[Amps] 8877 heater open circuit after shipping

[John Lyles]

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 at g3xaq.net>
> To: amps at 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 at gmail.com> To: amp <amps at 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.