1966 Fender champ amp output power tested on an oscilloscope
Signal is 0.05 - 0.08 VAC RMS at 440 Hz.
Treble and bass set on 5
O scope set at 2 volts / cm
I see it max out at 6 peak volts.
6 peak volts / 1.4 is 4.25 rms
4.25V * 4.25 V / 5 ohms = 3.6 Watts
the 66 champ and the 65 champ look almost identical.