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. If there is significant current in an arc, I have had
> the wirewounds rip apart, buckled the enamel coating.
I hope you [AMPS]-ites won't think this is tiresome, but I need a review of
exactly how the glitch resistor is supposed to function. I've seen some
posts implying that the glitch resistor is supposed to function as a fuse --
"Don't use cement wirewounds because they'll explode when they fail", "I
used big power fuses instead", etc. Other posts imply that they are *not*
supposed to act like fuses, but are intended to limit current. 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. But does the glitch resistor solve this problem by slowing down
the rise in current or by opening? 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? 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?
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?
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.
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?
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
--
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
|