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[AMPS] Anode current vs cathode current...

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Subject: [AMPS] Anode current vs cathode current...
From: (John Lyles)
Date: Tue, 3 Mar 1998 15:15:40 -0700
When putting an ammeter or shunt/voltmeter in the ground (negative) lead of
a HV DC supply, remember that this connection can be at a high potential
when there is a dump of energy in a sparkover in the B+ lead. This includes
tube arcs or choke arcs to chassis. During the instant of the arc, there
may be only hundreds of volts of potential drop across the ionized path of
the conducting spark. But the big capacitor with it's charge won't go to
zero instantly. So, like a see-saw with a lightweight kid on one side (your
meter), the fat boy drops the other end (an arc), and the kid flys into the
air.  This happens until the line fuse blows, unless your series R in the
B+ lead carries the difference. But that means it's value must be much
higher than the ammeter shunt, or else it would be a simple voltage divider
between each side of the power supply. If the arc is before the resistor,
at the power supply side, expect fireworks at the shunt or ammeter side. If
you do meter the negative lead, insulate the thing for HV on the power
supply side, and install some sort of MOV/transorb/spark gap to snub the
spike. It should go to earth/chassis/ground. Then the potential rise is
clipped before it breaks things. Otherwise it may do damage to your meter,
your wiring, or your body.

I learned about this in college with a home brew class C amplifier using a
4CX300A. It was on 90.7 MHz, our final PA for the radio station at WUVT.
Every once in a while, the milliammeter for plate current would shower
sparks and go boom right at the front panel. This happened occasionally, so
it was an intermittent problem that drove me (the student chief engineer)
to be fearful of touching the front panel when it was running. Finally one
day it blew out around the meter in a spectacular way, and we couldn't
recover to get on the air. Upon inspection, I discovered that the Eimac
SK600 socket was carbonized around top, so that plate voltage was sparking
to chassis. This was before we learned about noninductive energy limiting
resistors in the HV lead.

Years later, I noticed that so many radio and TV transmitters had the
ammeter in the B+ lead, and then mounted it about an inch behind the front
panel window for safety. I had a particular dusty radio station that
required cleaning that glass often, as the thing acted like a dust
collector. Good thing that they didn't have a hole so that I could stick my
screwdriver into and zero the movement!


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