[Amps] why a tube fails ?

[Roger (K8RI)]

On 8/27/2013 11:00 PM, Jim Garland wrote:
> Hi Charles,
> Very intersesting questions!  I can't help you with your question about tube
> failure modes, though I know at least one mode with indirectly heated
> cathode tubes is that excessive cathode current can cause the filament
> coating to evaporate and coat the grid, which then starts emitting electrons
> and quickly destroys the tube. .
>
> I can be a bit more informed about your second question. First, a
> nomenclature correction. Free electrons are not ions. Ions are atoms which
> have temporarily lost or gained one or more electrons, and thus have a net
> charge. For example, a hydrogen ion is just a proton, since a neutral
> hydrogen atom normally has a proton nucleus and a single bound electron.
> When that electron becomes unbound, the hydrogen atom becomes an ion.
>

As a tech this is my understanding:

Conventional current flow = + to -
Electron current = - to +

Conventional current flow and electron current flow are in general, 
nothing more than a convenience for the person tracing the circuit and 
conventional current generally confuses the technician.  There are 
places where only one or the other works

Electron current confuses engineers.

Most semiconductor circuits are far easier to trace using conventional 
current.

OTOH tubes work with electron current and are much easier to explain 
using electron current.  This can be proven using a magnet. Explain an 
X-Ray tube with conventional current when it's the impact of high speed 
electron (negative) into a copper or tungsten cathode (positive) that 
generate the X-Rays. Engineers insist on conventional current while the 
rest of us use electron current.

Electrons are emitted from the cathode and flow to the plate.
That flow is controlled by the grid and screen (if it's a tetrode)

There are a number of ways a tube can fail, outside of arcing

The cathode in indirectly heated tube like the 8877 and thoriated 
tungsten in tubes like the 3-500 will emit a given number of electrons 
at a given temperature at the rated filament voltage and current.

As tubes age this ability is finite. IOW as the cathode of either type 
ages it's ability to emit electrons decreases.

When the tube starts life it has a strong ability to emit electrons.  As 
they age the cathode material is eroded. A little goes a long way. 
eventually there is not enough of this material that easily emits 
electrons left so that the tub's current is limited and it can no longer 
reach its full rated plate current at the original cathode voltage and 
current and is refereed to as soft.  This is the normal method of failure.

Tubes, particularly glass one can also become gassy due to imperfect 
seals,  Getterin can "eat up" some gas, so if the tube is not too bad it 
may be saved for a few more hours.  Sometimes many more.

73

Roger (K8RI)