The top diagram would be better. The bottom one puts the secondary winding
at ground potential, with full high voltage potential between windings.
That means the insulation between primary and secondary must withstand the
full high voltage dc. The top diagram keeps both windings at high voltage,
so the potential difference between them is zero, assuming the same plate
voltage on modulator and final. To be on the safe side, I always mount my
mod transformers, mod reactors and filter chokes in insulation to avoid
unnecessary stress on the transformer insulation between windings and core.
Just be careful, and treat the transformer as "hot" with high voltage, since
it would become a death trap if the insulation failed and effectively
connected the transformer core to the high voltage. I make it a practice to
avoid touching these components when the high voltage is on.
The inductance should be 8 henries for every 1000 ohms of modulating
impedance. For example, running 2000 volts @ 500 ma to the final gives 4000
ohms using ohm's law. The minimum optimum inductance would be 4 X 8 henries
or 32 henries. You could go with less, but you would begin to lose low
frequency responce and see noticeable phase shift distortion in the lower to
midrange frequencies.
Don K4KYV
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