[Amps] SB220 Meter blown

Jim Thomson jim.thom at telus.net
Sun May 17 16:19:26 EDT 2020

Date: Sun, 17 May 2020 05:15:11 +1000
From: Adrian <vk4tux at gmail.com>
To: Gary Schafer <garyschafer at largeriver.net>, 'Jim'
<jimw7ry at gmail.com>, amps at contesting.com
Subject: Re: [Amps] SB220 Meter blown

## IF  say the anode arcs  to the  grounded grid in a GG triode amp,
thats  called a ..HARD  fault.  Even  with  2 x diodes  in series..so  meter
accuracy is  not affected in normal operation, the  diodes  will  be  into  
conduction  so  fast, the meter  wont even  begin to  deflect.  Been
there,  done that.

##  IF  say  you  apply way too  much  drive..and  bury the  plate
and grid meter past the end,  thats  called a ...SOFT  fault.  In the
case of the  ..soft  fault, the  diodes, even 1 or...  2  diodes  in series
will  not  protect the meter.  Correct  protection for..soft  faults
would  consist of  a  high speed  protection  circuit, aka .. plate  overcurrent,
grid  overcurrent.   Electronic Grid  overcurrent is  common in  GG  triodes
with  delicate  grids. 

##  Correct  place to  wire the  diodes  is  between  chassis..and   B-.
IE:  B-  right  at the  cold end of the  string of  HV lytics.   Done that
way, BOTH  plate and grid  meters  are  protected against hard  faults.

##   Triode Tubes like a  3-500Z,  3CX-3000A7,  or any other  GG triode
with a HD grid  structure, only  require protection for  HARD  faults. Triode 
tubes  like a  8877, 3CX-800A7, etc,  require  protection for  SOFT 
faults...fast electronic protection...  AND  also HARD faults.

##  I usually parallel 3-4   6A10s so end  up with a 1600A  surge rated assy.
Then a FAST   HV fuse  in series  with the  B+...followed  by a  50 ohm  glitch  resistor.
Also a 2nd  HV  fuse..in series with just one leg of the  secondary of the  plate xfmr.

##  anode to grid arc,  glitch R  LIMITS the  fault  current  to a  safer value.<1msec
later,  HV fuse opens  up.... event over.  No meters  damaged,  tube not  damaged.
No  diodes  in  HV  supply damaged.  Cro-bar  anything you want..and  nothing bad
happens..except a blown  HV fuse.    Been there,  done that.  The concept  works  good
and  I have tested  it  many  times.   Paralleled  6A10  protection diodes  always stay intact. 

##  50 ohm  glitch  resistor has to have its  wattage rating  sized correctly..and  ditto
with any  HV  fuse.   Don’t use a 3A  fuse on a 3-500Z  amp.  Use a 1A rated  HV  fuse.
3000 V  /  50 ohms  = 60A  fault  current.   60A  fault  current  will  open off a 1A  rated
HV  fuse in  < 1msec. 

##  Ebay is a cheap source for  5000 V rated  fuses, and  fuse container assys..used
in  microwave  ovens.   Typ  available in   750/800/900/1000/100  ma  ratings. 
2 x HV fuses, wired as I described  above..+ a  50 ohm glitch  resistor,  will
ensure  your    HV  supply is  fully protected.   In  my  case,  only diodes are used
for hard faults..to protect the  grid +  plate meter.

Jim   VE7RF

The diodes(s) should be direct across the meter, and enough in series as 
needed to excedd the full scale deflection

voltage required before forward bias is achieved in the diodes. It's all 
very simple, as stated here a few times now. re ;

I would rather follow Rich's advice on the subject as per my previous 
link. contained withjin ;

"It may take more than one diode to protect a meter shunt resistor. A 
silicon diode begins to conduct at a forward voltage of about 0.5V. To 
avoid affecting meter accuracy, the operating voltage per glitch 
protection diode should not exceed 0.5V. For example, a 1 ohm shunt, at 
a reading of 1A full-scale, has 1V across it. Thus, two protection 
diodes in series would be needed to preserve meter accuracy. Similarly, 
if the shunt resistor for a 1A full-scale meter is 1.5 ohm, the maximum 
shunt voltage is 1.5V--so three diodes are needed.

Glitch protection diodes should not be petite. Big, ugly diodes with a 
peak current rating of 200a or more are best. Smaller diodes--and the 
meter they were supposed to be protecting--can be destroyed during a 
glitch. Suitable glitch protection diodes are 1N5400 (50PIV) to 1N5408 
(1000PIV). In this application, PIV is not important. The 1N5400 family 
of diodes is rated at 200a for 8.3mS.

During an extremely high current surge, a glitch protection diode may 
short out--and by so doing protect the precious parts. Replacing a 
shorted protection diode instead of a kaput meter is almost fun."

ref; http://www.somis.org/D-amplifiers2.html

Once a petite signal diode blows apart it is no longer protecting the meter.

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