Testing emission without risk

[Keit]

@Keit I couldn't see any statement on statistics that might indicate whether any apparent correlation is likely to be real..........before discussing mechanisms for a correlation which might not be real perhaps ?

My test results have been very consistent - so far, on only one tube type. Two things need to happen before my method should be accepated by anyone:-

1) More tests done on more tubes, and on tubes of various types.
2) The theory needs to be critically examined.

I was hoping some nice persons on this forum had any thoughts on pros and cons. Or replicate the test and tell us how it went.

 

[Keit]

Not sure if this helps anyone here, RCA has a book on the design and construction of VT's. In reality it's a series of lectures on the physics and the applied techniques for mfg of them. worth a read, helps if you have a physics background and good math skills. Link here to PDF.
http://www.tubebooks.org/Books/Atwood/RCA 1940 Vacuum Tube Design.pdf

If you guys are familiar with or already know of this, then disregard.

Yes, I have a copy of this and have studied it extensively. But thanks for your helpful thought anyway.

 

[smoking-amp]

" I couldn't see any statement on statistics that might indicate whether any apparent correlation is likely to be real..........before discussing mechanisms for a correlation which might not be real perhaps ? "

I'm sure there would be some correlation, just that there might be some degradation factors that might disproportionately skew the relationship between thermal and electron emission. Data collection on the tubes of interest could resolve this.

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"Note that emitted electron velocity is not correlated with emission. To understand why, imagine a cathode with an excessive number of dead spots or excessive porosity. ........"

Yes, I see your point if emission degradation is a "knock-out" process. I have no idea what is going on microscopically at the cathode surface. Could there not be some gradual processes that slowly degrade the quality of emission? Say, for example, the evaporation of cathode material, maybe removes more of one oxide than another, causing the energy threshold to increase. So electrons would leave with lower average energy. One might be able to see a shift in the minimum positive plate voltage to get current, although contact potentials would probably overwhelm this from a practical measurement perspective. So then maybe a B field could be useful to resolve the residual electron energy.

 

[DF96]

So please go away and read a good text on thermal radiation, the Stefan-Botzmann Law, Plank's Law, Wein's Displacement Law, and understand the meaning of emissivity and spectral emissivity.

I just found "Equilibrium Thermodynamics" by Adkins on my shelf. It was one of my 2nd-year undergrad textbooks. I read the section on thermal radiation (which included a derivation of Wien's distribution law) and it contained no surprises. The Feynman lectures added some quantum mechanics. My postgrad textbooks are not helpful in this matter, as they deal with much more advanced matters such as quantum field theory and quantum gravity.

So I am left with your assertion that emissivity increases markedly with temperature yet requires no colour change, and that this increase is modified by the electron emission properties of the surface. I have been unable to confirm this either from my physics textbooks or from a little Googling. Perhaps you could assist by giving a publicly-accessible link, or even posting a brief extract from a relevant source?

 

[smoking-amp]

"So I am left with your assertion that emissivity increases markedly with temperature yet requires no colour change, and that this increase is modified by the electron emission properties of the surface. "


Is "increase is modified by the electron emission properties of the surface" being claimed here, or just that the two correlate well? I thought the idea was that thermal emission degradation would be a good indicator of cathode damage, and hence electron emission degrading.

 

[Keit]

Yes, absolutely there are also mechanisms of gradual change. Yes, there certainly are some mechanisms that skew the relationships. The trick is to work out what's negligible and what's not.

I covered three of the most important mechanisms for cathode deterioration in an earlier post. Cathode material does indeed evaporate. Some chemical constituents condense on cold parts of the tube, and some get absorbed by the gettering.

I don't think that a shift in the minimum anode voltage to get zero current would have any practical value, but I could be wrong. This is of course what is known as the retarding voltage and is a small negative voltage. Some cathode deterioration mechanisms will affect it and some will not, to a minute degree in theory. Worth testing - I'll give it a go when I get time.

It's such a simple test easy to do, that could have been done since the beginning of tube commercialisation, so it would be surprising that it has not been used if it works.

