A 548

[Paul_RK]

Incidentally, why were most of the rectifiers of the period 5v?   And why 6.3v and 12.6v, is there some magic reason?

Sam.

The usual reason offered for 6.3V is that it's the voltage provided by a fully charged 6V lead-acid battery... which only raises the question of why, as 4V and 2V heaters were commonplace in the heyday of lead-acid batteries, they're not quoted as 4.2V and 2.1V respectively.

It's also claimed that 5V was chosen as the minimum useful voltage from a 6V accumulator as it discharges, which seems to make even less sense as (a) there's no rationale for specifying a heater according to the minimum voltage it's going to see, and (b) rectifiers never run from lead-acid batteries anyway: ah well. The U50 like most other rectifiers is directly heated so has to have a transformer winding independent of the supply to the other valves, so pretty much anything could have been chosen and there was no advantage in powering it from the same winding as them.

In the very early days of mains radio, pretty much anything was chosen, hence I'll have to make a few changes if I ever try to get my 1929 Columbia radiogram up and running: its three RF valves are present and probably correct, all 4V heater types, but missing are the output valve, a PX650 power triode with 6V heater - highly coveted by audiophools, hence or otherwise almost unobtainable - and the rectifier, which was an oddity with an 8V heater.

Paul