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Re: [Amps] FW:   negative screen current

To: Matt <>,
Subject: Re: [Amps] FW:   negative screen current
From: peter chadwick <>
Date: Tue, 31 Dec 2013 12:46:35 +0100
List-post: <">>
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.


Peter G3RZP

 Message Received: Dec 31 2013, 03:40 AM
 From: "Matt" <>
 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
 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.
 -----Original Message-----
 From: Amps [] On Behalf Of Peter Voelpel
 Sent: Monday, December 30, 2013 9:25 PM
 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.
 -----Original Message-----
 From: Jim Garland [] 
 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
 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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