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.
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