I think my initial message was misleading. I think my brain was foggy,
too. I was conflating two different designs that I was thinking about!
The tube is a 4CX1000A, AB1 with a swamping resistor. The grid current
(if any) is measured in series with this resistor. The cathode is
grounded. I have put a 25-watt 0.25 ohm resistor from the negative side
of the power supply to ground, essentially a meter shunt. The plate
meter is a milliammeter with a series resistor that reads the voltage
across this shunt. The meter is in the RF section, so the wire from the
negative side of the power supply is not really the B- connection, it is
just the metering voltage.
I think all I need is a diode in the RF section to protect the meter in
case of an arc.
The power supply section is finished except for the cover, which I hope
to do this weekend. Then I can start the fun part.
On 1/19/13 10:23 AM, Jim Garland wrote:
Vic,
I'm a little puzzled by your proposed schematic. You didn't mention
what tube you'll be using in your new amplifier, but I'm guessing
you'll be using a grounded grid triode(s) design. The customary
practice is to measure the plate current with an ammeter in the B-
lead of the supply. The cathode of the tube is raised above ground by
the operating bias, so that the voltage developed across a low-value
resistor to ground from the B- lead is used to measure the grid
current, not the plate current.
There are basically three ways an arc can take place in the standard
design. First, there can be an arc to ground in the RF deck. Second,
there can be an arc to ground in the power supply. And third, there
can be an internal arc in the tube. In the first two cases, the arc
current flows from ground up into the B- lead and back into the power
supply. Each of these cases is usually addressed by means of a diode
in both the RF deck and power supply, each of which has it's anode
grounded and its cathode on the B-minus lead. Although there has been
endless debate on the subject, I've never seen any reason to use
back-to-back diodes, or multiple diodes in parallel. Since the grid
current flows from the B- lead to ground through a resistor, a diode
with it's anode connected to the B- line will clamp the voltage to
0.7V and seriously distort grid current readings. Normally, one wants
a 5-10 ohm resistor between B- to ground for grid current measuring
purposes. BTW, a single 6A10 diode has a surge rating of 250A, so
there's really no need to parallel more than one.
There's one other precaution one should make, and that's to put a 1 W
resistor (say 220 ohms) between B- and ground in the power supply.
This resistor has high enough value that it won't distort grid current
measurements significantly. It's purpose is to keep the B- from
soaring above ground if the power supply is turned on with the HV
cable to the amplifier disconnected. It's basically just to provide an
anchor for the B- side of the power supply. Normally, a negligigle
current flows through it. I checked my most recent power supply and
discovered that without the resistor, the B- lead drifted up to
several hundred volts above ground, when the RF deck was disconnected
from the power supply.
Aside from these precautions, and possibly placing protection diodes
across the plate meter terminals, I don't believe any other arc
suppression measures are needed or desirable.
73,
Jim W8ZR
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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