[Amps] Glitch resistor + fuse?

Jim Garland 4cx250b at miamioh.edu
Mon Nov 13 12:09:29 EST 2017

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:


Jim W8ZR

> -----Original Message----- From: Dick NY1E
> Sent: Monday, November 13, 2017 10:03
> To: amps at contesting.com
> Subject: [Amps] Glitch resistor + fuse?
> I have 2 amplifiers, an SB220 and a TL922A, they both had 10 watt 
> glitch resistors added before I got them, they have each done their 
> job but self destructed when they did. I replaced them with 25 watt 
> when it happened. My question is, one has a 1 amp fuse in series with 
> it. Is that necessary or is it going to blow needlessly when the 
> glitch resistor would handle it?
> Any opinions?de Dick NY1E
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