Speaking about doorknob caps, plate blockers etc:
Larry Sez
>Would I put TV doorknobs in a product I was going to market? No way! I
>wouldn't put a Centralab 850 in there either! But, for a lot of us this is
>just a hobby. Part of the hobby for me is pushing the limit. Making parts
>work in applications they were not designed for, just because I got them
>for free or cheap is part of that. So what if it blows up, I have a whole
>pile of replacement parts and more amps in the rack.
>The whole point here is that, for a lot of us, using parts in applications
>they were never intended for is a way of life. Careful attention to
>*practical* considerations can result in cost savings that are significant
>to most of us. As an interesting aside, I also take great pleasure in doing
>things the "staff scientists" say can not or should not be done.
>Larry - W7IUV
Its true. The big difference in building amps for fun at home and amps for
a living is sometimes in the amount of overdesign one puts in. And a lot of
this is strictly on paper too, stuff that you document in your notebook,
such and such capacitor chosen because its said to be an RF rated part, so
many amperers, etc. A lot of decisions have to be made on experience, like
how many caps got trashed just mounting them wrong. I have seen a lot of
broken 858s too, and even graduated to the bigger doorknobs, with 2 inch
length. But they are real fragile, mechanically. The little ones seem to
pop apart when the amplifiers sneeze. I learned that you better put some
flexibility on the mounting too.
If the plastic cased doorknobs work, use 'em. Your experience indicates
that they are adequate for blocker application. If I built an amp that has
to run 8000 hours a year, at high power, and skimped on some component to
save $$ but the stated application and rating were being pushed in my
amplifier, I'd look pretty stupid to my peers. But at home, hey, its a
totally different formula. I would hope that commercial companies don't cut
too many corners to save cost, but a few have, we've seen and heard about
them. Its amazing how Heathkit did a pretty good job with low cost too.
There is another major jump in components used as capacitors. HF amplifiers
need a lot of C for blockers to handle 160 meters. So we depend on mica or
ceramic for enough K to get high C in a small package. For VHF power, or
industrial heaters in the higher freq. one can get by with things like film
between plates, special homeade capacitors made with PTFE (Teflon), PET
(polyester), polyimide (Kapton). Here you are on your own. The doorknobs
are totally useless, even the ceramic 858 types. Boom. Blown to
smitherines.
73
John
K5PRO
--
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
|