[Amps] HV Diode String Question

[Radio WC6W]

Hi Jim,
  Avalanche spec'ed parts may be series'ed san snubbers & equalizers.  

  There is a question as to whether some small (typcially circa .001 uF) caps should be placed across them to prevent diode switching noise.  Though, there are probably far worse noises to shield and filter out in an amplifier. :-)

  You probably won't find those devices in any commercial amp due to their cost.

  And... aren't those rated at 5A?

73 & Good morning,
  Marv WC6W 

http://wc6w.50webs.com/

--- On Mon, 6/7/10, Jim Garland <4cx250b at muohio.edu> wrote:

> From: Jim Garland <4cx250b at muohio.edu>
> Subject: [Amps] HV Diode String Question
> To: amps at contesting.com
> Date: Monday, June 7, 2010, 8:12 PM
> Here's an old topic that's come up
> many times, but I'm still not sure of the
> best answer. In my homebrew amp that runs 4500V with a
> voltage doubler power
> supply, I have two series strings of rectifier
> diodes.  I had a flashover a
> couple of days ago which took out several diodes in each
> string.  (It never
> should have happened A grounded solder lug was touching the
> enamel coating
> on a 100K/100W bleeder resistor and arced through the
> coating. Evidently,
> when I tightened the screw on the solder lug, it rotated
> around and touched
> the bleeder, and I didn't notice it. It blew the bleeder in
> half and sounded
> like a shotgun blast.)
> 
>  
> 
> I built the diode strings in the 1970s. Each one used eight
> HEP-170 diodes
> (remember those?), each shunted by a .01uF cap and a 470K
> resistor. I'm
> replacing the diodes with 1N5553 diodes. These are mil spec
> glass
> encapsulated diodes rated at 3A/800PIV, with 2uA reverse
> current at rated
> PIV. (By comparison, 1N5408s are rated at 500uA reverse
> current.)
> 
>  
> 
> My question is whether I need to shunt these high quality
> diodes the same
> way I did the old HEP-170s?  I'm aware of the reasons
> for shunting, e;g;
> using caps to distribute switching transients, resistors to
> equalize the
> reverse voltage distribution. But is it really necessary
> with modern,
> controlled avalanche diodes, presumably made to tight
> tolerances?  For space
> reasons, I'd prefer not to shunt them unless it's really
> necessary.
> 
>  
> 
> I've looked at some modern commercial amplifier schematics,
> and I find no
> consistent approach. Some use resistor shunts only, some
> use capacitor
> shunts only. Those that use molded diode blocks don't use
> any shunts. So I'm
> interested in knowing the latest thinking on this subject.
> Thanks!
> 
> 73,
> 
> Jim W8ZR
> 
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