematically, the plate is 1/mu times as effective as the grid in controlling current flow through the triode (the cathode is [mu + 1] times as effective as the plate); and considering that a 300B has a mu of about 4, even being only one fourth as effective is something substantial.

The triode's plate, very much like a resistor, will resist an increase in plate voltage by increasing its current conduction and will resist an decrease in plate voltage by decreasing its current conduction; thus, the plate's opposition to voltage variations defines a negative feedback mechanism. In other words, the triode already enjoys a short, fast feedback loop from its plate to its cathode.

Applying a short, fast feedback loop to a pentode will bestow a much lower plate resistance to the pentode, while still retaining the pentode's wider useable current and voltage swing. Yes, it is a little like having one's cake and eating it, which was, if you will remember, the promise made by using an ultra-linear topology. (In many ways ultra-linear is a bestowing of triode like characteristics onto the pentode, while still preserving the pentode' features.)

However, nothing comes for free and the price to be paid for this enhanced performance is that transconductance will be needed to fuel the feedback mechanism. But before jumping to far ahead, let's start with the circuit's defining features and functioning.

Beginning with the feedback loop, but in greater detail, imagine either a triode (say, a 300B) or a pentode (say, a 350B or EL34) whose grid was shorted to its plate via a coupling capacitor, as shown in the following schematic. As this circuit is now configured, it is not an amplifier, instead and in terms of AC signals, it is a virtual resistor of sorts, and a low valued resistor at that. For example, a +1 volt pulse in plate voltage will be immediately relayed to the grid, which will prompt the tube's entire transconductance to counter this voltage by increasing the tube's conduction.