Rectifier Valve Heater Voltage

[Andy M0FYA]

Something that puzzles me - why did the filament or heater voltage of rectifier valves become standardised at 5v, particularly for octal-based valves, at a time when 6.3v had become the standard voltage for other valves?

I'm sure this is just some quirk of history as valves progressed from typically 2v to 4v battery filament supplies to 6.3v derived from a mains transformer, but I'm intrigued. Any ideas?

Andy

[pwdrive]

Good question Andy, to which I don't have an answer, most of those 5Y3, 5U4 types took quite a heavy heater current, maybe it was a trade off..heater material/temperature/emission, poss max ripple current forward voltage drop etc.
Would be interesting to find out for sure, something in the back of my mind thinks I saw the answer at sometime in the past but no idea now.

Lawrence.

[Radio Fixer]

I think the reason stems from the USA use of 2.5V for tube heaters back in the early 30's. A line up for a set I'm working on at the moment uses ones that draw 0.8A, 1A and the 2A5 O/P tubes are 1.75A. The O/P valves are in class A and so a beefy rectifier was needed. What else than the 80 which had been in development since the late 20's. Only draw back it needed a big filament and at 2.5V it would have consumed 4A so maybe it was a logical step to say OK double the voltage to 5V and then it only needs 2A (which it does).

I have an article by Peter Lankshear, who knows as much about early radios as anyone out there. In it he traces the history of the 80 all the way to valves like the 5Y3G which was an 80 with an octal base. It seems the 80 was such a successful design it lived on ... why change something that worked and as it was indirectly heated it needed a separate transformer winding anyway so the 5V was no problem.

Its a good article and I will find the time to scan it and add to the thread.

Gary