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 Post subject: True emission?
PostPosted: Mar Sat 21, 2020 11:27 pm 
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Examining most all old tube testers it seems that all are tied into the cathode of the tube circuit wise, even the Hickok patented circuit. It would also seem that true emission comes only from the electron cloud which is constantly replenished by the cathode setup within the tube, so then it stands to reason that that a true emission test should only be derived from the plate and control grids which tests only the pull from the cloud, totally isolating the cathode setup. Is this the failure in the old testers?


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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 12:10 am 
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Not sure where you see a "failure." There is a cloud of electrons surrounding the cathode in normal vacuum tube operation, but the cathode supports and constantly replenishes the cloud. If you put a milliammeter in the cathode circuit you will see that the current there is equal to the plate and screen currents (assuming no current in the first grid, or due to leakage or gas). The cathode and its cloud are in series with the rest of the tube and whatever circuit it is used in.

No commercial tube tester ever measured the full, maximum emission of a cathode. Trying to do that, even for an instant, can damage a cathode and change its characteristics forever. Tube testers have many other limitations due to the fact that it is impossible to foresee, let alone duplicate all the different conditions tubes are subjected to in operating equipment. But this is not a "failing," it is just a practical reality. Any tube tester worth the space it occupies should be able to tell you when a tube is bad, but only the equipment you intend to use it in can tell you if a tube is really good.

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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 12:42 am 
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I see the failure as not examining only the grid and plate structures pulling electrons to the plate as the final result, if the results are low, the tube is bad, it does not really matter where the electrons came from at that point, the cathode emission is mutually exclusive from how it is used, although it must be up to the task of supply.

.


Last edited by Brobertson on Mar Sun 22, 2020 1:07 am, edited 1 time in total.

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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 1:06 am 
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Still not sure what you are driving at. In nearly all emission tube testers there's a milliammeter in the plate circuit and the grids are tied to the plate so the tube operates as a diode. In a few models the grids are shorted to the cathode but the results are the same. There are shunt or load resistances in the circuit which are chosen so the cathode has to have normal emission in order for the meter to deflect into the "good" part of the scale. An interesting effect can be observed by testing tubes with weak cathodes, when the pointer initially deflects upscale but then nearly immediately starts moving downscale. The initial peak is due to the electron cloud being depleted through conduction to the plate, but if a cathode is worn out it cannot replenish the cloud fast enough and the reading falls off.

The classic definition of transconductance is plate current delta divided by the grid voltage delta that caused it, with the plate voltage held constant. This requires that the grid voltage delta be made as small as practical. In Hickok tube testers the grid voltage swing is actually quite large, relatively speaking. This ensures that the results reflect tube emission to some extent, as well as tube gain. Better for finding dud tubes--which was the purpose they were trying to accomplish--but they were not exactly measuring transconductance by the book.

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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 1:25 am 
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Your signal introduced into the cloud determines what drives the voltage swing that is accepted into the next stage - at that point in time and that point alone, has nothing to do with the cathode or how the electrons are supplied (although that has be there and good), most tube tester circuits include the cathode in the design and skew the results of testing, it hinders a true nature of a tube test.


Last edited by Brobertson on Mar Sun 22, 2020 1:45 am, edited 1 time in total.

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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 1:43 am 
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You seem to be overlooking a very fundamental fact of electric circuits, namely they have to be complete in order to work. The circuit is from the cathode to the plate. No cathode, no current. In typical operation there is very little current flowing into or out of the first grid, it sets up an electric field which moderates the flow of electrons from cathode to plate.

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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 1:47 am 
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Brobertson wrote:
I see the failure as not examining only the grid and plate structures pulling electrons to the plate as the final result, if the results are low, the tube is bad.


Not that simple. For instance, an 807 tube can test at better than 100% of its rated performance in a Hickok 539C, one of the "better" tube testers around, but it may yield substandard performance in a transmitter circuit because it wasn't tested at its rated voltages of, say, 750 vDC on the plate and 250 - 300 vDC on the screen grid.

