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Re: [Amps] load capacitor ratings?

To: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] load capacitor ratings?
From: Will Matney <craxd1@ezwv.com>
Date: Mon, 30 Aug 2004 20:37:47 -0400
List-post: <mailto:amps@contesting.com>
Rich,
Exactly. I talked to the owner of Oren Elliot (Orens son Stephen) several years ago about this and what he thought the rating should be. He had several thoughts on this but told me that most were using a X2 rating the same as the coupling capacitor for tune C. He recommended using a load C with a voltage rating some higher than the peak voltage rating across the load, generally having a 50 ohm load as standard. I think he figured by a sometimes varying load, it could take more voltage to create the same power across a mis-matched load. Of course this could sure be the case and is something I would not have thought of at the time. Not all antennas are a perfect 50 ohms or close to it. His capacitors are tested in open air with a hipot tester. They had a jig made up with a nut driver handle made onto a coupling to turn the rotor with. They applied the hipot and turned the rotor through its rotation to catch any arcing. My concern was the physical size of air variable capacitors and what was the least size I could safely get by with. He did mention that one was available where the plates had a "nylon dielectric" to insulate the stator plates raising a .016" air gap to 1100 V (X2). They were about X3 the cost too! I took parasitic arcing into consideration also, and he mentioned this too in his rating.


One thing to note about buying new capacitors from Elliot. If there doing a large run for any company, they always have several setting on the shelves and you can buy these cheaper than a custom or one that has to be made up. I was using a 32APL-300-DE which is a double ceramic end, 300 pF cap with .032" spaced plates. I think you'll find this at RF Parts, but I was buying them from they guy who sold them to RF Parts beforehand. He mentioned using some stock 32APL-250-DE or 32APL-208-DE's then adding a doorknob cap in parallel to bring it into tune. I did this several times without a failure. Before Merit got these caps, I was paying $11.00 each, and now they are $30.00 if I remember at RF Parts. The 32APL-208-DE was about $9.50 each from Oren Elliot.

The .032" air gap has a voltage rating of 1070 V max. This was the load capacitor with a 900 to 1000 Vdc plate voltage capable of 1800 watts PEP on a 50 ohm load. However, for 1800 watts, this only requires 300 volts PEP. 1070 V / 300 V = 3.56 : 1 safety ratio. In reality, a .016 air gap with a 640 v rating would have sufficed in my opinion. This would have gave about a X2 rating for load C. The tune C he recommended was to use a 48APL-41, which is a 41 pF single end cap with .042" spacing rated for 1500V max. Notice that this rating is not X2 either but only 1.5 times the plate voltage. I never did see the tune C arc in any of these amps I built. I have seen arcing in load C however. This is about the rating were talking about here now.

I'm also not for sure who the manufacturers were using a X2 rating, but they were amateur amplifiers using type M-73 capacitors (large frames). The M-73 is rated from a .030 air gap at 950 V to a .140 air gap rated for 3700 V. The M-73 is used in the QRO amps now. QRO wasn't in business when we were having the conversation though. I use the M-73 myself in the large tubes with a high plate voltage. These have real ceramic insulators with high amperage strap type wiper contacts. I've seen some by other manufacturers using epoxy glass board in place of the ceramic insulators.

Anyone interested in OEP products can visit their website at;

http://www.orenelliottproducts.com/index.html

Will Matney

R. Measures wrote:


On Aug 30, 2004, at 9:13 AM, Will Matney wrote:


Simple,
Determine the voltage across the 50 ohm load from the power in watts. Then use a load capacitor with a voltage rating equal to or up to two times the load voltage. The tune control is two times the plate voltage and so is the coupling capacitor.


Will - The peak neg.-going V on the Tune C is a bit less than the anode supply V. The peak pos. going V is the same. Thus, a Tune-C with the same breakdown V as the anode supply will not arc in normal operation. However, given a VHF parasitic, the Tune-C can arc.


Will Matney


How do I work out the voltage requirements for my load capacitor in a Pi-L?
This will be a fixed freq amp designed and set-up without the need for large
VSWR tolerances.



I've searched the amp archives and only found one ref to this from KM1H, suggesting around 1kv at 1.5kw.

What's the formula please?


Neil G0JHC



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Richard L. Measures, AG6K, 805.386.3734. www.somis.org


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