[Amps] Glitch resistor + fuse?

[Jim Thomson]

Date: Mon, 13 Nov 2017 10:09:29 -0700
From: Jim Garland <4cx250b at miamioh.edu>
To: amps at contesting.com
Subject: Re: [Amps] Glitch resistor + fuse?

<Nearly all commercial amplifiers have glitch resistors in the HV line, 
typically about 50 ohms. A 10W resistor is fine, and a larger wattage 
one is fine also, but the larger power rating is unnecessary. The glitch 
fuse is to limit the current pulse in the event of a short in the HV 
line, as might result from an internal tube arc. For example, a short to 
ground in a 2500V power supply will result in a peak current pulse of 50 
Amps. Obviously an HV power supply can't supply that much current, but 
the charge stored in the filter capacitor bank can produce a brief pulse 
with that much current.The glitch resistor is not intended to blow, but 
to absorb the charge stored in the filter capacitors, thus minimizing 
damage to other amplifier components.

?A fuse in the HV line is also a good idea. As mentioned? in another 
comment, RL Drake provided a 2W low value (less than an ohm) composition 
resistor with the L4B and L7 amps, and these would explode like a 
shotgun shell if there was an HV Short.

Modern fuses designed for the job are a better alternative (and easier 
to find) than the Drake solution, but with a caveat. An ordinary 
3AG-type fuse won't do the job. It will arc internally and won't 
interrupt the current flow. What I use are microwave oven fuses, rated 
at 5 kV. These look like ordinary 3AG fuses but are a bit longer.They 
cost about $2 each and come in various current ratings. Typically an 
850mA rating is a normal choice for ham amplifiers, but if more current 
is desired, they can be paralleled. You can buy them from dozens of 
sources and in multiple quantities on Amazon or eBay. Here's a link to 
one supplier:

https://www.amazon.com/Glass-Tube-Fuse-0-85A-Microwave/dp/B00X74LUJ6/ref=pd_day0_469_3?_encoding=UTF8&psc=1&refRID=CSR8VA8Q86YGRD5DJX5W

73,
Jim W8ZR

###  A 10 watt rated glitch will not work with a 50 ohm value.    1 A plate current into
50 ohms =  50 watts.... so your 10 watt  rated resistor is toast right there. 
A 50 watt wire wound, with a 50 ohm value will suffice.  Drake uses
a .82 ohm +  .5 watt carbon comp resistor.... for the... HV fuse. 
Drake L4 + L7 amps run at 800 ma into  .82 ohm =  .525 watts.  I use a 50 ohm +  50 watt
WW in series with the drake  .82 ohm.. hv fuse.  

##  A  25 watt rated glitch will work, provided its only 25  ohms.   1A into 25 ohms = 25 watts.
If no room for a 50 watt, 50 ohm glitch,  you can series  2  x 25 ohm, 25 watt resistors.

##  The purpose of the 50 ohm glitch is to LIMIT the fault current to a safe value, IF a HARD fault occurs. 
The purpose of a HV fuse, in series with the 50 ohm glitch, is to INTERRUPT the  fault current, IF a 
hard fault occurs.   Wire the HV fuse inboard of the 50 ohm glitch.   IF the glitch blows its brains out,
and dangles down and touches the chassis, you want the HV fuse on the inboard side of the glitch R. 

##  IF you get a short to chassis, etc,  inboard of the HV fuse and glitch R,   wire a 2nd  HV fuse, between
plate xfmr sec and input to diode bridge, or doubler,  or  full wave CT.  This 2nd HV fuse, can go in EITHER
leg of the plate xfmr secondary,  but only one leg.   Then your diode board is protected. 

##  IF a tube arc occurs..and say you have 3000 V through a 50 ohm glitch, fault current is just  60 A.   60A  will
open off a 1 A  rated fuse asap, like in less than  2 msecs.   With a 7700 vdc supply and a 3A rated fuse, and a 50 ohm glitch
R, the fault current is  154 A.   154 A will open off a 3A rated HV fuse in less than 2 msecs every time. 

