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In the schematic above, we see a cascode-like driver circuit. The bottom triode's gm realizes the voltage gain for both the top and bottom output tubes. With only one current path available to both 10k plate resistors, the signal voltages that are developed across these resistors must match. The driver stage's triodes, however, exhibit very different rp's. The bottom driver triode's rp remains unaltered, but the top triode's rp effectively becomes equal to:
rp´ = rp + (mu + 1)(rp + Ra),
which helps it to approximate the needed infinite value. With such a high effective rp, both top and bottom output tubes present only their rp in opposition to a back-reflected pulse and both equally realize the same voltage gain. Thus, both output tubes function as amplifiers. (Notice that the bottom drive tube's cathode resistor bypass capacitor does not connect to the negative rail, but rather to ground. This arrangement is needed to prevent the negative power supply noise from being amplified. For the same reason, removing the bypass capacitor will result in the negative power supply being amplified.)
There remain two disadvantages of this circuit. The first is the huge voltage differential between driver stage's cathodes, 170 volts in this example. This extreme value makes using a single twin triode envelope less desirable. In fact, the safest route would be to use separate envelopes and to give each its own heater power supply. The second disadvantage is the lack of a means to center the output stages voltage division. What we need is a means to adjust the voltage division between output tubes. Notice that we cannot adjust the idle current balance between the output tubes (something we would strive to do in a parallel single-ended amplifier), as the tubes must share an equal current draw, because they are in series with each other. The easiest solution is to add a potentiometer to the top driver tube's input. This potentiometer will allow a small change in the DC voltage that the bottom output tube will see, which will allow the plate voltage adjustment.
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