At 09:51 AM 6/25/99 -0600, John Lyles wrote:
> They might work, but
>the losses and subsequent internal heating are high. Not to mention the
>resonances in them, since they are high K dielectric and probably have a
>lot of internal inductance. But if you can get by with them, then so be it.
>I believe it is a gross misuse of a component in a circuit that should have
>a better one. If it fails, you get DC on your output network, and so on. Is
>that safe? Is there an RFC or L to ground the network? Worse yet, does the
>DC come out to your antenna feeder then?
Theoretically, John, I agree with you, but practically speaking, not so.
Every HF amp I have built in the last 30+ years has had TV doorknobs as the
anode DC block. Not a single failure in dozens of instances with almost
constant use over 30 years. Is that "getting by"? How can you have a
"better" component? Could you guarantee that a component rated for that use
would not fail in 30 years? Even more, I personally have had more failures
of the "world standard" Centralab 850's than TV doorknobs! (Miss using both
equally, of course <grin>)
In Amplifiers 101, I learned that anyone who values their life, puts a low
DC resistance to ground at the output side of the block cap. I go one step
further, I make sure my antenna designs also have a low DC resistance to
ground. You will have to trust me on this one John, but I am sure that I am
more paranoid about HV in my shack than you are at work. I got that way
after the second time (many years ago) that I woke up on the wrong side of
the room with smoke curling from finger tips. Do I trust my life to the TV
doorknobs? Well, at least as much as anything else I've ever seen. (Which
ain't saying much!)
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.
I really do envy those guys that have access to the "correct parts". I
often wonder what I might accomplish if I didn't have to scrounge for every
little damn thing.
Also,
>when placing caps in paralllel,use the same values and be sure to strap
>them so that the electrical length and inductance to each cap is identical,
>for current sharing. Never place them linearly along the strap or mounting
>plates, in line with the current flow, but place them on a T plate, which
>causes current to divide equally, with the caps all perpindicular to the
>ciruit.
Absolutely! This is a no-brainer. Select TV doorknobs (or any other caps)
from the same manufacturer, exact same values, mounting styles, even date
code if available. I screw mine in between two copper plates. When using 4,
(preferred) mount 2x2 in a square pattern. The copper helps dissipate the
(non-existent) heating problem supposedly caused by RF current through them.
>
> Centralab 858 Ceramic doorknob style rated 1000 pf, 5 kV, X5V dielectric
>Series Resonance at 28 MHz. much higher Q, nothing else beyond 81 MHz, but
>stays inductive there
I can destroy 850's and 858's with equal ease. 857's are a whole 'nother
story. Someday (if I ever have time) I should build an appropriate test
fixture and run a bunch of these things with my HP8753D. Bet the results
would be interesting, if not actually usefull.
When working on the YC156 conversion, I actually went out in the shop, dug
out my cache of TX caps and found some Sangamo mica TX caps. They even had
RF current ratings on them! Voltage ratings were barely adequate (by my
standards) and they were so huge, they wouldn't fit in the only place I
could mount the block cap. Gave up and put the TV doorknobs back in. Works
fine, lasts long time!
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.
By the way, John, I love your stories, reports, and photos. Please don't
stop doing what you are doing!
73 & HAVE FUN!
Larry - W7IUV
w7iuv@arrl.net
http://w7iuv@axtek.com/w7iuv
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