Thus far in the previous post we analyzed parameters of the 6922 using graphical charts. However, there is a mathematical method to obtain the 6922 parameters. There are three AC parameters that define the characteristic of 6922, as long as they are measured at the operating point. These parameters are:
Mu = amplification factor (no units)
Gm = transconductance (mA/V)
Ra = anode resistance (KOhm)
1. Determining Amplification
The amplification factor Mu of 6922 is ratio of the change in anode voltage Va to the change in grid voltage Vg with anode current held constant.
We can measure Mu at the operating point of 6922 by drawing a horizontal line through the operating point and calculating the gain.
Typically, Mu is one of the more stable tube parameters and varies little with anode current.
2. Determining Trasconductance
The transconductance Gm is the ratio of the change in anode current Ia to the change in grid voltage Vg with anode voltage held constant.
To find Gm we draw a vertical line through the operating point and measure the change in anode current.
3. Determining Anode Resistance
Anode resistance Ra is the ratio of the change in anode voltage Va to the change in anode current Ia with grid voltage held constant.
There is a very useful equation which links Mu, Gm, and Ra together:
Of course, we can arrange this equation as necessary to find the third parameter if we know the other two.