Doubts on cathode stripping in tubes

[teocc_1308]

Hi folks

I was told that cathode stripping only affect tubes that are indirectly heated. Tubes like 300B and 845 which are directly heated are quite safe from cathode stripping.

Would appreciate if someone could clarify.
Thank you. Regards

[SY]

Lots of heat and opinions on this question, precious little rigorous data. And much of that is conflicting.

My own experience has been that for most tubes, it isn't a problem except at higher plate to cathode voltages.

[EC8010]

There are three types of cathodes - it doesn't matter whether it's directly heated or indirectly heated, it's the emissive surface that is important.

Pure tungsten (brilliant white): Immune to cathode stripping
Thoriated tungsten (bright yellow): Susceptible to cathode stripping.
Oxide-coated (red): Very susceptible to cathode stripping.

So (as a quick way of remembering), the dimmer the filament, the more susceptible the cathode is to cathode stripping.

845 is thoriated tungsten, 300B is oxide-coated.

[Bandersnatch]

Hey-Hey!!!,
so how is the cathode stripped? apply HV across anode-cathode and warm up the heater? something else?
cheers,
Douglas

[EC8010]

Cathode stripping is perhaps a slightly misleading description. Cathode bombardment would be more accurate - remember that we are considering damage to the cathode emissive surface.

The vacuum in a valve is not perfect, so there are copious stray gas molecules randomly floating between the anode and cathode. If an electron should be accelerated towards the anode from the cathode, there is always a chance that it will strike a gas molecule and have sufficient energy to remove an electron from that molecule, rendering it positively charged and attracted to a lower potential such as the cathode. The problem is that the ion is not merely a single electron, it's a molecule with a nucleus composed of (heavy) neutrons having considerable momentum when it strikes the cathode emissive surface.

If the only force on particles between the cathode and anode was the accelerating electrostatic force of the anode potential, then the repellant force of the electron cloud above the cathode surface would be sufficient to completely prevent damaging cathode bombardment. However, Brownian motion means that some ions have additional momentum, and that tips the balance in favour of cathode bombardment.

We need to prevent/reduce ion bombardment, preferably by not producing positive ions, or by maintaining a protective space charge around the cathode emissive surface. In general, that means that we shouldn't allow the anode to become positive before the cathode is able to develop a space charge.

[SY]

What sorts of collision energies are required to ionize a typical gas molecule (nitrogen, oxygen, water, by and large)? What sort of electron kinetic energies result in a typical tube construction with typical voltages used in audio? Say, a 400V plate potential and a 4mm cathode-plate spacing? How do these compare?

Pedantic points, unimportant: you really mean "protons and neutrons." And you mean a Boltzman distribution of momenta, not Brownian motion, a related but different phenomenon.

[Circlotron]

Fluoro tubes also have directly-heated coated cathodes. Where I worked we made some really good quality inverters to allow 15 watt tubes to run from a 24 vdc supply. We paid close attention to pre-heating the cathodes before the high voltage was applied to the tube. In one particular test that ran for about 6 weeks, on a 3 seconds on, 7 seconds off basis we got over 350,000 starts(!) from the tube before we gave up and decided it was good enough. Tube was still running OK. We then tried it using using no preheat time and the tube would fail in about 2 days. During the test you could see the tube ends gradually going black from the cathode material being splattered on the inside surface of the glass. Gradually the emission would be concentrated in a smaller and smaller area of the filament and it would eventually melt and go open circuit at this point.

Not exactly the same as a high-vacuum valve but from that it would seem the best way to protect your expensive bottles is to get the cathodes up to operating temperature, then ease the HT on over several seconds.

[EC8010]

Circlotron: Wow, a genuine measurement rather than a theoretical speculation! Your failure description sounds like notching of the filament. Comments?

SY: I thought you were the physicist? I accept your corrections.

[gingertube]

How much enrgy is required to strip an electron from an atom?

Who knows BUT this experience sort of relates:

I run a 14 dynode photomultiplier tube with 2200 volts anode to cathode as the sensor in the receiver of a Laser Airborne Depth Sounder System (Hydrographic Survey).

As the tubes age and some gas gets in we see increasing "afterpulsing". This is when a "photo" electron being accelerated up the tube, toward the anode, hits a residual gas atom (probably Helium), stripping off an electron and turning it into a positively charged ion. This ion is then accelerated back the other direction and smashes into the photocathode, busting loose a few electrons which then acccelerate up to the anode, causing a false "after pulse".

Space charge saturation (electron cloud) around the cathode won't do anything to impede a positively charged ion getting back to the cathode.

I've been trying to get the photomultiplier tube manufacturer to add a Titanium getter for years - Titanium has a HUGE affinity for Helium.

As it is we just change the tube every 3 to 4 years (maximum).

Cheers,
Ian

[teocc_1308]

To confuse this issue a little.
I just read on following

Source: http://yarchive.net/electr/tube_time_delay.html

"One common argument used by tube rectifier aficionados is the so-called cathode stripping effect where high voltage applied to the plate of a tube before the cathode has warmed up can strip the cathode of emitting material.

Unfortunately, this effect only occurs at high voltage, typically above 10
kilovolts. It is not a factor in small receiving or transmitting tubes.
If it really were a problem, it would destroy the tube rectifiers which have plate voltage applied immediately at turn-on.

It's only affect tubes typically >10 KV, while not affecting small signal tubes or transmitting tubes.

Just wondering is there any official research or paper publication to precisely define this concern ?
Thank & regards