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Re: [Amps]  negative screen current

To: "'Peter Voelpel'" <>, <>
Subject: Re: [Amps]  negative screen current
From: "Gary Schafer" <>
Date: Mon, 30 Dec 2013 20:34:40 -0600
List-post: <">>
that explanation of what happens in the PLATE seems right on.

But notice that when they are talking about "secondary emissions from the
plate" reducing plate current, those are secondary emissions from the PLATE,
and not electrons collected by the plate. The plate is emitting the
secondary electrons supplied by the power supply. 

Lower plate current:
(difference between positive and negative power supply current.) in this
case the net positive plate current is greater than the negative current of
the secondary emissions so you see a reduced plate current due to the
negative current.

The article says nothing about secondary emissions from the SCREEN. It only
says that the screen can collect some of the electrons from the secondary
emissions from the PLATE along with some of the electrons directly from the

When it comes to secondary emissions from the SCREEN, that is electrons
emitted FROM the screen, not collected by the screen. 

The process of secondary emission from the screen is similar to that of the
plate. Electrons bombard the screen and knock lower level electrons from the
screen. If the screen should emit more secondary electrons than  electrons
collected it will result in negative screen current. 
Note that electrons collected by the screen can be from the cathode or the
electrons collected from the plate secondary emissions.

Ceramic tubes generally operate with a low screen voltage compared to plate

"Valves of this type are usually designed to operate with screen voltages
only about 15% of the anode voltage compared to about 50% in the early
types. This enables them to swing over a much larger anode excursion
increasing the power output and making them even more efficient. These
valves have a very small screen current usually near zero, because when the
anode is swung right down below the low screen potential, there is some
screen secondary emission and the screen current could be negative."

The last paragraph is quoted from the article that George T Daughters, K6GT
posted awhile back.

Gary  K4FMX

> -----Original Message----- 
> From: Amps [] On Behalf Of Peter
> Voelpel
> Sent: Monday, December 30, 2013 6:11 PM
> To:
> Subject: Re: [Amps]  negative screen current
> Extracted from
> "As the screen grid is positively charged, it collects electrons, which
> causes current to flow in the screen grid circuit. This uses power and
> heats
> the screen grid; if the screen heats up enough it can melt and destroy
> the
> tube. There are two sources of electrons collected by the screen grid?in
> addition to the electrons emitted by the cathode, the screen grid can
> also
> collect secondary electrons ejected from the anode by the impact of the
> energetic primary electrons. Secondary emission can increase enough to
> decrease the anode current, since a single primary electron can eject
> more
> than one secondary electron. The reduction in anode current is because
> the
> external anode current (through the connection pin) is due to the
> cathode-to-anode current minus the secondary emission current. This can
> give
> the tetrode valve a distinctive negative resistance characteristic,
> sometimes called "tetrode kink". This is usually undesirable, although
> it
> can be exploited as in the dynatron oscillator. The secondary emission
> can
> be suppressed by adding a suppressor grid, making a pentode, or beam
> plates
> to make a beam tetrode/kinkless tetrode."
> That explanation is identical to that found in German textbooks
> 73
> Peter

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