As requested, here is the DC Balance circuit I devised for the HF-81 project of this thread. In reality however, the basic configuration could work with any of the small push-pull 6BQ5 amplifiers from Eico, and many other manufacturers for that matter. Depending on the operating voltages in the output stage, the 165 ohm resistor might require adjustment to maintain the original current flow of a given design, and of course other designs may use other values of grid return resistors than the ones shown here (330K), but the basic configuration will work just fine.
For push-pull-parallel designs like the HF-30, the design will still balance the quiescent current flow through the primary winding of the OPT (which is the important thing) as long as the two tubes on each side of the push-pull signal are treated as "one" tube. However, the design won't balance the two tubes to each other that are on each side of the signal. That would require basically a duplication of this circuit, plus the addition of third high wattage pot to achieve a perfect balance between all four tubes. Such designs are best implemented when fixed bias is used.
Dave
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