Steve, the answer IS obvious (hope my shaky maths is correct here):
The area of the 6 mm tube is 28.28 sq. mm.
The area of the 6x1 mm wire is 4.7 sq. mm....
Around 8 sq. mm of good copper tube sounds about right for 1 kW on 6m
(guessing).
Also: the 6 mm tube does not suffer the phenomenon of RF eddy currents
to the same extent that plaited conductors would. Betcha that plaited
conductors of the same cross-sectional area get hotter than a monolithic
tube of material.
Did the 6 mm copper pipe also have varnish on it? Point is, some
varnishes have poor RF properties. Any substance likely to gas off when
hot is bad news, the gasses may be poisonous, in any event may be
conductive to some extent, encouraging RF arcs through the plasma.
Another point, not mentioned so far on this thread, some heating of the
series plate components is in fact desirable. Heating=resistive losses,
which effectively places some "R" in series with the output, reducing
gain (heating of components in parallel also reduces gain), one can then
rely less on the performance of suppressors, and maybe even say good bye
to Rich's resistance wire...
NOTE: THIS IS NOT A CUE TO RE-START THE GRATE
RESISTANCE-WIRE-ON-THE-ANODE DEBATE.
Ciao,
Ian ZS6BTE
Steve Thompson wrote:
>
> An experience I had while messing with a 6m anode circuit: I wanted to try a
> different coil size and fighting with 6mm (1/4") copper pipe was getting
> tiresome. Allowing for skin effect, I figured that 6 x 1mm wires in parallel
> (like litz wire) would give the same area for current flow and be much
> easier to handle. I made some twisted wire with a cordless drill and tried
> it. After about 20 secs. key down, huge plumes of smoke came out with the
> exhaust air - when the coil was removed, the varnish had burned and all
> strands had started to fuse together.
>
> Maybe I'm missing something obvious, but I can't see why the losses should
> have been much higher. The copper pipe will shift heat away more easily, but
> even after prolonged use, the final pipe coil is not discoloured, so I feel
> sure the difference was in actual dissipation.
>
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