Why do rectifier valves have 5-volt heaters?
The first mass produced consumer tubes, ’00 and ’01, had 5-volt filaments to allow reasonable service from a 6-volt wet-cell battery via a rheostat. Then, early AC tubes went to multiples or sub-multiples of 5: 1.25, 2.5, 5, 7.5, 10 etc. The 2.5 and 1.25 volt units were designed that way to help control AC hum. The later development of the car radio precipitated the more familiar 6.3 volt standard which is more appropriate for a 3-cell battery being float-charged by the vehicle’s generator (actually more like 7 volts).
