I did a quick search for the specs, and, if I am reading it right, Wikipedia [1] gives the filament current as being 350mA at 6.3V, a dissipation of 2.2W. As th

I did a quick search for the specs, and, if I am reading it right, Wikipedia [1] gives the filament current as being 350mA at 6.3V, a dissipation of 2.2W. As that current sums to 196A for the 560 tubes, I suspect this is what the 200A figure refers to. As the tubes are at high vacuum, they are already well-insulated, so I imagine that most of the heat loss is via infra-red radiation. I have a very vague recollection that, in thermionic tubes, the anode has to be kept reasonably cool so that it is not emitting electrons itself. I would be surprised if there are any low-hanging fruit to be plucked here, especially given that vacuum tubes were important technology for a half-century. [1] https://en.wikipedia.org/wiki/6N3P https://en.wikipedia.org/wiki/6N3P