16 or 8 ohm speaker ?

[Dick.1]

A quick search suggests it might be similar to the 2038. If so, that's running from 24V off-load, and the 18VAC supply is coming from an overwind on the motor. That'll sag considerably under load. The 3 watt rating is "speech and music", which means "peak power", which means the average sine wave power will be more like 1.5 watts.

The output capacitor is 400uF, which gives a -3dB point of 50Hz into an 8 ohm resistive load. That's plenty low-enough for something like this. Assuming that capacitor is still healthy, of course

Hi Mark

The rail is measured by me at 23.5v (off load) which would appear in line with your thinking.

The docs I downloaded agree that 2028/2038 are  more or less the same as is the Ultra 6024.

Dick.