heater voltage

[trem]

Sorry if this is an obvious question, but......... How/Why did 5 Volts become a common voltage for rectifier filaments, when so many other tubes use a 6 Volt heater.?
I realize there were a few 6 Volt recters, but the majority.......
Thank You

[olddawg]

Sorry if this is an obvious question, but......... How/Why did 5 Volts become a common voltage for rectifier filaments, when so many other tubes use a 6 Volt heater.?
I realize there were a few 6 Volt recters, but the majority.......
Thank You

Why 5, 6.3 and 7 volt heaters?

[DRH1958]

FWIW, here is this subject on another forum: Antique Radio Forums ? View topic - 5 volt and 6.3 volt secondaries?

[trem]

FWIW, here is this subject on another forum: Antique Radio Forums ? View topic - 5 volt and 6.3 volt secondaries?

Thanks....... Some interesting comments:

"This same question was asked by Brian in Australia on the Tube Collectors Association forum a few days ago. There was one suggestion that it was optimum filament length for the tube design. My opinion is that it just happened. Here's my reply on that forum for what it's worth. I hope others will give their thoughts too:

Didn't it just happen? The original 5 volt standard came about because
it worked well from a 6 volt accumulator; it allowed the tubes to
operate when the battery discharged below 6 volts. The 6.3 volts arrived
to cope with an auto's 6 volt system drooping below 6 volts when idle
and rising over 7 volts on charge. Since the rectifier using a directly
heated filament couldn't run off an auto's battery (it would short the
B+) there was never a need to change it from the original 5 volts which
had become some sort of standard. Surely the filament design just
required selecting the right gage wire for the conditions. The range of
5 volt rectifiers covered a large variety of output currents and in
Europe the standard was 4 volts.

Don Black.

PS Rich is quite right but 5 volt directly heated rectifiers were used right up till the end of the tube era and there were also 5 volt indirectly heated tubes such as the 5AR4 and 5V4 that had the cathode tied to one heater pin, necessitating an insulated 5 volt winding. That removed the requirement for high voltage heater cathode insulation. There were 6.3 volt rectifiers made with isolated cathodes such as 6X5 and it's miniature version 6X4 for use in auto sets and AC sets with a single 6.3 volt winding."

[J M Fahey]

I guess a "different from what all other tubes use" voltage was chosen so there was not even the least doubt or temptation to make the mistake of connecting that cathode/filament together with others.
Direct heated filaments *require* very well insulated windings well separated from all others (and the core) .

Besides that, designing heaters was everyday work for those guys, I have seen many tubes which come with different filament types, and the datasheet is the same for all, save for filament voltage and current, so they clearly found the way .