There is a table on the diagram. It lists different values of RG which result in different values of gain. It lists as follows.
RG in k ohms, Gain in dB.
1, plus 20.
3.3, plus 10.
10, zero.
33, minus 10.
100, minus 20.

Tube amplifier.
The inverting input goes to the grid of the left hand triode, one half of a 12AX7. It connects to ground through a 1 meg ohm resistor. The grid of the right hand triode is the noninverting input. It also goes to ground through a 1 meg ohm resistor. This is not a contradiction. If the noninverting input is grounded on the outside of the amplifier it doesn't matter what else may be connected between the grid and ground. The grid is grounded. The two cathodes connect together and to one end of a 5.6 k ohm resistor. The other end connects to the drain of a depletion mode M O S F E T, IXCP10M45S. The gate goes through a 1 k ohm resistor to minus 105 volts. The source goes through a 100 ohm resistor, then a 5 k ohm pot connected as a rheostat to minus 105 volts. The plate of the left hand triode goes through a 270 k ohm resistor to a point we will call B plus two. The right hand plate goes through another 270 k ohm resistor to B plus two. B plus two connects through a 22 u f 450 volt capacitor to ground. This is an electrolytic cap and the plus side goes to B plus two. A 47 k ohm resistor connects between B plus two and plus 255 volts. The right hand plate connects directly to the grid of the lower triode in the cascode amplifier, a 12 A V 7. The cathode of this triode goes to ground through the parallel combination of a 27 k ohm resistor and a 100 u f 100 volt capacitor. The plate of the lower triode connects directly to the cathode of the upper triode in the same tube. The grid of the upper triode connects through a 100 k ohm resistor to plus 255 volts and another 100 k ohm resistor to ground. The grid also connects through a 0.1 u f capacitor to ground. The plate of the upper triode goes through a 27 k ohm resistor to plus 255 volts. The plate also goes through the parallel combination of a 330 k ohm resistor and a 5 pf capacitor to the grid of the cathode follower, 1/2 of a 12 A U 7 or a 6 C 4. This grid also connects through a 1.2 n f, 0.0012 u f, capacitor to ground. This grid also goes through a 150 k ohm resistor and then a 100 k ohm pot connected as a rheostat to minus 105 volts. Minus 105 volts also goes through a 0.1 u f capacitor to ground. The cathode of this triode goes through an 18 k ohm resistor to minus 105 volts. The cathode is the output point of the amplifier.
End of verbal description.
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