Checking heater current ratios on the other hand requires a level of accuracy that is not a problem today with 4-1/2 and 5-1/2 digit DVMs cheaply available, but would have been labour-intensive standards lab job back in the tube era.

 

[Keit]

"So I am left with your assertion that emissivity increases markedly with temperature yet requires no colour change, and that this increase is modified by the electron emission properties of the surface. "


Is "increase is modified by the electron emission properties of the surface" being claimed here, or just that the two correlate well? I thought the idea was that thermal emission degradation would be a good indicator of cathode damage, and hence electron emission degrading.

Thermal and electron emissivities are just well correlated by being driven by another change. A bit like me getting physicaly fitter while my bike tyres wear out. It's not the wear in the tyres that makes me fitter. Its my muscular effort that's driving both.

Note that the correlation is a negative correlation- thermal emissivity increases while electron emissivity goes down.

 

[Keit]

So I am left with your assertion that emissivity increases markedly with temperature yet requires no colour change, and that this increase is modified by the electron emission properties of the surface. I have been unable to confirm this either from my physics textbooks or from a little Googling. Perhaps you could assist by giving a publicly-accessible link, or even posting a brief extract from a relevant source?

I'm not suprised. I made no such assertion. You are just twisting up words.

Electron emissivity change does not drive thermal emisivity change, and I didn't say it did. Both changes are driven by another set of changes - the destruction/alteration of the cathode surface by several mechanisms.

Your word twisting nonsense does nobody any favours. As does your mis-use of terminology.

 

[luckythedog]

My test results have been very consistent - so far, on only one tube type. Two things need to happen before my method should be accepated by anyone:-

1) More tests done on more tubes, and on tubes of various types.
2) The theory needs to be critically examined.

I was hoping some nice persons on this forum had any thoughts on pros and cons. Or replicate the test and tell us how it went.

Keit, OK so at this time we don't know whether any correlation is real. How many of this type have you tested, and what are the measurements - just to give some indication of the likelihood of a random coincidence ? Thx !

 

[smoking-amp]

"Note that the correlation is a negative correlation- thermal emissivity improves while electron emissivity goes down. "

OK, understood.

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Just as a matter of possible interest to the discussion. I had always thought that RCA's "Dark Heater" tubes were some improved cathode coating that allowed cooler running heaters. But on recently delving into the matter, I found that this refers to a better thermal emission coating on the heater instead. (better heat transfer to the indirectly heated cathode.) The heater is coated with a dark ceramic insulator rather than the usual white ceramic coating.

Maybe this could provide a new type of data point in the heater resistance discussion? Or at least a confounding factor to look out for. A good proportion of TV tubes (but not reliably consistant) made after 1962 seem to have this feature. One can see it by simple inspection of the heater wires.

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Seeing as power output tubes are the most seriously affected by declining electron emission, it would be useful to perform some checks of this filament/heater resistance method on one of those types to confirm some measurable effect. (most being indirect heated) The "Dark Heater" factor is quite prevalent in the later TV horizontal output tubes. (used by many for audio now)

 

[Keit]

That's an excellent suggestion.

I would expect the increase in the heater current ratio to be slightly more marked due to the tighter thermal coupling between heater and cathode. But I don't have any of these dark heater tubes, although I've read about them.

The trick would be in obtaining some. It would be hit and miss (mostly miss) trying to get them on eBay. Any suggestions on where to get?

I have a moderately large collection of TV tubes, but they are nearly all Australian production and all have standard white heater/cathode insulation.

 

[smoking-amp]

Tubes designed by RCA after 1962 are typically spec'd for dark heaters, even if made by someone else. But the best bet would be an RCA design, made by RCA.

TV outputs: 6LZ6 (RCA 1971), 6JF6/6JG6 (RCA 1964), 6KM6 (RCA 1965), 6JR6/6JU6 (RCA 1966), 6KV6 (RCA 1967), 6JT6/6GT5/6GJ5/6JB6 (RCA 1964)

In the smaller signal size: 6JC6, 6KV8, 6LQ8

Higher heater voltage versions of course will typically be cheaper.

