[Boatanchors] Tube filaments

[Al Klase]

Dave,

5 volts was chosen for the original mass-produced tubes like the 201
because it was around number, and appropriate for extracting a
reasonable percentage of the available energy from a 6-volt wet-cell
battery.  (A filament rheostat was generally used to compensate for
sagging battery voltage, as well as to control gain.)  When AC receivers
were introduced,  the voltage was halved to 2.5 volts for heater type
small signal tubes like 24A and 27 as well as filament type power tubes
like 45 to help control hum.  You need a separate filament supply for
the rectifier because it sits at full B+, so it was just as easy to
stick with five volts.  So the receiving tube world was largely 2.5 and
5.0 until the advent of car radios, which, BTW, had to wait for the
development of AVC technology.  6.3 is the nominal voltage of a fully
charged three cell lead-acid battery.  The 6.3 volt tubes are designed
to tolerate the 7 volts present when the engine is running.  The car
radio also required the introduction of 6.3-volt heater-type rectifiers
like the 84/6Z4 and 6X5 or cold-cathode rectifiers like 0Z4 because
there was only one filament supply.

Al

David Knepper wrote:
> Does anyone know why RCA, etc. chose 5 volts for their rectifier tubes and 
> 6.3 volts for their receiving tubes?
>
> Just curious.
>
> Thank you
>
> David Knepper, W3ST/W3CRA
> Publisher of the Collins Journal
> Secretary of the Collins Radio Association
> www.collinsra.com
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-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/