The complete amplifier circuit is relatively simple,
inexpensive, and efficient. With a 370-volt power transformer at
130-ma peak requirement, power output is almost as high as for a
tetrode amplifier and twice that of a triode amplifier with cathode
bias and the same power
supply. No adjustments are necessary for
balance either of the phase inverter or of the output-stage plate
current, and there are no critical values of capacitors or resistors
required. The amplifier is driven to 20 watts of output with an
input of only 0.7 volt. Feedback is carried around the complete
circuit in an external loop. There are 20 db of feedback in this
loop as measured under load conditions (about 30 db based on open
circuit gain), and a

safe margin is maintained. A small capacitor across the
feedback resistor increases the feedback in the region above 100 kHz
to smooth the high-frequency response. This capacitor is not
required to keep the amplifier stable though it does add to the
stability margin. Performance of the Amplifier.
All stages of
the amplifier have been adjusted for minimum intermodulation, and
the IM curves based on sine-wave power output are shown in Fig. 4.
These curves were run using frequencies of 40 and 2,000; 40 and
7,000; 40 and 12,000; 100 and 2,000; and 60 and 7,000, all mixed
four to one. The IM is almost identical under all conditions of test
indicating that it is completely independent of frequency,

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