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You can connect any analog audio source to any AM radio receiver, tube or transistor, by simply attaching the hot lead from the source to the center wiper contact of the receiver's volume control, and running a lead between the grounds of both sets.
For line-powered tube radios, an electrolytic cap between the source hot lead and the receiver's volume control would be a good precaution, if the source is also line-powered.
I have used this method as a quick and dirty way to check the audio stages of mal-functioning receivers as well, to isolate the problem between the RF/IF and audio stages.
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