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
-----Original Message----- From: Dick NY1E
Sent: Monday, November 13, 2017 10:03
To: amps@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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