Pentode TC - progress (fwd)

---------- Forwarded message ----------
Date: Sun, 26 Jul 1998 20:45:03 +0100
From: R M Craven <craven-at-globalnet.co.uk>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Pentode TC - progress

Here is a quick note to let people know how my work is progressing, on my
5CX1500A pentode Tesla coil.

I have finally managed to sort out a problem with my pentode powered TC
which was preventing me from developing much power in the tank circuit. Now
that that problem is behind me, as soon as i can get some time, I should be
able to set up the rest of the system.

This weekend I only had time to test my theory as to why I wasn't developing
much power. That issue is now behind me, and I started to get momentary
brush-like discharges approx 10" long, which then settled back to a reliable
5-6". The diharge was either silent or rasping/roaring like a gas flame on a
propane torch, depending on the tune of the tank/secondary.

Three things were noted which may be of general interest:

1. The tank inductor, wound from 8mm Cu microbore, got noticeably warm to
the touch after maybe 40-60s of running. I am glad I took Dave Sharpe's
advice to go for Cu pipe; PVC insulated stranded wire would have packed up
fairly quickly, I reckon.

2. The secondary also got noticeably warm within 60s, in that area which was
in the immediate vicinty of the tank coil. I feel that this is mainly due to
dielectric loss in the form and in the wire insulation and coatings. The
rest of the secondary also warmed up after maybe 3 mins, presumably due to
the high current that was being sunk into the brush discharge. BTW, that
brush discharge was energetic enough to easily melt the 0.71mm (22swg)
copper wire.

3. The topload, an 8" x 3" toroid, got very warm on its underside within
about 30s of running. This suggests that eddy current heating is
significant. The topload was made from the usual aluminium flexible hose,
with the ridges/valleys evened out using polyester filler. This was then
sprayed with Ni shielding spray before being given an aluminium foil tape

Remembering that heating effects are due to the rms values, it is easy to
see that CW coils which develop high everage powers will cuase such heating,
whereas spark-gap driven TCs tend to have lower average powers for the same
input power. In those cases the heating effect is lessened.

Point no. (3) above is interesting: I remember a couple of years ago when
the issue of "shorted turn" toroids arose, the excellent debates that sprung
up. I seem to remember that Greg Leyh used a slotted toroid on his 160kW
system which he later felt wasn't necessary and did various experiments to
show that the eddy current losses were small. Many people contributed to the
debate and supported the conclusion that slotting the toroid was an
overkill. Well, maybe after all it is necessary (for CW coils  but not spark
gap driven ones).