Bill -- I don't think it was the external box this time, but yes, they are vulnerable to RF and ground loops. More testing needed. Mystery, intermittent hum is not my friend, especially since I'm not the only operator of the station over there.
Pete -- the external mic box dressed up the stock audio just enough, without asking too much of an unmodified amateur transmitter. There's a wide range of voices gonna be on that rig, so I may make the EQ settings even more generic than what you heard.
Most of the "demonstration" emphasis is on the receive end since the audience of museum visitors will be right there in the room. The transmit part of the equation can be shortchanged a little to help guarantee a good operating experience among the volunteer staff with licenses. Only a few have ever run tube-type transmitters, and as mentioned on here earlier, it's a challenge to cook up a standardized set of tuneup procedures.
Like how and why to zero-beat ?!?!? That's one I got the other week.
Here's a snippet of some video I'm putting together from today. I can hear the autotransformer hum getting into the camera mic, so it's easy to figure the transmitter mic caught some of it too.
http://youtu.be/3x51W8gP754