1965 Fender champ amp output power tested on an oscillscope
Signal is 0.03 - 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 65 champ looks almost identical to the 66 champ.