[Amps] high voltage fuses

[Jim Thomson]

Date: Thu, 10 Oct 2013 07:44:08 -0700
From: Bill Turner <dezrat1242 at yahoo.com>
To: Amps <amps at contesting.com>
Subject: Re: [Amps] high voltage fuses
ORIGINAL MESSAGE:          (may be snipped)


On Thu, 10 Oct 2013 03:15:25 -0700, Jim wrote:

>  The grid meter wont be damaged at all.   Even with rvs
>connected diodes across the grid meter, the predominant fault path is from chassis through
>the 4-6 x paralleled diodes, then directly to the B- of the filter caps. 

REPLY:

For what seems like the millionth time, placing diodes directly across the
grid meter will not protect it. You must place a resistor in series with the
meter and connected the diodes across the meter + resistor combination. 

The diodes don't conduct until the voltage across them reaches about .6
volts. If they are connected directly across the meter with no resistor, the
meter will be destroyed long before the .6 volts is reached. 

Choose the value of the resistor so that the .6 volt figure is reached at
about 110% of full scale. Don't forget to include the meter resistance in
the calculation. 

This also applies to the plate meter or any low-resistance current meter,
which nearly all of them are. 

73, Bill W6WRT

###  Your scheme will  work for stuff like  over driving  an amp, and pulling 
1.3A of plate current  on a  0-1A  plate current meter. 

##  what I’m talking about is a hard fault...like B+  arcs to the chassis...or B+
arcs inside the tube..to the grounded grid, etc.   In those cases, the predominant
path is from chassis, UP through the safety diodes and directly  to the B-
completing the loop.   The safety diodes I’m talking about are the RVS pair
of 6A10s..wired between  chassis  and B- of filter cap. 

##  I will also use a RVS  connected pair of 6A10s  across the grid meter.  Short the
B+  to chassis..and all that happens is the HV fuse in the B+  supply opens up...in <2 msecs. 
Event over.  Grid meter never gets damaged.... and nor does the plate current meter.  
I use several 6A10’s in parallel to increase the peak current rating. 

##  We actually tested this with a 6800 vdc supply  which has a 150 uf  filter cap  in it...
plus the usual HV fuse + 50 ohm glitch R.   I could sit there and blow HV  fuses all day long
and nothing happens..except a blown HV fuse.   I used a vac contactor to short the B+  to the
chassis for testing. 

##  OK,  what I don’t have  protection for,  is for cases where the plate current +  grid meter is just driven
beyond their ranges.  Electronic grid + plate over current protection  will easily solve that situation..since the
trip thresholds can be set to any current..which will also typ be less than the full scale meter current.  
I don’t bother with it since I don’t use tubes with delicate floozy grids in em. 

Jim  VE7RF