On Thu, 13 May 1999 17:33:15 -0400 "Dick Green" <dick.green@valley.net>
writes:
>
> John Lyles wrote:
>
> > Series R for current limiting
> >
> > I would recommend that the wirewounds be avoided in this
> application if
> you
> > can afford to use a Globar style ceramic tube resistor of similar
> value to
> > what KM1H suggested.
KM1H never suggested a Globar in the HV path, only as an alternative to a
parasitic suppressor resistor.
If there is significant current in an arc, I
> have had
> > the wirewounds rip apart, buckled the enamel coating.
Broken enamel is not a concern as long as the resistance wire did not
open. I suspect that a few here expect a multi hundred amp dump to leave
no visible sign on the resistor? Get serious please, if it survived that
much of a hit spend a few pennies on a new one .
>
> I hope you [AMPS]-ites won't think this is tiresome,
Non stop and repetitive maybe but not tiresome....
but I need a
> review of
> exactly how the glitch resistor is supposed to function.
Why? Are you designing a new amp or just refurbing something like a
LK-550??
I've seen
> some
> posts implying that the glitch resistor is supposed to function as a
> fuse --
Wrong
> "Don't use cement wirewounds because they'll explode when they
> fail",
Yes
"I
> used big power fuses instead",
even worse
etc. Other posts imply that they are
> *not*
> supposed to act like fuses, but are intended to limit current.
Gee, really!
My
> understanding of the problem is that when an arc or short occurs,
> the HV ps
> filter caps can dump a lot of current into the plate circuit before
> the AC
> fuses blow.
Beaucoup amps
But does the glitch resistor solve this problem by
> slowing down
> the rise in current or by opening?
Explain that "slow down" theory please.
During the microseconds or
> milliseconds
> of a tube arc, is it possible for the glitch resistor to limit the
> rise in
> current enough to prevent damage to the amp, yet still survive?
Now you used the correct word "limit current". If sized properly it
will survive and so should all the parts it is designed to protect.
> During a
> long-duration short, like opening the cover of an amp with an
> interlock, is
> the protection provided by slowing the rise in current long enough
> for the
> AC fuses to blow or is it provided by failure of the glitch
> resistor?
Figure it out yourself; all it takes is Ohms Law. The answer varies by
the application.
BTW, there is no HV shorting bar in a LK-550 so why the question? Anyone
who has spent a few minutes in a SB-220 type amp gets rid of that
shorting bar anyway; it has probably contributed to a high percentage of
multimeter failures.
>
> This figures into part selection in a big way. If it's supposed to
> act like
> a fuse, do I really care about the problems with an inductive
> wirewound that
> John points out? Do I really need to spend $35 or more for a
> non-inductive
> power resistor?
John is incorrect if that indeed was his intent; possibly you
misunderstood also.
A wirewound resistor in the HV path is what you want. Maybe at levels way
above ham limits something else is appropriate but dont waste your time
thinking about it.
>
> Although I'm ready to take Carl's recommendation, I'd like to know
> the best
> procedure for determining glitch resistor values. The LK550 no-load
> plate
> voltage is 3200VDC (around 3150VDC in mine), and the transformer is
> rated at
> 1.75A. Under load, my ps sags to about 2700VDC. I figure typical
> operating
> current at the 1500W legal limit would be about 1000 mA. Seems like
> it would
> be a good idea to design the glitch resistor to allow the amp to be
> pushed
> to at least 1200 mA (about 1750W), if not the full max of 1400 mA
> (2KW). An
> Ohmite 15-ohm 25W ceramic-coated wirewound has a max current rating
> of 1291
> mA, which is pretty close. Carl says the ceramic units should be
> able to
> withstand brief tuneup periods at the max, which is all I really
> care about.
The math works just fine, if ur into a panic attack go to 50 W, price
differential is small as long as you have the room to stuff it in.
> Any other opinions? Should I design it so that if the next owner
> wants to
> run RTTY at full tilt the glitch resistor won't blow? If so, what
> values
> will work?
RTTY full tilt in a LK550, ya gotta be kidding!
You remind me of someone else who spends all his time asking questions,
second guessing the replies, and spends most of his time going around in
circles.
Exercise your Extra Class License and have at it.
Norm
>
> John -- Thanks for your comments on mounting techniques. I had the
> same
> concerns about the screws loosening up on the ceramic standoff
> insulators,
> or possibly breaking the insulators by tightening the screws too
> much. One
> reply suggested using the spring brackets, but mounting them on a
> piece of
> 1/16" or 1/8" plastic. Then the piece of plastic could be set on a
> pair of
> ceramic standoff insulators, keeping the spring brackets well away
> from
> chassis ground. I suppose a metal retaining clip or a piece of wire
> attached
> to the plastic could prevent the resistor from rolling. Sound good?
>
> 73, Dick, WC1M
>
>
>
> --
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>
>
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