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RE: The caps in question

If you examine this stage in isolation, a much smaller cap could have been used. The input impedance at the cathodyne is even higher than 1 meg, due to being bootstrapped. However, achieving good stability in a pentode design with 20 dB or more of global feedback requires every node to exhibit as little low frequency phase shift as possible. The cathodyne won't draw grid current under normal conditions, because output power is limited first by the output stage grids. I have to assume that because of this, the designers believed a large cap at the driver grid would do no harm.





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