Perhaps a better example: I have a non-working HQ-145A. After recapping and repairing a couple of strange "errors", I aligned the IF and starting testing the front end. The upper two bands were dead. I pulled the 6C4 HFO and tested it in my 572A. It tested very good, but IT DIDN'T WORK IN THE RADIO. Replaced it with a mil-spec 6C4WA. That solved THAT problem.

A tube tester doesn't test any part of the tube in isolation. It tests, as best as it can, the tube as a whole. When I was first in the tube equipment business (young corporal, Marine, Barstow MCSC, 1967-1969), I hardly ever used a tube tester (a TV-7) for troubleshooting. I had a collection of known good tubes I swapped in whenever I had a questionable tube. Of course, I had that luxury there.

The final test is always the circuit you are going to use the tube in. That 6C4 I mentioned about would have worked great in an audio amp. Unfortunately, I wasn't repairing an audio amp.

John


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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 1:50 am 
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You are modulating the cloud.


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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 1:59 am 
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Some of the other people on the forum will almost certainly understand what you are talking about better than I do, so I am signing off now. :roll:

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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 2:01 am 
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'Perhaps a better example: I have a non-working HQ-145A. After recapping and repairing a couple of strange "errors", I aligned the IF and starting testing the front end. The upper two bands were dead. I pulled the 6C4 HFO and tested it in my 572A. It tested very good, but IT DIDN'T WORK IN THE RADIO. Replaced
it with a mil-spec 6C4WA. That solved THAT problem.'

My point exactly.


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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 3:00 am 
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You seem to be starting from the theoretical viewpoint that tube testers are an absolute. None have ever been made that could do what you suggest. The tube testers were designed to give an essential go/no go indication of emission, and several steps better with the transconductance models.

I have to agree with Chris.... what do you expect a tube tester do do? The only way to REALLY test a tube is to put it into the circuit that it's expected to function with. There's no tube tester that can mimic all real world conditions of an operating circuit. Even if one could be designed, how would you be able to test all possible variables within all possible circuit designs for that particular tube?

Bottom line is that the tube testers of old, and new, for the most part do exactly what they were designed to do, and surprisingly well.

Step up to a curve tracer, and you have yet another set of parameters telling you how that specific tube is behaving. But it still may or may not work in any given circuit.

Tubes were never precision instruments. Neither were tube testers.

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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 3:30 am 
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Chris108 wrote:
Some of the other people on the forum will almost certainly understand what you are talking about better than I do, so I am signing off now. :roll:

I just read through 3 times and still do not understand...

How about "back to basics"?

For ANY kind of tube test, you have to do 3 things:
--Apply a potential from plate to cathode
--Do something to control the potential from grid to cathode
--Measure the cathode current

To be sure, every tester does this differently---and some also check for shorts, or maybe grid emission, etc.

Tube testers, DMMs, scopes--or just plain old radios--don't operate on anything more complex than the basic curves on the data sheet. The existence of a cloud of electrons at the cathode is not relevant. What counts is whether you get the expected plate current for a specified set of conditions.

All the above for triodes---add a few more words for tetrodes, pentodes, heptodes, octodes, etc........

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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 12:21 pm 
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Thanks for all replies, very interesting and helps put it into better perspective for me.


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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 4:19 pm 
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What an emissions tester really tests is the cathode. That is a useful test to make but as you are saying that is not a complete test of a tube. The fancier transconductance testers make a more sophisticated test but they too have their limitations. Like the simpler emissions testers they only test the tube in one configuration under one set of conditions. The problem is that those conditions and that configuration may not tell you enough about the tube to predict its performance in any given circuit. Thus the common advice here that the best test of any tube is the circuit it will be used in.

There are two problems with that advice.

