Ronald wrote:

Hey Tim ,

Just look up "cathode follower", set up a 6080/6AS7 circuit with maybe 30
to
50mA flowing.

Bin there , done that ..... (30mA)
As I said works great for 300 ohms load , but not on 8 ohms ......

Ronald .

The 6080/6AS7 is good for 20 watts of dissipation, both halves, so if
Ea was 50v, then Ia could be 400 mA.
I know this because I use a couple as pass elements in a HV regulator.
However, one would never run a tube flat out in class A for a headphone amp,
because only a small output power is wanted.
But using a supply of +50v to +100v would be fine with
100mA of anode current, and use the tube as a cathode follower,
driven by almost any medium U triode..
Then use a choke as the cathode load to pass the DC to 0V, and some resistance,

so that you can bias the 6080 grid to 0V, and still have the cathode voltage
able to regulate Ia as in the case of a normally cathode biased stage.
The phones are then cap coupled, for 8 ohms or higher via a 4,700 uF x 50 or 63
volt rated cap;
something above the cathode voltage. ( The cap should be biased with some DV).

If you have 70mA rms max of output current into 8 ohms,
that will make 40 milliwatts of power, and I suggest that
would waken the dead with headphones.
The voltage swing is only 0.56 vrms,
but if Ia was 200 mA, you'd get 1.12 vrms and 0.16 watts of power max.

An alternative to the choke is a transistor CCS.
And an alternative to a cap coupled output is a balanced output using
the two halves of the 6080 with a pair of CCS, so the phones connect
cathode to cathode.
Or an ungapped auto transformer with a CT to an RC biasing circuit is the rolls
royce,
because you can have a variety of load choices.
The drive to the 6080 is from the same twin triode as the SE amp, except
it can be set up as an LTP.

Such an LTP would need only an SE input if one side is grounded
and a CCS tail is used to the cathodes.

The PP OPT for the phones can have taps for
300, 150, 100, 32, 8 ohms easily placed along it,
and can be wound on a toroidal core using a core rated for say
50VA mains use.
The winding should have about 1,000 turns.

The transformer coupling reduces thd, since the tubes are matched
so they will always see a correct load, which never occurs with
OTL amps connected to 8 ohm loads.
But the OTL amp with headphones is OK because the power is so low,
and thus thd is low because the power is low.

Its possible to use a full SE CF 6080, with a CCS, and cap couple an AC
transformer
say using something out of one of those low power horrid transformer coupled
transistor amps
of the 50s and early 60s.
The turn ratio and primary inductance should be about right.

Patrick Turner.