[Tube-Swap] Why 6.3V heater?

[wolfbob]

During the outgassing phase of the tube manufacture abnd while it is still
on the vaccuum pumps (two) the tube elements are heated to slight color to
drive out the gasses in the metal structure. With the elements cooled, but
still on the pumps the cathode is activated by running the heater voltage to
twice the rated voltage. This gets the cathode very hot, driving off
volatile gasses that are picked up and sets up the cathode surface
chemically to be the big electron donor. The heater is at this elevated
voltage for several minutes. The final operation on the pumps is to activate
the getter by heating it to do some vaporization of the metal (again by
induction heating). The metal is vaporized upon the inside of the envelope a
very thin film that will continue to suck up ions throughout the life of the
tube.

Raising the heater voltage above the 6.3 will indeed shorten the life, but
not much. Using the variation in AC line voltage is a poor choice, you
should look at the typicat automobile application where the battery voltage
will vary from about 11.5 volts to well over 15 volts. on a daily basis. I
guess what I am saqying is that the heater can run at 7-7.5 volts without
serious damage.

WBob

----- Original Message ----- 
From: "Brian Carling" <bcarling@cfl.rr.com>
To: <tube-swap@mailman.qth.net>
Sent: Monday, May 03, 2004 3:01 PM
Subject: Re: [Tube-Swap] Why 6.3V heater?


> ** TUBE-SWAP REFLECTOR - Please do NOT cross-post messages to multiple
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>
> On 2 May 2004 at 11:26, W0rw@aol.com wrote:
>
> > See http://www.harpamps.com/micKtubes/Why-6.3V-Filaments.html
>
>
> Paul - part of his explanation is pretty incomplete or
> even misleading though...
> The claim is that using more than 6.3V will shorten the life or
> even damage the tubes.
>
> In my many years of working on tube audio amplifiers,
> I see no evidence that a small increase in filament voltage
> will have these deleterious effects.
>
> Many 6V tube heaters are fed with 6.5 to 7 volts AC, especially
> since the older equipment was designed to run on 110V
> or 115-117V AC and todays line voltage is typically
> anywhere from 118 to 128V AC in most areas.
>
> I have seen 6v tubes run happily on 7V AC or even more
> for years.  However, it's better to be safe than sorry and
> it IS best to keep them down around 6.3 to 6.5V.
>
> Applying 5V AC might in some cases reduce emission to
> the point that an oscillator stops, or that bad distortion
> might ensue etc.
>
> YMMV - 73 - AF4K
>
> QUOTE:
>
> 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
>
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