John,
On homebrewed amps, it's better to think about mounting the tubes as far away
from the chassis sides/back/front as possible. In commercial amps you have to
take what they make, but when building one, that increased space helps a lot. I
think that Tony showed a piece of PVC that burnt just because of this reason,
as the burnt place was between the tube and a close chassis wall. Heck, they
use RF to solder and braze with too. It can make certain things hot as a
firecracker.
The newer fish paper, (the blue stuff) is supposed to be moisture resistant
because of moisture in transformers causing problems. That was what one
manufacturer told me when he sent me some smaples of it. Another ham on here
used it sucessfully several times with no problems. The old untreated stuff
like the brown paper will absorb moisture easy. It's not any good until it's
soaked in varnish and dried. At that it can still cause problems.
What did you find good for use in the plastics as I'm interested about that
myself?
Best,
Will
*********** REPLY SEPARATOR ***********
On 4/14/06 at 11:03 AM John T. M. Lyles wrote:
>Air cooled tube chimneys seem to be a big topic these days, as much
>as discussion of EPAY and that other service. I want to suggest that
>people consider that these chimneys are typically situated at the
>highest impedance of an RF amplifier, across the anode to ground
>space. With 2-3 kV of peak RF swing there, at, say 28.25 MHz, one can
>expect a minimal but measureable dielectric heating effect. With a
>thin cross section, like transformer paper that Will suggests, this
>may not cause appreciable heating in such a narrow piece of
>dielectric. As long as moisture doesn't ingress into the paper, it
>should be OK. Vinyl chloride-containing polymers (like PVC) are just
>the opposite of what you want for a low loss dielectric. According to
>Arthur Von Hipple's classic compendium of dielectric data from MIT in
>the 1950s, (Dielectric Materials and Applications, available in
>reprint from Artech house now), PVC formulations have typically 100 x
>the loss tangent of polyethylene at 1 - 10 MHz range. RF sealing
>process is used to stamp PVC articles together, like the patches on
>auto carpets under the pedals, like inflatable rafts, like door
>panels for autos, packs and raincoats. The harder polymers such as
>used in pipe may be a bit more resistant than the flexible variety,
>but they still have a lot of ionic content. We don't use PVC pipe
>fittings in commercial Deionized water systems as they tend to
>release ions into the solution.
>
>As an RF engineer I try to stick by rules of things which are
>acceptable and not, and apply them universally. I spent 6 years doing
>just dielectric heating experiments for applications at E. I. DuPont
>Co, so I got very familiar with what plastics I could trust or not.
>My overly conservative stance (go ahead AG6K, fire away!) often gets
>criticism from those who say it will work fine at lower voltage, or
>for low duty operation like amateur transmissions. It true that cost
>is no object in many high powered systems where you don't want to run
>the risk of a burn-out. In low Q applications like pi-networks, the
>voltage is well defined. In cavity and line circuits, beware, as the
>Q can be higher, the voltage rises significant. I still suggest that
>we don't tempt Mr. Murphy too far, as one may be sitting up on 10
>meters (or worst yet, 6) key down and wonder why output power seems
>to be dropping while smoke begins to rise from the PA compartment
>(and that hideous smell of melted PVC). Think of it this way, would
>you use the same material as a standoff insulator in an RF network
>with high voltage? Do you mind the lowering of Q that lossy
>dielectrics can introduce? 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. Be careful; check your homeowner insurance policy
>if you care to experiment and take materials close to their limits.
>
>John
>K5PRO
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|