Thanks Thomas, I'm exploring other OTL alternatives.
You will not be able to drive 80 ohms phones to reasonable levels using a single tube like the 6SN7 or ECC88 in an OTL configuration. To output 10mW into an 80 ohm load requires a peak current of nearly 16mA which means the idle current of an OTL stage has to be at least this value. The thing is you only need a voltage of 900mV so you are asking a tube to produce a high current at a low voltage, exactly the opposite of what it does well. The answer is a transformer and with a suitable turns ration an ECC88 will happily drive a pair of 80 ohms phones to high levels at low distortion and with plenty of headroom.
Phones are 300 ohms and I want to avoid OTL due to expensive prices. If necessary I can use more tubes.
Oops, must have been thinking of another thread when I used 80 ohms in my calculation. For 10mW into 300 ohms you need a peak current of just over 8mA. You might just about get this from an ECC88 SRPP.
If you use 100 ohm cathode bias resistors and a 200V supply you can run at an idle current of 15mA and keep within the allowed plate dissipation and output 2.66 volts rms into a 300 ohm load with about 1.6% distortion according to LTspice which is just under 25mW. For 10mW you need about 1.75V rms which this circuit will do into 300 ohms with a distortion of just over 1%.
In practice I have found that LTspice overestimates the second harmonic distortion and underestimates the third.
You could reduce the distortion by an order of magnitude with suitable negative feedback.
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