It can be used without any limitation as preamplifier. It can drive any kind of power amplifier, because it has enough output voltage swing, enough gain (about 30db) and a very low output impedance (about 30 ohms).
The output stage is a paralleled-triode cathode follower. For the output tubes, I tried the E182CC and E288CC and nothing else. I used the Mullard E182CC and suggest only tubes by Mullard, JAN-Philips or Siemens in the output. The amplifier will drive headphones of 200 to 600 ohms impedance. For more output to drive 150 ohm headphones, add another 22K ohm resistor (9) to Rk to increase the bias current of the E182CC.
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