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RE: The question is why 200 henries?

I have done a few experiments, enough to say with confidence that "more is better" with regard to inductance of a plate choke for an output stage.

A recent, specific example is increasing the inductance for a 300B amp from 24 henries to 40 henries, both with a 3000 ohm output transformer. The bass improvement is clearly audible. (This is equivalent to changing from 80 to 133 henries for a 10K output, which is typical for a Type 211 tube.)

In an older experiment, I made a preamp with a 6BL7 parafeed into a 5K output transformer; I set it up with four different inductances plus a current source, which could be selected with a switch. I no longer recall the specific inductances, but they covered about a 2.5:1 ratio. The smallest inductance gave perfectly acceptable measured bass extension, but every increase was audibly better, and the current source was still better than the largest inductance, indicating room for more inductance.

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