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. 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. 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.
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.
> >
>
> --
> FAQ on WWW: http://www.contesting.com/ampfaq.html
> Submissions: amps@contesting.com
> Administrative requests: amps-REQUEST@contesting.com
> Problems: owner-amps@contesting.com
> Search: http://www.contesting.com/km9p/search.htm
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|