Fig. 2.
Comparison of Ultra-linear operation with triode and tetrode
operation using a push-pull stage without feedback.

The situation is demonstrated graphically in
Fig 2 where it can be seen that over a narrow band of operation
where screen load impedance is about 18.5 per cent of plate load
impedance, the new arrangement provides the high power output of
tetrodes with low internal impedance such as is normally obtained
from triodes, while distortion figures are equal or better than the
extremes of operation. We have achieved a new tube type without
designing a new tube. This tube is neither triode nor tetrode, but
its improved linearity over either of those types justifies the
designation "ultra linear."

The Complete Amplifier
Circuit
In
applying the ultra-linear output arrangement to complete amplifier
circuits, it was found that the simple version of (B), Fig. 1 could
be used to advantage. By feeding DC to the screens through a
properly placed tap on the primary of the output transformer, the
operating conditions are preserved, and the close coupling between
screen and plate is advantageous when feedback is carried around the
stage. The disadvantage of this simpler arrangement is that screen
and plate must operate at the same DC potential. In the particular
arrangement used the screen and plate are operated at the same

potential (350 volts plate to cathode) without exceeding
dissipation requirements, either quiescent or at maximum output.
This new output coupling arrangement reduces screen dissipation at
high levels and is a safe mode of operation with respect to tube
life.
A circuit arrangement has been designed to take full
advantage of the ultra linear output stage. This circuit, Fig. 3,
takes into account the necessity for complete stability under
feedback conditions so as to eliminate tendencies toward transient
instability under any type of load, including the varying impedance
of loudspeaker systems.  This complete circuit offers linearity
of operation of a very high order. It is based around a special
output transformer, the Acrosound TO-300, which is 6600 ohms primary
impedance and has taps at the optimum point indicated in Fig. 2. A
special seven-section symmetrical winding arrangement placed on a
substantial grain-oriented lamination of unique shape permits a
ratio of primary inductance to leakage reactance in excess of 15,000
to 1. The response of the transformer alone is within ± 1 db from 10
to 100,000 cps with extremely low phase shift and no resonances
within this band.

pg.
3

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