Could I get away with lighting up a #47 bulb (6.3V) using a 9V supply and 3V zener? Trying to use a tube amp bulb in a pedal in a pinch :icon_idea:.
Seems alright to me :) Incandescent bulbs aren't to fussy about voltage, it might shine a little brighter/dimmer if the voltage is off, but you'll get the added (dis)advantage of shorter/longer lifetime.
A quick search says it need 0.15A. 0.15A * 3V drop over the zener = 0.45W power being dissipated, so use at least a 1/2 watt zener diode.
Use a resistor.
Aim for like 5V. (Life of #47 is short on 6V; and modern #47s are not what they used to be.)
9V-5V= 4V to lose.
#47 is nominal 0.150 Amps.
4V/0.150A = 27 Ohms
4V*0.150A = 0.6 Watts
Try two 47-Ohm 1/2W in parallel to each other, series with lamp.
Note that 0.150 Amps, 150mA, is a LOT of current by pedal standards.
With an LED you could get away with well under one mA to power the indicator light, saves a lot on batteries.