On Sun, 24 Jan 1999 07:54:27 -0800 dan hearn <dhearn@ix.netcom.com>
writes:
>
>I am surprised that no mention has been made of the temperature
>coefficients of
>Centralab doorknob capacitors. These are usually stamped right on the
>cap. They
>are also in the Centralab catalog.
Unfortunately NPO is only available up to 65pf in the HT-58 series by
HEC. Not usually a useful range of values unless you parallel a bunch of
them.
They range from NPO (zero) thru
>N500 (minus
>500 ppm/degree C) to Hi K ( uncontrolled but high ). Even the NPO's
>are stable
>only over a limited temp range. Heating with r.f. current flow is
>inevitable.
>Transmitting micas with adequate rating are certainly superior but the
>cost
>discourages most of us from using them unless we get them via flea
>markets.
And then you have to be concerned with moisture migration over the years.
I once had a beautiful 1000pf 10KV Aerovox literally blow apart and take
out a 4-1000 chimney.
Ameritron appears to be having good results with a monolithic style cap,
I remember W8JI discussing them a year or so ago.
I
>talked a friend who is a CD rep out of a catalog which has r.f.
>current ratings
>for the small 500v and 1000v micas. I was surprised how much current
>some of
>these small micas can handle.
Dont pass up the larger size mica compression trimmers such as the Arco
302M series. They are rated 1000V and come in values from 15 to 2500pf.
The large open construction will undoubtedly offer better cooling and
higher current as compared to a sealed unit.
Bring a hand held capacity meter to the fleamarkets.
73 Carl KM1H
> By the way, my guess that the Alpha 77DX grid meter is 250 ma f.s.
>in spite of
>the 500 ma scale has been confirmed by two people. This means that the
>80-100 ma
>I get with proper tuneup is ok, and I do know how to tune it up, Hi .
>73. Dan,
>N5AR
>
>Roy Koeppe wrote:
>
>> Hi Bill,
>>
>> Well, I experimented long and hard, testing even high-grade
>xmitting
>> ceramics in the plate tuning and loading locations. They all drifted
>to an
>> unacceptable degree. I presume from the tube heat alone.
>>
>> So, have sworn them all off. Big micas rated 8-amps do the trick.
>They don't
>> budge one iota! Lesser current ratings are OK in the loading
>section, but
>> still need to be micas.
>>
>> Good investigating & 73, K6XK
>>
>> -----Original Message-----
>> From: WB6WCW@aol.com <WB6WCW@aol.com>
>> To: royanjoy@ncn.net <royanjoy@ncn.net>
To: <amps@contesting.com>
>> Date: Saturday, January 23, 1999 7:26 PM
>> Subject: Re: [AMPS] To Ten-Tec Inc, et al, Re: Titan II
>>
>> >Hi Roy:
>> >
>> >My experience in fixing-up an Amp Supply LK-550,
>> >which uses (3) 3-500Z tubes, is that the 'padding'
>> >capacitors get quite hot on 80 and 160.
>> >
>> >I believe it is due more to a misuse of the parts, than the fact
>> >that they are ceramics.
>> >
>> >If you look at <http://www.rfparts.com>, under "Products", look
>> >at "Ceramic Bathtub" capacitors. It lists the current ratings
>> >of the HT-50/HH58/850S family of capacitors. At 1 MHz, the
>> >1000 pf caps are only good for 1.6 amps (and they do overheat).
>> >
>> >If you replace the 1000 pf with a pair of 500 pf caps, then the
>current
>> >rating is 6.4 amps (and will not overheat). Carl (KM1H)
>recommended
>> >that I use 750 pf units, it would require less parts (cheaper), but
>haven't
>> >been able to find any of them.....
>> >
>> >After fixing all of the "Tune" padding capacitors, I still had a
>problem
>> >on 160 mtrs with the ceramics they used on the output side of the
>> >pi-network. Same problem; 1000 pf capacitors overheating. Am
>> >presently replacing them with 500 pf units-----and I sure hope
>that
>> >solves the final problem (it's getting rather expensive).
>> >
>> >Good Luck & 73's
>> >
>> >Bill W6WCW (E-Mail: WB6WCW@aol.com)
>> >Santa Barbara, CA.
>> >
>>
>> --
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>
>
>
>
>--
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>
>
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