I run 50 ohms at 50W for current limiting and a hunk of #30 wire for a HV fuse.
'guess I don't intend the resistor to be the fuse. . .
Art k9xf
Dick Green wrote:
> 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
--
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
|