[Amps] Question about separate power supply/RF section

Jim Garland 4cx250b at miamioh.edu
Sat Jan 19 13:23:40 EST 2013


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.



Jim W8ZR 

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