[Captainclock]

I wouldn't put it past that Smoke Detector to of had a vacuum tube in it for part of the circuit, seeing as from what I can see of the circuitboard there are no transistors in there just capacitors and resistors (which is all a vacuum tube circuit needs to run) plus if its running off of AC like most hardwired Smoke detectors do you already have the 120volts it needs to power the circuit, more than likely it has a silicone diode for its recifier circuit and then the tube is probably for the control circuit to make sure the smoke detector goes off like it should when it detects smoke, more than likely on the smoke detector chassis somewhere there's probably a tube diagram that tells what kind of tube it needs to power it.

This kind of reminds me of an old Slave Clock I saw at the antique store once where the clock's correction circuit was powered by 3 vacuum tubes (not somehthing you'd expect to find in a time only wall clock but then again the slave clock's correction circuitry had to be powered by something right?)