See

Thanks for this very interesting post. It should definitely be adapted to a 3-500ZG (graphite anode) because the photograph shows a 3-500Z with metal anode

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So 900 V 250 mA (225 W) and the author states:

Watch the color of tube anodes. Anodes should be a very faintly detectable red. Do not run the tubes too hot.

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In the case of the L-7 which uses two tubes in parallel, what would be the best solution to achieve cooking? (remove one tube?)