This was one of the more difficult procedures for me to figure out, and I think I might have given in a little to the “it looks like the book so I’ll roll with it” mentality. I understand that it is designed just like a regular preamp gain stage, but there seemed to be a little magic that I couldn’t quite grasp. Despite that, I know that this section is pivotal to the push-pull output stage, so I had to dive in eventually. I saved this section for last because I wasn’t ready to wrap my mind around it yet. Now’s that time. Using The Valve Wizard again for this, you can read up on it with me by using the link in this sentence.
The first thing to do is devote some voltage to the tail. I don’t have a high voltage power supply in my design, so I don’t have as much as I could give it. I figured that having 235 volts for the anodes would be sufficient, and since I have a 277 volt supply, this gives the tail about 42 volts. I picked the 82k and 100k resistors for the anodes based on many common designs. This gives me 2.8mA and 2.3mA respectively. Now I can draw my load lines from these point to 235V and find a bias point.
I chose my bias point to be around 1.4V. The only unfortunate result from this point is the awkward resistor values needed in the tail. The quiescent current is about 1.6mA so 42V/1.6mA gives me a total tail resistance of 26.2k ohms. In order to have my bias point at 1.4V, I need to divide this by the same quiescent current which gives me 875 ohms for the cathode resistor. Subtracting this from 26.2k ohms leaves 25.3k ohms for the remainder of the tail. (I’ll use a 820 ohm and 56 ohm resistor for 875 ohms, and a 22k ohm and 3.3k ohm resistor for the 25.3k ohm resistor.)
The grid resistors are 470k ohms, which since there are two this gives an input impedance of almost 1M ohms. The DC blocking caps at the input are chosen to be .01uF to satisfy the -3dB cutoff at 20Hz. You can see The Valve Wizard’s calculation and explanation of this.