More than 50years ago, I was taught that it is very bad practice to rely on
mounting screws to ground a PCB. You should always use a lead, or if necessary
for RF, a suitable grounding strap. Especially with boards that can shrink or
creep, such as Duroid.
73
Peter G3RZP
========================================
Message Received: Dec 31 2013, 03:40 AM
From: "Matt" <maflukey@gmail.com>
To: amps@contesting.com
Cc:
Subject: [Amps] FW: negative screen current
I don’t know that this info would have any bearing on the discussion but for
what it's worth...
Several years ago after moving across country, I experienced a problem when
I tried to use my AL-1500 wherein the meter showed negative grid current as
soon as filament voltage was applied but before the B+ relay engaged. I
shut the amp off before the B+ timer ran out so as to avoid any unpleasant
surprises. The problem was very repeatable and I was afraid the tube may
have been internally damaged in shipping although the amp was disassembled
and the tube and transformer shipped separately in their original
packaging...
With much troubleshooting it turned out the problem was caused by a bad
ground connection at the HV rectifier board. The board in that amp design
grounds via its standoff hardware and the screw that held the standoff to
the case had become loosened during shipping. Tightening up the screw
completely fixed the problem. I later added a secondary ground strap from
the board to the chassis for peace of mind.
Matt
KM5VI
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Peter Voelpel
Sent: Monday, December 30, 2013 9:25 PM
To: amps@contesting.com
Subject: Re: [Amps] negative screen current
Yes Jim,
You are totally right, of course the electrons are always negative, if not
there would be no flow from anode to screen or either.
Its already a bit late ;-))
It is only the direction of electron flow which changes.
73
Peter
-----Original Message-----
From: Jim Garland [mailto:4cx250b@miamioh.edu]
Peter,
I may have misunderstood yout comment, and if so I apologize, but if not
then I believe you are harboring a misconception about charges and current
flow.
Electrons always have a negative charge, and there's nothing we can do to
them that will change that fact (except annihilate them with a positron,
which is a postively charged electron, and which turns them into a gamma
ray. Or we can combine them with a proton, which turns them into a
chargeless neutron. Neither of these processes are believed to occur in
vacuum tubes.) This means that if electrons are absorbed by the screen grid,
no matter where they come from, then that corresponds to a positive grid
current. The only exception is if the screen grid is floating (not connected
to any external circuitry), in which case no screen current flows and the
screen voltage becomes more negative, thus creating an electric field that
repels the approaching electrons until no more of them accumulate. The
bottom line is that electrons coming from the anode are indistinguishable
from electrons coming from the cathode, so far as their charge, mass, and
spin are concerned. They will differ only in their kinetic energy, which is
basically how fast they are moving.
If the screen grid is grounded, then there is no charge accumulation on the
screen. Electrons striking the screen from the cathode or anode flow
immediately to ground, with no charge buildup. If the screen is connected
to a positive power supply, then in equilibrium there will be a charge
deficit on the screen (resulting in an outward electric field), but this
charge will neither decrease nor increase as cuurent flows into the screen.
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