I have offered this suggestion before but I will offer it again: Ameritron
(MFJ) will sell the chimneys that are used in the AL-800/800H amps. These
fit nice and snug around the 3cxp800a7 tubes used in said amps.
Hope the above is helpful,
73,
Gary...wa6fgi
----- Original Message -----
From: "Ian White GM3SEK" <gm3sek@ifwtech.co.uk>
To: <amps@contesting.com>
Sent: Friday, April 14, 2006 12:50 PM
Subject: Re: [Amps] Chimney materials - Loss or not TSPA
> John T. M. Lyles wrote:
>>Unfortunately, the old microwave oven test doesn't work well enough to
>>apply the results, as constant fields are not produced, they purposely
>>move them around to cook a chicken uniformly. Applying a fixed electric
>>field in one direction through a thick dielectric, with minimal spaces
>>(air gaps) between it and the metal, is the exact method used to get
>>temperature rise in dielectric heating industry.
>
> I think he microwave oven test does provide useful information - within
> its limitations, of course. If you test similar samples of various
> materials at the same time (relying on the turntable or mode-stirring to
> subject them to similar average power densities) then you can tell by
> touch which material is better or worse than the next one.
>
> Loss tangents will be lower at lower frequencies, but (for the types of
> materials that you'd ever consider as RF insulators) the ranking order -
> from best to worst of the samples tested - is very unlikely to change.
>
> What the microwave oven test will NOT tell you is whether a given
> material will be OK in your particular application. It all depends on
> the localized RF field strength, which can be very difficult to predict,
> or even to visualize in your mind's eye.
>
> There are examples where materials with a 'poor' reputation are
> perfectly OK, because the RF field strength where they're being used is
> actually quite low... but if we're dumb enough, it is possible to burn
> out even a 'superb' material like Teflon.
>
> In the end, it's probably better to play safe like John does. By all
> means use materials with good RF properties (and IMO the microwave oven
> test can be helpful in selecting plastics) and use the minimum possible
> thickness to avoid internal heating.
>
> There seems no reason to make a chimney out of thick material. The only
> reason to use thick-walled tubing would be if you think it might begin
> to soften - which says it's the wrong material anyway. Materials that
> have worked for me in VHF/UHF amps include rolled Teflon sheet or mylar
> sheet, with the vertical seam secured by thin mylar parcel tape; and
> various kinds of glass tubing, glued to the metalwork with silicone
> sealant. Like John, I wouldn't willingly use PVC... it might be OK in
> your particular application, but why risk it when better materials are
> easily available?
>
> Will's fish paper might be fine, too, in many situations. (Worst case,
> it would leave only a small amount of ash :-)
>
>
>
> --
> 73 from Ian GM3SEK
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>
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