 

[QuantumFoam]

coming in late here and thinking of this from the quantum side. Electrons are not 'particles' as we know, it seems that what is trying to be correlated is the release of energy from the material due to the increase from the heater. It would seem then that we would need to know the degradation loss on the heater and it's material and the material of the cathode and anode and importantly the band structure as well.
Is the thought here that the heater emissivity current density is correlated with the coatings on the plate so that by measuring the heater it would give an indication of the plates?

temperature plays a big part in the life of the cathode. http://www.cathode.com/pdf/temp_paper.pdf
worth a read on the measurements of the cathode plate and heater.

It's an interesting thought, outside the box. lots of variables at play though, esp considering the thermal distribution as the above paper shows.

 

[QuantumFoam]

If I'm off track here let me know, I found another good paper on cathodes that may be useful to the research.
http://www.cathode.com/pdf/tb-135.pdf

And on a similar note, but very neat ION and HALL thrusters using hollow cathodes.
http://descanso.jpl.nasa.gov/SciTechBook/series1/Goebel_06_Chap6_cathodes.pdf

 

[smoking-amp]

Any information available on the new room temperature thin film cathodes?

http://www.diyaudio.com/forums/tube...idea-dont-say-i-didnt-warn-2.html#post2354828

 

[luckythedog]

Hmmm. So we don't know whether any apparent correlation is real, nor any hard numbers or statistics to indicate how likely or not it's coincidence. And we're trying to apply physics (with unsurprising difficulty) about two observations which might not even be linked at all and on the face of it are quite probably/possibly independent ?

Even IF there was some correlation, there need not be a causal link - for example a certain supplier of filament might (for business reasons) be paired up with a certain supplier of cathodes which go on to have poor emission......

Seems to me some discipline is needed to avoid a lovely goose chase here !

 

[Keit]

there need not be a causal link - for example a certain supplier of filament might (for business reasons) be paired up with a certain supplier of cathodes which go on to have poor emission

I can rule that out I think. As I said in a much earlier post in this thread, I tested NOS and old tubes from all the major US, UK, and Australian manufacturers - KenRad/GE, RCA, AWV, Philips/Mullard, Brimar/STC, and more. Manufactuer specific issues was the firtst thing I thought of - that's why I confined the test to RF pentodes - the only type that I have both sufficinet numbers and from a wide range of manufacturers.

Keit

 

[Keit]

QuantumFoam,

Thanks for the links - interesting, and I'll study them carefully later. But note that dispenser cathodes are not relavent to the tubes I've been testing. Receiving tubes use simple slip-coated or spray-coated metal cathodes. Occaisonally you get an electrophoresis-coated metal cathode.

Keit

 

[QuantumFoam]

QuantumFoam,

Thanks for the links - interesting, and I'll study them carefully later. But note that dispenser cathodes are not relavent to the tubes I've been testing. Receiving tubes use simple slip-coated or spray-coated metal cathodes. Occaisonally you get an electrophoresis-coated metal cathode.

Keit

No prob, I dug up the paper as it deals with degradation loss and thermal fluctuations that shorten the life span of the cathode. It also has good info on coatings.

 

[luckythedog]

I can rule that out I think. As I said in a much earlier post in this thread, I tested NOS and old tubes from all the major US, UK, and Australian manufacturers - KenRad/GE, RCA, AWV, Philips/Mullard, Brimar/STC, and more. Manufactuer specific issues was the firtst thing I thought of - that's why I confined the test to RF pentodes - the only type that I have both sufficinet numbers and from a wide range of manufacturers.

But how many have you tested and how many were low emission ? Personally, I haven't got that many low emission examples of any type. If you catch my drift.

And one could rapidly speculate on many non-causal links IF there really were some correlation. Surely any such speculation on a link, be it circumstantial or physics based, is underpinned by there being some reasonable statistical base for it? I don't mean 'medical clinical trial standard', just reasonable armchair stuff and we don't even know that unfortunately.