Tnx for those tricks Carl--I looked at mine and fortunately they all
look okay. I guess they just are not high current RF doorknobs.
I also have some 500 pF brown ones about 3/4 inch diam. and 3/4 inch
long maybe 20 or 30 KV which I used as padding caps in a 160 m. L
network tuner. The inductor in series and the caps to ground. After
tx on 160 for 5 or 10 minutes with 250 to 300 w. (but with audio) they
feel very slightly warm so at this point if I had it to over again I'd
use micas there too, but I'm not going to worry about it now.
73
Rob
K5UJ
On Wed, Mar 2, 2011 at 6:53 PM, Carl <km1h@jeremy.mv.com> wrote:
> Those are pricey when new but even NOS 15KV 857's are often $5-10 at
> hamfests and the small ones can be had for $1-3 in perfect condition.
>
> Appearance counts, but dirt doesnt. Huh you say? An RF abused cap will show
> signs of overheating on the side or ends, there may even be cracks in the
> casing. Look at the terminals for reflowed solder. Bring 6-32 screws for
> the small ones, thread in each side and give a pull, some sleazebags will
> try and glue a bad one back together.
>
> Carl
> KM1H
>
>
> ----- Original Message ----- From: "Rob Atkinson" <ranchorobbo@gmail.com>
> To: "Carl" <km1h@jeremy.mv.com>
> Cc: <amps@contesting.com>
> Sent: Wednesday, March 02, 2011 6:21 PM
> Subject: Re: [Amps] door knob capacitors
>
>
> Thanks Carl; mine were probably 15 KV then. I looked at the High
> Energy website. Those caps look great but they also look kind of
> pricey. Well you get what you pay for.
>
> I'm gg to give some micas I have a try for loading on 160. A guy had
> a half dozen for sale at the Ft.Wayne hamfest last November for around
> $2 or $3 each. Sangamos and Cornel Dubliers--3 or 4 A 200 to 500 pF,
> the kind in a plastic body 2" x 1" x 1.5" with the lugs on top.
>
> Glenn, I don't intend to steal the thread away from you so go ahead.
>
> 73
>
> Rob
> K5UJ
>
> On Wed, Mar 2, 2011 at 2:32 PM, Carl <km1h@jeremy.mv.com> wrote:
>>
>> Real Centralab and other US RF doorknobs are mostly 5, 7.5, and 15KV in
>> the
>> most common 2 sizes. The caps in Heath SB amps are cheap non RF rated that
>> will often do bad things on 10M.
>>
>> Check out High Energy Corp for what is currently in production.
>>
>> There are also Teflon disc caps being used but I dont have any
>> manufactrurers names handy.
>>
>> Carl
>> KM1H
>>
>>
>> ----- Original Message ----- From: "Rob Atkinson" <ranchorobbo@gmail.com>
>> To: <amps@contesting.com>
>> Sent: Tuesday, March 01, 2011 12:32 PM
>> Subject: Re: [Amps] door knob capacitors
>>
>>
>>> Yeah but how do you tell a fleamarket doorknob is okay for RF. If you
>>> have a lot of do re mi and can get them new I guess it's easy, but
>>> your typical $2 doorknob in a box sitting on the pavement in a parking
>>> lot is a different situation. The markings are often cryptic and vary
>>> by mfr., -- It's better to be safe than sorry. OTOH, I did not know
>>> micas, eg. the big brown jobs with the binding posts on top ever
>>> leaked. I test mine with my el cheapo Eico 950 cap tester, one of
>>> those things with the magic eye tube but it only puts up to 500 v. on
>>> one. If it says a cap is good, I'm not sure it is really good or not
>>> except the eye opens up at or near the spec'd value.
>>>
>>> So I'm not sure what to reccomend for 160. I thought the 1 MHz tx
>>> micas were a safe bet. the trick is getting them to fit inside.
>>> Some amps are cramped for space.
>>>
>>> The Centralab doorknob I used was about 3/4 inch diameter coated with
>>> blue green glaze and 250 pF at 20 KV.
>>>
>>> Rob
>>> K5UJ
>>>
>>> <<<
>>> Rob,
>>>
>>> It depends on the capacitor. The ones intended for high RF
>>> currents are marked to show the maximum reactive current
>>> they can handle. Some time ago I had the same experience
>>> as you, with a cap literary exploding. Now I use only caps
>>> that are spec'd for high RF current. I have had no trouble
>>> wiht those.
>>>
>>> 73,
>>>
>>> George, AA7JV
>>>>>>
>>> _______________________________________________
>>> Amps mailing list
>>> Amps@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>
>
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