Great comments, Jim.
You're right, of course, about a 50ohm/10W glitch resistor being
inadequate. I panicked when I read your comment and checked the
schematic of my latest two power supplies. It turns out I used 10 ohms@
50W, not 50 ohms@10W. I got things backwards in my comment. Whew! I
realize that 10 ohms may be a bit too low for most power supplies, but
these particular supplies can source a little over two amps, so 10 ohms
isn't unreasonably small in worst case conditions. In addition to the
glitch resistor and HV fuses (two or three in parallel, depending on the
RF deck), I also have have an overcurrent trip circuit and, like you, a
double pole magnetic circuit breaker in the primary line.
Re vacuum relays, I agree that that most any moderately sized vacuum
relay in the HV line can stick if subjected to a DC short to ground.
I've tried to minimize that problem with Gigavac relays that are
designed to hot switch DC voltages of 10kV at 10A, if I"m remembering
right. I had an internal arc in a new GU74B, which tripped everything,
but the Gigavacs worked fine. (The Gigavac relays aren't intended as
protection devices, but just to route the HV around to different RF
decks.) Years ago, I learned the hard way that ordinary vacuum relays
designed for RF switching should NEVER be used to interrupt DC currents.
Here's a tip, I learned at the same time. Sticking vacuum relays can
often be fixed by holding them near a powerful magnet. Once freed, they
seem to work normally, but I imagine they don't meet full specs for
current capacity.
BTW, are you sure that the Drake .82 ohm fuse resistors are 1/2W? I
looked in my resistor bin and found 2W resistors. Probably doesn't make
any difference. Whatever, they sure make a lot of noise when they blow!
73,
Jim W8ZR
On 11/14/2017 4:32 PM, Jim Thomson wrote:
Date: Mon, 13 Nov 2017 10:09:29 -0700
From: Jim Garland <4cx250b@miamioh.edu>
To: amps@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
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