Schematic below shows a typical push-pull amplifier built with one 12AX7 and two EL34 tubes.
Additional pairs of output tubes may be used but calculations remain the same. The output stage is fixed biased, but cathode biased amplifiers are usually fully cathode bypassed so treatment would also be the same. We will start with finding the open-loop gain from input to output before applying feedback. For a push-pull amplifier we can consider one half of the output stage when calculating gain.
Strictly speaking, because the pair of EL34 is not perfectly balanced, its gain will be different depending on which of its grids and anodes we consider. Nevertheless, for the sake of simplicity, we will assume the balance is fair and take the gain to one anode as a half the differential gain:
R = R5 = R6 = 100KOhm || 220KOhm = 69KOhm
Ra = 65KOhm
The voltage gain of each EL34 pentode is -GmZpri. The output transformer will usually be specified in terms of its anode-to-anode impedance, which might be 3.5KOhm for a pair of EL34, but we are only considering one half of the output stage. Assuming the output stage operates in class A or AB, the impedance seen by either tube is half the anode-to-anode impedance under small signal conditions.
From EL34 datasheet the Gm is about 10mA/V. Assuming this is a class AB amplifier with a 3.5KOhm output transformer, the magnitude of the gain of one side is:
If feedback is taken from the 8Ohm speaker tap then we find the turns ratio to be:
Thus, the voltage gain of the transformer is 1/20.9 = 0.048. The total open-loop voltage gain of the circuit is:
If we aim for a feedback factor of 10dB, then the gain of the amplifier will be reduced by a factor of:
Now we can find the feedback coefficient B by rearranging the universal feedback equation:
In other words, for every volt of signal appearing at the speaker, 0.1 volts will be fed back to the phase inverter via the potential divider formed by R2 and R10 on the schematic above. Rearranging the formula for a potential divider to find R10 gives:
The nearest standard resistor is 43KOhm. A variable resistor could be used to set the exact value to taste, and presence, resonance and variable feedback controls finally added.