6AS7 maximum DC current is 250mA per tube (0.25 Amps).

20 Watts rms into an 8 Ohm load requires 1.58 Amps rms.

For a sine wave, Peak amps = Root (2) times rms Amps. Root (2) ~ 1.414

1.58 Amps * 1.414 = 2.235 Amps peak.

A 20 Watt rms amplifier into 8 Ohms requires 2.235 Amps peak output.

If you want to use only 2 6AS7 tubes to get 20 Watts rms, you will need an output transformer.

An output transformer can reduce the output impedance, and more importantly in this case increases current.

6AS7 tubes have a mu (u) of 2. The driver will have to swing lots of volts.

The driver will also have to drive the capacitance of parallel triodes.

Probably the most efficient 6AS7 amplifier will be push pull. Two 6AS7 could produce 20 Watts

with the plates driving the push pull transformer.

When you parallel the 2 triodes in a 6AS7, you should use individual self bias resistors, with individual bypass caps. Thats 4 Rs and 4 Cs.

I am certain there are examples in this forum using 6AS7 in push pull with an output transformer. Perhaps one of them produces 10 Watts.

You are half way there.

There may not be any examples of Parallel push pull with an output transformer, but you need that.

So . . . to go from a single 6AS7 to two 6AS7 will most simply require:

Doubling the B+ current, paralleling the two triodes in each 6AS7s (one 6AS7 for Push, and one 6AS7 for Pull); and using an output transformer that is rated for 2X power (more current and laminations) and also rated at 0.707 of the primary impedance of the 10

Watt single 6AS7 amplifier.