Example 4.1
A 12AX7 has a plate load resistor, Rb, of 220 k ohms and the grid resistor of the following stage, Rcf, is 470 k ohms. Tube manual data on the 12AX7 gives Amplification factor = 100 and plate resistance = 80 k ohms. What is the gain of this 12AX7 amplifierSolution.
First we calculate Rbc.Rbc = Rb Rcf / (Rb + Rcf) = ( 220 k x 470 k ) / ( 220 k + 470 k ) = 150 k ohms. Now we calculate the gain.Av = mu Rbc / ( rp + Rbc ) = 100 x 150 k / ( 80 k + 150 k ) = 65.2
Example 4.2
A triode tube is to be used as a single ended low power amplifier. The output transformer has a primary impedance of 3000 ohms. Think of Rb being 3 k ohms and Rcf as not present. Which of the following tubes will give the greatest voltage gain?12AT7, Amplification Factor = 60, Plate Resistance = 10900
12AU7, Amplification Factor = 17, Plate Resistance = 7700
12AV7, Amplification Factor = 41, Plate Resistance = 4800
12AX7, Amplification Factor = 100, Plate Resistance = 80000
Solution.
The gain for each is given by,Av = mu Rb / ( rp + Rb ) For the 12AT7 it is,Av = 60 x 3000 / ( 10900 + 3000 ) = 12.9 For the 12AU7 Av = 4.77
For the 12AV7 Av = 15.8
for the 12AX7 Av = 3.61
The 12AV7 comes in first with 15.8 and the 12AT7 is runner-up at 12.9. Notice that the 12AX7 with its impressive amplification factor of 100 comes in last at 3.61. A high amplification factor may not guarantee a high gain amplifier.
Example 4.3
A 12AX7 which is without a cathode bypass capacitor has a plate load resistor, Rb, of 220 k ohms, a cathode resistor, Rk, of 3300 ohms, and the grid resistor of the following stage, Rcf, is 470 k ohms. Tube manual data on the 12AX7 gives Amplification factor = 100 and plate resistance = 80 k ohms. What is the gain of this 12AX7 amplifierSolution
Without a cathode bypass capacitor the gain is given by,Av = mu Rbc / (rp + Rbc + Rk (mu + 1)) Remembering from Example 4.1 the parallel combination of Rb and Rcf = 150 k ohms and substituting all values gives,Av = 100 150 k / (80 k + 150 k + 3300 x 101 ) = 26.6 BACK