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substantially different bandwidth and waveform fidelity than its plate leads. Transformers are the loose cannons of an amplifier's design.
On the other hand, if the transformer is removed from the circuit, but the ultra-linear mode is retained, we could more fairly judge the ultra-linear topology. Because grid 2 is at a more positive voltage than the cathode, it receives some of the electron flow from the cathode. Consequently this grid requires a low impedance source no matter how it is configured in a circuit. In other words, because it is conducting current, a simple two resistor voltage divider will not work, as the current flow into the resistor would shift the grid 2 voltage too low. No, what is needed, unfortunately, is a more complex circuit.
Line Stage Amplifier
As a design example of how the ultra-linear mode can be used in a circuit that is not a power amplifier, a line stage amplifier proves useful. Of course, the technique used here can be applied to phono stages, single-ended power amplifiers, and virtually any other audio circuit. While straight gain and a fairly low output impedance are basic requirements of any line stage amplifier, we will add ultra-linear mode. Remember, the goal here is to mimic the ultra-linear transformer functionality by using resistors, capacitors, and active devices, such as triodes or MOSFETs.
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Ultra-linear operation of a pentode requires two sub-functions: a sampling of the alterations in the cathode-to-plate voltage and voltage division of this voltage, and a low impedance means of driving grid 2 with this signal.
Deriving the ultra-linear ratio of the output tube's cathode-to-plate voltage requires only a two resistor voltage divider. Ratios between 10% to 50% have been used by ultra-linear transformers. Consequently, the resistor ratio can be anywhere between 9:1 to 1:1. If we stick to Hafler and Keroes's ratio of 20% (actually, they specified 18%), the resistor ratio will be 4:1. Thus, a 400k and a 100k resistor will divide the plate voltage swing down to 20%. High value resistor were chosen to prevent excessive loading of the plate. Of course, if the plate load used is substantially lower than usual, then substantially lower values could be used for these resistors.
The obvious circuit choice for driving this grid, the Cathode Follower, is the best choice. In fact, this circuit works extremely well in this application because the current flowing out of grid 2 will flow into the Cathode Follower's cathode and then to its plate on its way to the power supply B+ connection. This means the Cathode Follower's cathode resistor need not be as low in value as might be first imagined, as the Cathode Follower will source most of its current flow from the current flowing out of grid 2.
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