The AL series uses a separate tapped transformer for the filament.
On Sat, Apr 1, 2023, 12:14 PM John Lyles <jtml@losalamos.com> wrote:
> Alan,
>
> I don't know the AL-1500 but It isn't likely that running the filament
> alone without HV will damage the filament of a 3CX1500A7/8877 triode -
> unless the filament voltage rose up too high, when the anode power
> supply was unloaded. That would be a poor transformer design though.
> Besides, in normal operation with no RF drive the anode current is the
> idling current which also partially unloads the transformer also.
>
>
> > Date: Sat, 1 Apr 2023 12:46:44 +0100
> > From: Alan Ibbetson <alan@g3xaq.net>
> > To: amps@contesting.com
> > Subject: Re: [Amps] 8877 heater open circuit after shipping
> >
> > The guy sent the open-heater 8877 back. Yes, the heater is open and yes,
> it
> > is the tube I sent him (date code and serial numbers match). So maybe the
> > valve really was broken in transit.
> >
> > One final question, and clutching at straws. I see on the Ameritron
> AL1500
> > circuit diagram one side of the heater is hard wired to the cathode. On
> my
> > suggestion the guy had pushed the anode connection to one side and run
> his
> > AL1500 like this for a few hours "heater only, to fully getter the tube".
> > Is there any conceivable way this could have burned out the heater? The
> > circuit diagram in the manual is rather hard to follow!
> >
> > 73, Alan G3XAQ
> >
> The discussion from W4BIN (below) is for a thoriated tungsten (TT) tube.
> The 8877 is a oxide coated cathode tube so the operating temperature is
> much lower, around 800 deg C. No thorium is involved in this design. So
> the brittle filament concern is not there for your 8877.
>
> It is very true that old TT filaments become brittle and can break with
> a mechanical shock. It is risky to ship an old TT tube (>20,000 hours)
> around as filament breaks can happen. If the tube uses bar or strip
> filaments, one can break loose and short to the nearby grid. If it is a
> mesh or basket filament, a stray filament wire may touch the grid also.
>
> The radioactivity of thorium doesn't enhance the electron emission. The
> TT filament is an alloy of thorium oxide and tungsten. The thorium is
> the active emissive element, whereas the tungsten is the support
> material and the source of resistive heat (~1900K). At higher
> temperature, the tungsten alone is a good electron emitter, but it is
> mechanically weak and would have a short life. Old tubes with pure
> tungsten did not last many thousands of hours. After being exhausted and
> baked out, the TT filament is flashed at a high temperature to drive
> some of the thorium oxide to the surface. The surface layer is
> evaporating during flashing. If the process is continued indefinitely,
> more thorium is brought to the surface but the monolayer is one atom
> deep. The additional thorium merely displaces some which evaporates so
> that the supply of thorium is reduced in the base alloy. Normal
> operation of a tube throughout its lifetime should not deplete the
> thorium. The emission would drop quickly when the thorium layer is
> depleted. when the thorium coating covers only 1/2 of the tungsten, the
> emission will drop to ~1% of its original value.
>
> To protect TT filaments from the destructive effects of positive ion
> bombardment, they are carbonized. This also reduces the evaporation rate
> of thorium at high temperatures. Molecules of hydrocarbon vapor
> (alcohol, benzene, or naphthalene typically) decompose upon striking the
> hot filament during this carburization step, leaving tungsten carbide.
> This carbide then reduces the thorium oxide within the filament to
> thorium which forms the monolayer on the emitting surface.
>
> This is a brittle material, so care must be taken to limit its formation
> to 2% or less. Evaporation of thorium from a carbonized filament is only
> 15% of what it would be without the carbide layer. The small amount of
> thorium which evaporates is replaced by diffusion of fresh thorium from
> inside the alloy. The carbide layer on the surface is also depleted over
> time (decarbing). This is the normal failure mode of TT filaments in a
> properly designed and properly operated power tube.
>
> A work function of the cathode surface increases as the carbide layer
> leaves the emitting surface due to the reduction of thorium that
> migrates over the surface. When the carbide layer is depleted , the
> reduction of thorium oxide reduces and the thorium cannot replenish the
> thorium that is lost to emission at a rate sufficient to maintain the 3
> Amperes/cm^2 emission current required to operate the tube at normal
> power output. As the thorium diminishes, the filament current must be
> increased to bring the rate of replenishment back to the normal 3
> Amperes/cm^2. An additional supply of thorium is still dissolved in the
> filament in the form of thorium oxide, which may be brought to the
> surface and reduced to thorium, by raising the temperature of the
> filament momentarily. This is what we note when we turn a filament up
> higher than normal for 10 minutes. When we turn off the filament, the
> mechanical deformation of the filament (from cold to hot) probably
> releases more thorium to the surface from the internal tungsten, but it
> evaporates rapidly when we are running. Hence the emission falls back
> off again.
>
> 73
>
> John
>
> K5PRO
>
>
> > Message: 3 Date: Sat, 1 Apr 2023 11:55:09 -0400 From: Ron W4BIN
> > <ka4inm@gmail.com> To: amp <amps@contesting.com> Subject: Re: [Amps]
> > 8877 heater open circuit after shipping
> > Electrons are produced when the filament wire is heated above 2200 ^o C.
> > Adding small amounts of thorium to the tungsten in the filament wire
> *reduces this
> > temperature substantially*, to about 1700 ^o C.? This increases the
> efficiency of
> > electron production and increases the life of the filament wire.
> >
> > ? Unfortunately after many hours of operation the thorium moves around
> > in the tungsten, eventually the molecules align in lines, gradually the
> tungsten becomes
> > very brittle. This has for years caused it to be inadvisable to move a
> tungsten light
> > "bulb" that shows a blackening (even ever so slight) between lamps.? I
> believe the increase
> > in electron production results from the fact that thorium is mildly
> radioactive.
> > Since thorium-232, has a half-life of about 14,050,000,000 years the
> > ability to increase the electron production will not fade over the
> lifetime of vacuum tubes.
> >
> > This has for many years made it imperative to handle "emergency spare"
> > broadcast transmitting tubes extremely genitally, without any bumping,
> when it is
> > time to test and re-degassing.
>
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|