[Amps] door knob capacitors

[Fuqua, Bill L]

I have a number of old door knob capacitors. One is much larger than a real doorknob. 
It is 1000pf  fixed vacuum capacitor. Was used in a HF amplifier with a 3CX10,000A7 as the plate coupling capacitor.
73
Bill wa4lav
________________________________________
From: amps-bounces at contesting.com [amps-bounces at contesting.com] On Behalf Of Carl [km1h at jeremy.mv.com]
Sent: Wednesday, March 02, 2011 7:53 PM
To: Rob Atkinson
Cc: amps at contesting.com
Subject: Re: [Amps] door knob capacitors

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 at gmail.com>
To: "Carl" <km1h at jeremy.mv.com>
Cc: <amps at 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 at 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 at gmail.com>
> To: <amps at 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
>>>>>
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>

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