##  You have to distinguish between a hard fault and a soft fault.   A soft fault would be like when plate current is a tiny 
bit above the tubes rated max plate current.   Ditto with grid current, esp on tubes with delicate grids like a 3CX-800A7, etc. 
Fast electronic shut down, either opening the key line or shutting down the drive level asap  for delicate tubes is required. 

##  On tubes like  3-500Z,  3CX-3000A7 etc,  a simple  FAST grid fuse, like a 3agc will suffice, wired between chassis  and 
positive terminal of grid meter...or grid shunt.  Suck too much dc grid current, and grid fuse open up.  Amp cant be driven, since 
no path for dc grid current.  Power output drops to zero watts, and input swr, between xcvr and input of amp rises to near infinity,
xcvr shuts itself down asap, dure to high swr, event over. 

##  I have several of those DPST, sky high V rated open frame relays that are mentioned in  marks. K5AM articles.   They use 
a  120 vac solenoid coil.   Opening off  B+ during a soft fault is one thing, but trying to open off the B+  during a hard fault is another matter, esp if
you have a 5000 to 7700 vdc B+ supply and  a few hundred uf  filter cap.   
A vac relay was tried for  the same idea in the old orr book, using the w8zr  8877  design.  Works ok for soft faults, but with a hard fault, you can really fry a 
small vac relay per jennings and kilovac engs.   marks other spst vac relay, contactor, was rated at 50 kv and 50A... but they are
rare..and huge things, at a foot long.   The open frame solenoid   uses a 120 vac coil.   After trying various the above schemes through the years, and robust tubes
in GG, I came to the conclusion that for my application, the simple, CORRECTLY rated  HV fuse, in series with the HD  50 ohm glitch R..and a second
HV fuse, also correctly rated, between  xfmr and diode board... provides for ample and fast protection.   The 120 vac solenoid dpst open frame relay will
not open in less than 2 msecs... like a HV fuse will.   I would use the med sized vac relay or  120 vac solenoid open frame relay for soft faults, but not hard faults,
at least for my particular applications. 

##  I also use a magnetic hydrualic two pole breaker in the incoming 240 vac line, inside the HV supply.  These are the type that are known as.... instant trip, with
little to no delay at all.  The fluid has been removed from the internal oil reservoir, by the factory, to provide for the instant trip function.  These types are much faster than
the  run of the mill heat activated, thermal  breakers.  Typ thermal breaker, like you have in your main 200A panel, will open off with a hard fault, in typ 35 to 40 msecs.

## another method is to use a large contactor, with an adjustable current trip threshold.    These can be used for a soft fault,  but beware of their use for a hard fault.

##  During a hard fault, you not only have a huge amount of energy stored in the HV filter caps, which is only current limited by the internal resistance of the cap, + the Z of 
any HV wiring.... you also have the ..follow on current   coming from the plate xfmr and diode board.   I have seen  paralleled  100 ohm wire wounds, 225 watts  each, used for
a 50 ohm glitch, on a 7800 vdc supply with almost 180 uf filter cap.  Incorrect HV fuse used, and the 2 x 225 watt resistors look like you hit them with a propane torch ! 
Replaced with 4 x 200 ohm, 225 watt wire wounds... in parallel, to make the 50 ohm, 900 watt rated glitch assy, air cooled by fan, and this time, the correct HV fuse used. 
Crow bar it all you want, nothing happens, blown HV fuse, thats it.  Buts thats a one off case. 

##  W8ZRs use of simple 5 kv microwave oven HV fuses, rated at 850 ma, is a good one.  just one would suffice for most amps, and 2 can be used in parallel for more current. 
I have also tried using them in series , for more B+..and that too works, or 4-8  of  em in series parallel for more B+  and more current. 

## HB hv fuses also work, provided the wire fusing element is either inside loose heat shrink, teflon spagetti tubing, or wrapped in 33 or 88 tape..then suspended  between
stand offs.   But test em on the bench..1st..with a variable low Vdc supply, like a 0-60 vdc at 5A type supply. 

Jim   VE7RF