What does the NFB do to the output impedance? (Ignoring the 50k pot)
The NFB will lower the small signal output impedance by the amount of feedback. The amount of feedback in dB is the difference between the open loop gain and the closed loop gain. Given the 6N3 has a mu of abut 35 and a rp of about 7K or so then.
1. the gain of the first stage is about 35 * 80/(80+85) = 17 times
2. The gain of the second stage is about 35 * 33/(33+25) = 20 times
So open loop gain is about 17 * 20 = 340 = 50dB
Closed loop gain will be close to (10K+2K2)/2K2 = 5.5 times = 15dB
Therefore the amount of feedback is 50 - 15dB = 35dB
OP impedance without NFB is about 33K//25K which is about 15K
NFB will reduce this by about 56 times to about 266 ohms
Remember that small signal output impedance is no indicator of drive capability, especially at large signal levels.