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[Amps] HV Fuse

To: <>
Subject: [Amps] HV Fuse
From: "Jim Thomson" <>
Date: Sat, 21 Sep 2019 14:38:43 -0700
List-post: <>

From: Jim Thomson 
Sent: Saturday, September 21, 2019 10:53 AM
Subject: HV Fuse

Date: Sat, 21 Sep 2019 09:50:11 -0400
From: "Paul Christensen" <>
To: "'amps'" <>
Subject: Re: [Amps] HV Fuse

<The bigger issue is the use of a HV fuse between the transformer secondary and 
C-input filter in an attempt to save the rectifier stacks during a fault event 
that occurs after the filter cap and before the glitch resistor.  I've not seen 
it implemented anywhere but was <mentioned by VE7RF in this post:


<Even with step-start, I'm skeptical of its effectiveness due to the high 
initial surge current at start-up.  So, perhaps just relying on the 20A ABC 
mains fuses is still best. 

<The Alpha 70V uses four Semtech SCH5000 HV rectifier blocks.  They're rated by 
Semtech at 5KV PIV but only 0.5A at 55 degs C, or 1.0 A with continuous forced 
air at 55 degs C.  This is a 1960s design and barely meets the margins for this 
amp.  I have <spares so I used them in the repair.  But, if it occurs again, 
I'll either use modern equivalents with better specs or a string of 6A10s on a 
new, custom PC board.  

<Paul, W9AC

##  6A10 diodes.  Rated at  1kv at 6A  CCS,  400A  half  cycle  surge.   IE,  
8.3  msec.   6A10  typ  will  HI  pot  test  to 1.4 kv. 
## 10A10  diodes.    1  kv  at 10A  CCS,  400A  surge  rated.   They  too  
hi-pot test  to  1.3  to  1.4  kv.  
## 10A07  diodes.  1  kv  at  10A  CCS,   but  600A  surge  rated.   hi-pot  to 
 1.3  to  1.4 kv. 

##  Mouser   sells  all 3  types   above.   I  typ  buy  either  100 /  250  at 
 a time. 

##   Both  the  6A  and  10A  diodes  above  can  easily  be  paralleled.   The 
 current  split is  typ   50-50....  and rare worse  case is   49-51....  or 

##  I  also  recently   tried   paralleling  2  x  complete   FWB  assys.    
Each  assy  uses 24 x 6A10  diodes  in  each  of  the  4 x legs.   Equal  
lengths  of  HV wire  used  from
xfmr  to  each  FWB  assy.  Again, the   current  split   was  50-50. 

##  years  ago,  I built a   6.8 kv  supply  with  60 x 1N5408s.   15  per  
leg...  in  a  FWB.   10  kva,  4.8 kv  hypersil pole pig  used.   One  day,  I 
 had  the  diode  board on a swivel
mount, positioned wrong, at an  angle   and  the  B+  arced to  chassis....  on 
the output  of the  diode  board  assy.   Took  out  all  60 x diodes in  a 
split  second....  followed  by  the
240 vac – 50A  rated  mag- hydraulic breaker.....  which  was the....  instant 
trip  version,  with the  oil  removed.... tripped  off.    Replaced  with  60 
x 6A10s.   But  this  time, a 2nd  HV
fuse  installed...  between xfmr sec.... and  FWB.   2nd  HV fuse can  go into 
either xfmr sec  leg.     I  tested the  config,  several times  using a HD   
open  frame, 30  kv, 60 A  HV  relay. Short  the  B+
anywhere  inboard  of  the   1st  HV  fuse....  IE:  B+  HV  fuse  + 50 ohm  
glitch R.......and  all  that happens  is an  open  HV  fuse  #2.....  between  
xfmr  and  FWB  assy.   Diodes 
survive  just  fine.     Test repeated   times. 

##  step  start  consisted of  2 x paralleled 50  ohm,  100  watt  wire  
wounds, and a 0-15  sec  adjustable  time  delay  +  contactor.   150uf  filter 
 cap  used  on  the  B+.   Runs  smooth.
Sold  B+  supply  to a buddy,   who  uses it  daily  with  his  YC-156  amp  
for  the  last 5  yrs.   

## The  only  time  that  HV  AC  fuse,  between  xfmr  and  FWB,   caused   
issues,  was  on another  buddys  8  kv  supply.   Everything  ran  fine,  till 
 he  increased  filter  C  to  163  uf.
Once  the  step  start  completed..after 15 secs....  the  HV AC  fuse opened  
up.    The  secondary surge  was  too  much  for  the  BUSS  HVU-3   fuse.   
The  fix  was to  replace the  20  ohm
step  start  resistor...  with a 10  ohm  unit.    Problem  was  the 1.5 A  of  
 xfmr  pri  magnetizing current  flows  through the  step start  resistor....  
with a V  drop  across  it.    Thats  why  the  B+  would
only  increase  to  7 kv.... then  jump to  8  kv    with  step  start R  

##  In  Pauls  case.....  use  some  6A10s,  etc...  and  call  it a day.   The 
 HV  fuse between  xfmr  and  diode  board  is just  extra  protection  for  
the  diode  board. 

Jim   VE7RF

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