It is not essential to have a constant gain in an amplifier to have
acceptable distortion. The G2DAF circuit is variable in gain, but it is
simply one of a number of variables that can be explored.
Although RCA does not explore the G2DAF circuitry, they certainly do discuss
variable gain amplifiers. In the 1959 RCA Receiving Tube Manual, they
discuss a volume expander for the use in high quality audio circuits. They
say: "A volume-expander amplifier has a variable gain which is greater for a
high amplitude signal than for a low-amplitude signal. The volume expander,
therefore, amplifies loud passage more than soft passages". In their
circuit, they apply the signal to an amplifier tube and also amplify that
signal in a separate stage and feed it to a rectifier and then apply that
rectified voltage to grid no. 3 of the amplifier tube. They go on to state:
"the gain of the amplifier increases with increase in signal amplitude and
thus produces volume expansion of the signal. The voltage gain of the
expander varies from 5 to 20."
I could not find any caution that the circuit would not work, or that it was
put in the book to mislead. Or that it causes severe distortion. And, this
circuit sounds very similar to the G2DAF circuitry. Perhaps G2DAF was
misled by RCA.
Rf and if amplifiers in receivers use variable gain tubes. And, although
they can distort, they can also be made to operate with little distortion.
Although the original G2DAF circuitry does does have a voltage on the
control grid which is 1/2 that of the screen, later circuits tapped down on
the 300 ohm load resistor. Thus, drive to the control grid could be
adjusted downward so that the screen voltage and the control grid voltage
would be similar to a circuit using a stabilized screen voltage. Rf
amplifiers can be operated in A, AB1, AB2 or B, Most rf amplifiers do draw
grid current, so the real question is what was the distortion, and if there
was distortion, would tapping down on the input resistor and reducing the
drive change the distortion.
Unfortunately, Eimac apparently never tested the G2DAF amplifier, but
assumed throughout The Care and Feeding of Power Grid Tubes that the screen
voltage was constant. (Rich might say that they never thought anyone would
be so stupid as to vary the screen voltage - making the test pointless).
However, they do have an interesting comment in 4.4.4 (Effects of Screen
Voltage) "... Usually, the lowest value of d.c. screen voltage compatible
with a given power output will produce the lowest intermodulation
distortion." The G2DAF follows this statement closely by varying the screen
voltage with the power output.
The big concern regarding screen voltage is for it to drop as the drive and
output increases. This will cause severe flattening and overload and is the
reason that screen voltage in the normal circuit must be regulated. The
G2DAF circuit reverses this.
Colin K7FM
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