Gary Schwartz wrote:
"For cooling, copper tubing is used as the RF conductor (skin effect
only) and water is pumped through the inside."

That`s exactly the structure of the final amplifier tank coil of the
100KW GE watercooled shortwave transmitters used at RFE. The coil was
silver plated. We distilled our own cooling water and the water
circulation system was connected by Saran tubing. Though many KV were
applied to the plates, d-c leakage was insignificant.

Best regards, Richard Harrison, KB5WZI