Tried searching but could not find a description of how the sound worked. From what I could gather, the phonograph signal is mixed with 60 Hz (or DC) track voltage. And from what I can see the tender had not much more than a speaker in terms of electrical components. So given the technology of the period, I'm thinking the tender speaker is simply playing the 60 Hz track voltage along with the higher frequency audio from the phonograph. And the speaker did not have sufficient low-frequency response to the 60 Hz "hum."
From the instruction sheet posted earlier, it appears tubes (vacuum) were required perhaps to amplify the weak phonograph signal to a voltage level comparable to the 60 Hz track voltage?
