From jv–(at)–eq1.ebr.ericsson.se Tue Nov 19 09:53:34 CST 1996
Article: 18584 of rec.audio.tubes
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From: Jos Van Dyck
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Subject: Re: Filament voltages, historically…
Date: Tue, 19 Nov 1996 14:41:14 -0800
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S. Husky Hoskulds wrote:
>
> I wonder how the number (value) 6,3 came up. It always seemed like an
> odd number to me. And also, how accurate does that have to be? Is 6.5
> ok? or 7? Just wondering…
>
> Thanks, H.
> —
>

This dates from the time equipment was battery powered.
Filaments (heaters) were driven from three lead acid batteries, giving
the voltage 3 * 2.1 V = 6.3 V.

Applying more than the nominal voltage will shorten the filament life, or
simply melt it. Applying slightly less (e.g. 5 V) will extend the tube
(valve) life, but lower output power and amplification.

Jos

From cign–(at)–elios.phy.OhioU.Edu Tue Nov 19 19:25:25 CST 1996
Article: 18635 of rec.audio.tubes
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From: cign–(at)–elios.phy.OhioU.Edu (Dave Cigna)
Subject: Re: Filament voltages, historically…
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S. Husky Hoskulds wrote:
>I wonder how the number (value) 6,3 came up. It always seemed like an
>odd number to me. And also, how accurate does that have to be? Is 6.5
>ok? or 7? Just wondering…

The only source of information that I have on this is Robert B. Tomer,
“Getting the most out of Vacuum Tubes”, Howard . Sams & Co., 1960. He
put in an interesting chapter called “Why So Many Tube Types?”. I’ll
summarize what he says about heater voltages (leaving out a lot).

The 1.1 volt filament tubes were designed to be run on 1.5V dry cells.
The reason they didn’t have 1.5V filaments is that dry cells won’t
produce a full 1.5V under load. In some systems a rheostat was used
to adjust the output to 1.1 volts.

Next, 6 volt wet batteries were adopted. These were typically run
partially discharged because the only way to charge them was to take
them to the local service center. Consequently, 5 volt filaments
were employed and rheostats were again installed to adjust the final
voltage at the tube.

The 2V filaments were originally designed for farms with wind driven
electrical systems. Supposedly, the batteries were kept at nearly
full charge, and so “the battery terminal voltage stayed fairly close
to the theoretical 2 volts per cell of the normal lead-type storage
battery.” Don’t ask me why 1.5V cells were suddenly 2V.

After that, people wanted radios in their cars. Here, the batteries
were kept at full charge most of the time, so you could count on
a full 6 volts. Evidently, somebody measured a bunch of car batteries,
because they decided on 6.3V as the average. According to Tomer, this
was the first effort at standardization. Later, the current draw
of heaters were standardized to increments of 300mA so that they could
be used in series. Subsequently, there were also tube types with heater
voltages that ran in multiples of 6.3. This allowed series and parallel
combinations to be built up in TV’s and radios.

Tomer also says, in a different chapter, that running heaters 10% over
their rated voltage will reduce the heater life by 50%. Likewise,
a 10% decrease in heater voltage is likely to double the lifetime. Of
course cathode emission will also be reduced, so performance might
suffer depending on the application.

— Dave Cigna