Output Impedance?

[Alastair E]

Must be a little dense this afternoon, but Im trying to figure the O/P impedance of my recent OTL.

At 8 ohm and 10V p-p o/p into 8 ohm.

At 4 ohm I get 9V p-p o/p without altering amp input level from sig-gen. (used WW resistors as load)

This was not done at full amp O/P, as I didnt want to over-stress my single pair of 6C33 in the O/P stage, although I have achieved 30W P-P before the onset of visible distortion into 8Ohm WW resistor......

Could some kind and more intellegent guy give me some clue as to the O/P impedance and maybe the damping factor?

[tubelab.com]

I don't know about the intelligence part, but:

A theoretically perfect voltage source of 11.25V p-p with a 1 ohm resistor in series with it will give the same results as your amp. This is known as the Thevenin equivalent circuit. The 1 ohm part is the output impedance. Which would indicate a damping factor of 4 with a 4 ohm load, and 8 with an 8 ohm load.