Paul,
In this case, you could stop the chimney just short of the ring, or make
the chimney about 1/8" to1/4" diameter larger than the ring and anode
fins. This would allow the air to pass on each side of the ring. The
chimney don't have to fit snug to the fins, especially with this style
tube. As long as the air flows by the fins at the correct amount, it
will cool the tube as it should. Keep in mind, there's about three small
bumps sticking out off those fins, on the top, at least 1/16"-1/8"
thick, which would require a larger chimney to fit over them anyhow.
This will actually work on all the steel tubes, a tight fit, or the
chimney to the bottom of the fins fitting tight against it, is not
needed if the chimney goes over the O.D. with a small amount of
clearance around it. This actually cools the tube more than just air
passing by the fins alone, since the air is passing over the O.D.
surface of the fin body. That makes up that much more surface area
receiving cooling.
Will Matney
Paul Whatton wrote:
Hi Will
Many thanks for the reply. I don't think I explained myself very well.
I'm blowing into the anode compartment so the chimneys run from the
fins up to the top cover. Air goes into the anode compartment and
escapes through the bottom of the fins and up through the two holes in
the top cover (with a proportion diverted downwards for cathode
cooling). So the whole anode compartment is sealed except for the two
chimney exits. The chimneys are fixed to the top cover and drop over
the tubes when the cover is put on. The W2GN/K2RIW article explaining
this method is at http://www.newsvhf.com/w2gn.html. This is different
to the "old fashioned" way I, and pretty much everyone else pre K2RIW,
used to blow tubes into the grid box using air system sockets and
conventional chimneys.
With a 3CX or 4CX tube the radial cooling fins terminate in a tube and
so all the air must go in from the bottom of the cooler through the
tube and out. But with the Russian tubes the fins are connected
together in a ring at the bottom but then have gaps at the sides up
their length. If I roll a PTFE tube for the chimneys I can make it
reach right down to the ring at the bottom, enclosing the whole cooler
in a tube just like a 4CX. Or I can make the chimney a bit shorter
giving the air two routes into the cooler, from the bottom and also in
from the sides. There's a nice picture of the GI-7b at
http://www.nd2x.net/gi7b.html which shows the cooling fins.
The conventional way of making the plate connection is to unscrew the
cooler, drop a bracket over the stud, and screw the cooler back on.
But this inevitably means some of the gaps at the bottom of the fins
are blocked. Not good. So I wondered as I was playing with the tubes
and PTFE sheet today if it would be better if the chimney didn't reach
right down to the ring, but instead stopped short and allowed some air
in throught he resulting gaps at the bottom of the sides of the fins
as well as through the bottom. Less air resistance & better air flow
through the fins blocked by the tank circuit bracket would seem to be
the advantages.
Boy, I'm not sure I articulated that all very well? Hope it made sense?
Thanks
Paul
craxd wrote:
Paul,
You can run the chimney from the chassis to the vent hole in the top
cover. It doesn't have to be under the fins like most short chimneys.
You can make one that reaches all the way to the top of the fins from
the chassis if you dont want to run it on up to the cover vent. Just
as long as the air is forced around the fins is all thats needed. You
want to make sure that the holes trough the chassis for the air flow
are large enough that the tube gets the proper air flow.
Will Matney
Paul Whatton wrote:
Hi all, I'd appreciate some advice.
I'm building an amp with a pair of GI-7bs and will be blowing into
the anode compartment and using two PTFE chimneys in the usual
(K2RIW) way venting out of the top of the case. But because the
Russian tubes have open sided anode fins I wondered if it would be
better to take the PTFE chimney right down to the bottom of the
cooler, thus creating a tunnel, or only part way down the fins to
allow some air flow through the sides as well? The latter would
certainly appear to help where the underside slots in the fins are
obstructed by the tank circuit bracket, but how about all round?
Your thought much appreciated.
73
Paul G4DCV
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