1LN5 (1LN5) Pentode-Tetrode Curve sweeps

Plate I/V Curve sweeps conducted on Tektronix 575 curve tracer.

In Pentode connection:

In Tetrode connection:

In Tetrode connection, but different bias levels at G2 and G3:

Regards,

-Joe

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With the help of Prof. Rudolph and Joe Sousa, here an explanation to the curves:

The first tubes were triodes with just one
control grid. The disadvantage of this device was an
unwanted capacity ( Cag ) between anode and grid.

This capacity has two disadvantages :
1. Cag appears raised by the triode gain, in
parallel to the input capacity ( Miller – Effect ). The
effective input capacity is significantly raised.
2. In selective RF amplifiers, the Miller effect
on inductive plate loads, easily causes unwanted
oscillations. Oscillation countermeasures include
neutralization as was usually found in RF amplifiers of the 20’ and 30’.

An obvious idea was to add a second grid between
control grid and anode, called the screen grid. The
German name is ‘Schirmgitter, which means shield
grid. The tetrode was thus born. The control
grid is now shielded, or screened, from the anode, by the screen grid.

Tetrodes suffered from a new disadvantage, which
one can see easily in Joes plots.
There is a kind of saddle in the Ia(Ua) curve.
This causes nonlinearities and harmonics in the
output signal.

The areas, where the curve falls, have negative
resistance. This can cause unwanted oscillations in the output signal.

The explanation of this effect is the secondary
emission of electrons from the anode plate.

The positive charge of the screen causes some of
the secondary emission electrons to be trapped by this grid, so they
cannot be part of the anode current. This happens
when the screen is more positive than
the anode.

Although the anode voltage rises, the anode
current decreases in the downward-sloping negative resistance region.

The next development was the pentode, with one
more grid. This grid is usually connected to the
cathode voltage. The secondary emission electrons
form the anode are deflected by this grid and moved back to the anode.

Now, the curve has no saddle.
 

Georg Beckmann

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