Both are an issue.
If it is floating I guess the capacitance is not an issue, usually. But the air
flow thru the base is enough to dissipate the
heat at the seals and cool the glass some. Most of the radiant energy is short
wavelength IR and passes thru the glass.
It will heat the metal shield and some will reflect. Now since the metal shield
is circular it the heat reflected will go straight back
to the plate. If you keep the seals cool , maybe add a fan on the side to blow
some air across the plate and envelope you
should be OK.
I love to see bright red glow of the plated. That is why sometimes I prefer
to run 4-250A's instead of 4-400A's.
They glow better. I rember sometimes seeing the shadow of the gird on the red
glow of 250TH tubes years ago.
I thought that was neat. That was when I ran a pair of 250TH tubes in grounded
grid (-60v grid bias) driven by a pair
of 6146Bs with a CE 10B exciter.
73
Bill wa4lav
________________________________________
From: amps-bounces@contesting.com [amps-bounces@contesting.com] On Behalf Of
Roger (K8RI) [sub1@rogerhalstead.com]
Sent: Saturday, December 25, 2010 11:48 PM
To: amps@contesting.com
Subject: Re: [Amps] Cracked Chimney
On 12/25/2010 9:18 PM, Ron Youvan wrote:
>>> I have an obviously commercially made 4-1000 chimney that is aluminum,
>>> so that could be fashioned. Shame to imprison a beauty such as a 4-1000.
>>> Using a metal chimney might bring some comments out of the woodwork,
>>> though, after all the discussion on heat and reflections. At least it
>>> might be easier to fabricate.
With the metal chimney I'd be more concerned about stray capacity than
absorption or cooling.
>>> Remember, I am just reporting - not advocating, although I do plan it
>>> use it in my unfinished 4-1000 amp.
> Several commercial cavities that I worked on had a very thin "fiber glass
> + epoxy" cylinder that
> dropped down around the fins, it was a sheet that was glued into a cylinder
> with a white cement I
> was once told it was done with "dental cement." ??? They were 1/32" thick or
> thinner.
> G-10 has been known to support combustion, most of the fiber glass + epoxy
> that I have found near
> high Voltage was made with a white hard glue that looked like white epoxy,
> but obviously is different.
There are many ways of making or repairing chimneys. I was not joking
when I mentioned earlier about a ceramics class. A non conductive
chimney made of fired clay would be quite practical and with a bit of
reinforcement quite strong. Another is the chimney made of silicone
rubber wrapped and glued using silactic RTV (TM). These chimneys are
durable, long lasting, and rugged. Fiberglass and resin OTOH are not at
all rugged when heat is taken into consideration. it depends on the
resin used as to the "critical temp", or softening point. Vinyl Ester
which has a much lower viscosity (is thinner or more runny) allows for a
thicker cloth or more dense weave making a much stronger layup as well
as having a notably higher critical temps. Dow Derakane is one of these.
None these make a really good replacement for a glass chimney for what
should be obvious reasons. The glass transmits radiant energy freely.
Silicone rubber and fiberglass both absorb this energy almost as
readily. That means they need a wider spacing from the tube envelope and
more air to keep the new chimney cool. How well the forced air cools
the chimney remains to be seen, but fiberglass, silastic rubber (TM) and
even ceramic used on a glass tube is an entirely different proposition
compared to using them on external anode/metal tubes.
I think the earlier suggestion make by some one to use cut large glass
jars such as pickle jars is probably both the best and cheapest approach
although it does take a bit of practice to make a good one. Done with
care you can get the straight sides and slight curve in at the top to
cool the anode seal..
73
Roger (K8RI)
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