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AC-Coupled
Cathode Follower Accordion
Once again, we have a two-triode driver stage. In the schematic below, we see once again a cascode-like driver circuit. The bottom triode controls the gain for both the top and bottom output tubes. With only one current path available to both driver tubes and 50k plate resistors, the signal voltage developed across these plate resistors must be equal.
The top output tube is configured as simple cathode follower, with cathode following grid. The coupling capacitor and two-resistor voltage divider establish its DC operating point. The bottom output tube is also configured as a cathode follower in that 100% of its plate's movement is returned to its grid via the top 5965. For example, if a positive pulse is applied to the output transformer's secondary, that pulse will be reflected back into the primary, expanding the voltage across it, and causing the bottom output tube's plate to move down in voltage. This downward movement is relayed to the top 5965 and it too moves its cathode down in voltage, which in turn forces the end of its 50k cathode resistor down in equal measure, turning off the bottom output tube to the same degree as the top output is turned off from having its cathode move more positive relative to its grid. Thus, with both output tubes functioning as cathode followers, both output tubes buck that voltage expansion across the primary.
Note, the bottom driver tube must not have its cathode resistor bypassed, as this tube's rp must be effectively increased to limit the voltage divider action between it and its plate resistor. (In fact, more unbypassed cathode resistance or even a pentode might be needed.)
Especially note that both driver tubes cannot be located in one tube envelope. The voltage differential between the top and bottom driver tubes is a whopping 550 volts, far in excess of any tube's cathode-to-heater rating; two separate envelopes, two separate heater power supplies.
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