WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I'm curious to know what will happen to a rectifier tube of filament voltage of 6.3 volt (e.g. 6CJ3 TV damper diode with cathode) if it is lit by a voltage of 5 volt instead.
the cathode will not generate enough electrons because it hasn't reached optimum temperature, so therefore it will not be able to deliver the desired current.
Eventually it will destroy the cathode (cathode stripping)
this can take a while though...
It won't get hot enough to emit significant electrons. The anode will then drag all available electrons from it and expose the cathode surface to ion bombardment which will destroy it in short order. Give the heater 6.3V - and if that means buying a cheap and cheerful 6V toroid, then do it.
The oversized filament in that tube will emit plenty of electrons. On my tube tester with 300 volts and a 100ma load, no dofference from 6.3 to 5 volts. Do expect to derate current to say 80% & expect some lower tube life- no big deal on these cheap tubes.
However, you have to ask, "Is it good engineering to deliberately run a valve in a way that theory says is damaging?" I can't help feel that this kludge will come back and bite you on the foot at some point in the future. A 6V toroid is cheap.
Others have done such without any issue reported on the TV damper rectifier tubes. The designer should just bite it cost wise and use a Mullard GZ34 or a pair if current is excessive for one tube.