The first and I suppose most important to us is that we are trying to restore radios. If the radio isn't working well we want to figure out if the circuit or the tube has a flaw. Putting the same tube back in the same circuit won't tell us that, since we already know that THAT tube is not working in THAT circuit. Some of us have a stock of old tubes that can be swapped out or a stock of working radios we can try a tube in, some of us don't. The advice has a lot of truth to it but not all of us have the means to implement it. Well, the good news is that you can always just order a new tube and when it comes if it turns out the circuit was the issue you have not wasted your money on a new tube, if nothing else you now have the beginnings of a stock of tubes to use next time!

The other issue is that the advice is not strictly true. IFF you knew the tube parameters that are important for a particular circuit AND you have a tube tester that is flexible enough to test the tube under the conditions it will see in the circuit you should be able to test the tube adequately independent of the circuit it will be used in. Vintage tube testers will not do that. Modern tube tracers will certainly do that if the tube voltages and currents that you need lie within their capabilities.


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 Post subject: Re: True emission?
PostPosted: Mar Sun 22, 2020 11:14 pm 
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In the case of the 6C4 it could be the circuit was marginal with off spec components or poor design so only a "Hot" tube would function.


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 Post subject: Re: True emission?
PostPosted: Mar Mon 23, 2020 12:16 am 
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'For ANY kind of tube test, you have to do 3 things:
--Apply a potential from plate to cathode
--Do something to control the potential from grid to cathode
--Measure the cathode current'

I still makes me think that is the problem I mention, right at this point you speak of, measuring or allowing the cathode current into the testing circuit does not indicate any potential problems with how the tubes grid and plate structure handles the current supplied (the cloud of electrons it has to deal with at that point), it only says 'that's the best I can do from this point giving the supply' - failures of the grids, misalignment's, and such are not considered in this type of setup. Branches of circuit tests derived only off the grids and plate would solve the problem, could it be done?


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 Post subject: Re: True emission?
PostPosted: Mar Mon 23, 2020 12:27 am 
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Doubtful. Tube testers measuring emission don't tell you if the emission is coming from a single spot on the cathode, or the entire cathode. Nor, for that matter, the condition of any particular section of the grid. It's just testing what gets "through" from cathode to plate. Some testers either include, or can be modified to include, an measurement of plate current as well, which also adds to what you know from testing that tube.

Transconductance testers give you a little more information as opposed to just a static emission test by using an actual signal into the grid.

Curve tracer waveforms can show you the tube behavoir at various grid states.

I still do not understand your reference to "the cloud of electrons". How is that relevant? Sure, there's a cloud. There's also a few elements other than the control grid that can effect that cloud. Screen and Suppressor grids for instance.

What are you trying to accomplish here? Tube testers have functioned more or less the same way (or ways) since the beginning of time to the most recent gyration, and they do their job pretty well.

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 Post subject: Re: True emission?
PostPosted: Mar Mon 23, 2020 12:44 am 
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From my research. controlling the cloud is what a tube is all about, unlike solid state. In the end it seems an emission tester is just as good as a mutual conductance tester, all things considered, but it's all fun thinking about it as I listen to my tube setup :lol: .


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 Post subject: Re: True emission?
PostPosted: Mar Mon 23, 2020 12:55 am 
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I'm now back with the group that cannot understand what problem we are trying to solve.

My personal opinion is that there is ZERO market for a "do everything" tester. With a curve tracer you can verify the characteristic curves published in the data sheets, but you can't measure all the small inter-electrode capacitances--and maybe some other subtle parameters.

If we're just trying to make radios work, many of us don't even use a tube tester unless it's part of trouble-shooting a specific symptom.

It the problem is circuit design and/or product development, then the normal drill is to start with the tube specifications, crank out some candidate designs, maybe some modelling---but, most important---test, test, and TEST.

So--once again--what problem needs to be solved?

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 Post subject: Re: True emission?
PostPosted: Mar Mon 23, 2020 1:14 am 
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Are you trying to invent a better mousetrap? If so, you're a little late to the party, and as was said, there wouldn't be a market for it.


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