[Amps] load capacitor ratings?

[Will Matney]

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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