Resistor value to use 8 ohm speaker on older Loksound 100 ohm

[RAILCAT]

The ESU decoder tester 51900 has a switch for the speaker to allow older decoders 100-ohms
or later decoders 8/16ohm. This seems to have a set of 4x330 ohm SMD resistors
and a selector switch.

Is it then safe to use an 8-ohm speaker with an older 100-ohm output decoder
with a suitable resistor and if so what value.?

The plan is to use the sugar cube-type speakers in place of the originals.

[lyled1117]

while it is 'safe', it is counterproductive. You will be placing nearly all of the energy into the resistor. Roughly 90% of the audio is being turned into heat in the resistor, you may not hear anything. This presumes you use a 91 ohm resistor (standard value) to add up to close to 100 ohms. The voltage, and thus power divide proportionately across the two resistances. In round numbers, the 91 ohm resistor gets 90% of the energy

Lyle D