On 12/25/2010 9:45 PM, Jeff Blaine wrote:
> I would be worried about using a plastic around this tube where the HDT (heat
> deformation temp) is unknown - the "problem" with the
> glass tubes is that a good amount of their disipation is by radiation rather
> than conduction - which means, no matter how fierce are
> the cooling winds that are blowing, the plastic is going to be bombed with a
> ton of IR from the tube.
With fiberglass and epoxy resin this is likely less than 250deg and less
than 300 using Vinyl Ester resin. Then there is the added problem of the
material being a good absorber of direct radiation from a glass tube.
73
Roger (K8RI)
> The cooling fan flow will help to keep the surface of the plastic facing the
> tube from melting in the worst case, but I have got to
> wonder what the long term prospect would be for the plastic surround
> material. The more transparent it is, the less the problem
> would be (because it will absorb less heat) - but what's transparent to
> visible light does not necessary mean IR transparency. And
> in the event the fan were to fail, you would have your own melt-down of that
> envelope even if it's made from PC (HDT 125c?). What a
> mess it could be in the wost case...
>
> A glass envelope of some kind seems the way to go for this application - and
> it has the advantage of letting the sweet glow out into
> the shack if there were to be a suitable window into the RF cage.
>
> 73, Jeff ACØC
> www.ac0c.com
>
> -----Original Message-----
> From: Ron Youvan
> Sent: Saturday, December 25, 2010 8:18 PM
> To: amps
> Subject: Re: [Amps] Cracked Chimney
>
>>> 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.
>>> 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.
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