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EF86 Screen Grid Effects Explained

The document discusses the effect of screen grid voltage on pentode vacuum tubes like the EF86. It shows graphs of the tube characteristics with and without a screen stopper resistor, demonstrating how the screen stopper provides screen current feedback to reduce nonlinearity. It also shows oscilloscope traces showing how reducing the screen voltage progressively lowers the transconductance while increasing input sensitivity, demonstrating this fundamental effect of pentode screen grids. Understanding this effect is important for designing guitar amplifier circuits using pentodes and tetrodes.

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Diego García Medina
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96 views3 pages

EF86 Screen Grid Effects Explained

The document discusses the effect of screen grid voltage on pentode vacuum tubes like the EF86. It shows graphs of the tube characteristics with and without a screen stopper resistor, demonstrating how the screen stopper provides screen current feedback to reduce nonlinearity. It also shows oscilloscope traces showing how reducing the screen voltage progressively lowers the transconductance while increasing input sensitivity, demonstrating this fundamental effect of pentode screen grids. Understanding this effect is important for designing guitar amplifier circuits using pentodes and tetrodes.

Uploaded by

Diego García Medina
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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23/3/2019 The Valve Wizard

The Valve Wizard


to design valve guitar amplifiers!

Menu The Effect of the Screen Grid


Home

Above left: The EF86 characteristics at a fixed


screen voltage of 100V. Note that the knee is well
defined, and the input sensitiviy is lowest: If centre
biased it would take roughly a 4Vp-p input signal to
cause clipping.
Above right: Using the same screen voltage but
inserting a 100k screen-stopper. The screen
current causes a drop across the resistor during
signal conditions, dynamically reducing the screen
voltage in antiphase with the input signal. The knee
is now softer, and the transconductance is now
quite constant, indicating a great reduction in non-
linearity thanks to this screen current feedback. The
input sensitivity is reduced, however, and the
possible output signal swing may also be reduced
when using an unbypassed screen resistor,
depending on the load being used. Also note the
small 'kinks' in the curves around Va = 20V, these
are probably due to brief 'spurts' of secondary
emission causing the average screen current to
increase momentarily. The same 'kinks' can be
seen in the published characteristics of many
power pentodes. Note: These are dynamic
characteristics; they only apply during signal
www.valvewizard.co.uk/EF86.html 1/3
23/3/2019 The Valve Wizard

conditions. For chosing quiescent conditions /


bias points we must still use the
characteristics above left. Why? Because
there can't be any screen compression effects
unless there is a signal to compress!

Below: These oscillograms show the effect of


altering the screen voltage on a pentode. As it is
progressively reduced, the gm (and therefore the
voltage gain) progressively falls, while the input
sensitivity increases. The basic 'shape' of the
curves remains broadly the same, but the grid
curves get 'squashed down'. Recognising and
understanding this effect is fundamental to
understanding pentodes / tetrodes, and
immediately makes you about twice as
knowledgeable as most guitar 'amp techs'!

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23/3/2019 The Valve Wizard

www.valvewizard.co.uk/EF86.html 3/3

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