Why This Is Not A Technical Manual
This is a little more than a technical manual.
There is a substantial amount of misinformation and confusion surrounding tube testing. That goes from the basics of what test parameters to use, down to what test methodology to employ. The confusion was born out of a need for a quick and cheap tester. The differential tester (intended in terms of transconductance) might be able to tell you if a tube is fit for use or not, but it will not tell you more than that. It will not tell you anything of value.
The confusion comes from this last point. Tube testers of yesteryear were designed as in circuit testers to test tubes in such a manner that would allow the user some objective criteria of usefulness in a particular circuit. But considering the thousands of applications that existed in the tube’s heyday and the hundreds of audio circuits one will be confronted with today, it seems almost pointless to try to address the “in circuit” viability of test. In any one test there are too many variables to look at for one to be able to isolate the contribution of a single tube. The tube must be functionally isolated from the circuit for it to be a valid test of any one tube.
The circuit centric nature of tube testing also extends to the data sheet by which one tries to derive parametric test functional points. The specifications call for values that are only meaningful within a certain circuit topology. Those values are meaningless outside of the exact reference circuit under investigation. What is being examined is not the tube itself, but the circuit itself. The implication is that if the circuit works in this manner, and consequently the output of the circuit is on spec, then it follows that the tube must be on spec.
That is of course an absurd proposition. The circuit itself has to be external to the object under test. It cannot be the other way around.
The biggest problem with most databooks, or your average single page specification sheet for a tube, is that it is hard to figure out who the intended reader should be. This is a very important question. Is the specification written for a circuit designer? For a circuit designer in the employ of an audio manufacturer or some other industry? Is it principally designed for the service technician in the field? If the later is the case, the manufacturer of the tube might be providing too much information. If it’s the former he surely would have provided too little information. Just as important is the means by which to communicate. Does one make extensive use of mathematics to convey the parameters? Clearly if you are writing to impress your professional peers, brevity and precision of exposition is very important. They are conversant with the context specific technical jargon, and do not need an introduction to the subject matter.
The reason that this manual is not conceived as your standard technical manual is because its not intended for a technical audience.
Of course we designed the tester so that it would be usefull to an audio engineer.
In which case we could convey to somebody conversant in the art the basic idea and its application in less than one page:
DC PARAMETRIC TUBE TESTER:
All power supplies are DC regulated constant current or constant voltage.
Supply A, Heater Supply: 0-26V DC 3A, one decimal voltage and current meters
Supply B, Cathode Supply: 0-300V DC 500mA, two decimal high precision voltage and current meters, with by pass.
Supply C, Grid supply: 0-26V DC 3A, one decimal voltage and current meters
B Supply voltage meter is upstream of cathode, mA current meter is downstream of anode.
Insert tube under test in appropriate tube socket assembly and connect direct current leads from supply(s) within the test fixture to reference pin, and alter voltage to get current measurements.
Reference Circuit Design: Vh fixed 0-26 DC, Vk + Vg DC variable = Ia.
The above would suffice. There is indeed very little to add to the above.
We decided to build this tester because we could not buy a tester that did what we needed it to do. My WWII (Date Code 1948) US SIGNAL CORP I-177B, had always faithfully told me if the tube was good or bad. But based on what? And outside of the circuit considerations how useful was that Hickok voltage shift method? What does that number mean and how was it arrived at? If you were in a trench in some God Forsaken corner of the planet trying to quickly trouble shoot your radio so you could communicate your position before some eager flight engineer decided to drop some friendly fire on your head, would you care why a tube was malfunctioning? The object was to quickly identify the defective tube, and replace it with a new one. The why was irrelevant: “beyond your pay grade” in MIL Spec terms. And provided you don’t care, any tester could tell you what to replace. As a matter of fact, with a decent amount of inventory, you could simply swap out tubes one at a time (assuming you had enough fuses to blow) until you found the defective one (or replace them all).
When I was tasked with the writing of this “manual” I decided to give myself a refresher on thermionic tube circuit design. But before I did that I pulled off the shelf my old trusted “Theory and Applications of Electron Tubes” by Reich and jumped to the section entitled “Determination of Static Tube Characteristics”. The first sentence is typical techy parlance: “It would seem at first thought that the determination of static tube characteristics by means of direct voltages and d-c meters is such a simple procedure that it requires no discussion”.
And there you have it: how do you describe something that does not require discussion?
The first draft was an exploration of circuit design with tubes. Heavy on the mathematics and all the variables needed to get a proper tube circuit to work, and how those parameters relate to vacuum tubes. But then I paused and asked myself the question: Who will be reading this? The electrical engineer will simply ask “Why are you wasting my time with a primer on circuit design?” Indeed that is the question.
The user of the VssBurst Parametric Tube Tester might be a designer but could just as easily be an end user. And said user might out of intellectual curiosity be interested in what circuit topology the amp he buys has- but will he actually design an amp himself? He might “build one in his mind” in order to comprehend the principles, and he might even draw it down so he can place some values along side the resistors and caps. Inquisitive minds are never idle, or bored.
Regardless, it was at this point that I realized that the real reader of this “manual” would not be a designer. It would be an end user with expensive audio equipment, or the shop or consultant that sold it to him. If you have an expensive piece of gear and you choose to trouble shoot the equipment, how do you test the tubes? If you are set to replace those tubes with new ones or vintage ones, how do you know they are any good? How do you know if they got damaged in transit?
Thus this non manual was created for a non technical audience.
It was decided that this manual would not in any way concern itself with circuit design but only with the subject at hand: what is a thermionic tube, how does it work, and how can you test it?
I finally took the decision that I would not use a single mathematical formula. I decided that a bare minimum of electrical engineering terms had to be employed but I would stick to those general principles that had a real person’s name attached to them. That of course led directly to a very brief history of electricity and how the thermionic tube came into existence; that would be both useful and interesting. Again, its not a history of science- only that which relates to the terminology necessary to test thermionic tubes.
Then I would briefly describe how electronic testing occurs without getting into the heavy statistical methodologies testing engineers usually employ. Again, not testing in a circuit centric world with circuit reference design attached but a tube centric, tube only explanation. Ultimately you always test any component in isolation. So its no stretch to test a tube as such.
Having established testing parameters and what they meant it would not be unreasonable for the reasonably skillful user to use the tester, test a tube, and come to some reasonable understanding of how it operates, if its within spec, in short if its “good or bad”. Better yet, give the user the ability to qualify similar tubes, or compare them to tubes in the same design family but from different batches or manufacturers.
Chapter one: A brief history of Electricity.
Chapter two: General Considerations on Testing Thermionic Valves
Chapter three: On Specifications
Chapter four: TUT: Tube under test.
Appendix: Further Reading
I trust this is useful.
If it is not please let me know what you would consider useful.
Hugo M. Fuxa
Test & Measurement of Thermionic Valves / Electron Tubes / Vacuum Tubes
The VssBurst Parametric Static Tube Tester
By Hugo M. Fuxa