I'm now a C-3 Tech...
At church we have a Hammond C-3 Organ and a leslie speaker. a genuine original. the real deal, with the spinnin' speakers, and vacuum tubes!
The C-3 and/or Leslie Speaker decided to make an ugly distorted sound during rehersal tonight. (it's been doing it intermittently this last week)
Get in there...start tapping around carefully, find that tapping a tube makes the Leslie Speaker start getting distorted and popping. Easy. Pop the back of the organ console where the box of spare tubes live. (Watch the worship leader look at you funny as you remove the back of the organ during rehersal at this point..)
(The old tube is lying next to the mic stand)
Grab a 6550, go to the back of the speaker, power down the organ, unplug the speaker, replace the tube, plug the speaker back in, power the organ back up...and off it goes. nice.