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Old 8th Feb 2019, 12:58 pm   #1
methusela
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Default Valve Testers

Many valve measurements described in the Radiotron Designer’s Handbook of 1954 are beyond my needs as a vintage wireless enthusiast. But mutual conductance, anode resistance and amplification factor measurements, for direct comparison with manufacturers’ data, seem readily available via several affordable self built units like :-

http://www.valveheaven.com/wp-conten...lve-tester.pdf (An Australian design built on a baking tin)

http://www.vintage-radio.net/forum/s...ad.php?t=48853 (The famous Sussex tester)

https://www.radioelec.com/en/vacuum-...-352-1512.html (The French DuoVac, pre-soldered, also available on eBay)

The Dutch uTracer was discounted, since it is slightly too expensive, requires a computer and requires too much fine soldering.

I understand that even brand new valves can vary hugely in their measured characteristics and that any one example is likely to measure differently on different testers, but one has to start somewhere! And the picture painted by www.jacmusic.com of testers is a little depressing, especially for those interested in one slightly more refined than a Go/No Go type.

For vintage equipment users, however, one of a valve’s most importance qualities is its likely future life. Radiotron describes an emission test and Gary Steinbaugh in his Make your own Tube Testers, aimed mainly at constructing laboratory grade instruments, states that “As a tube ages, mutual conductance, anode resistance and amplification factor remain stable, except in rare cases, so an emission test is typically a good test of a tube’s overall health”. Both sources emphasise that heater voltage should be reduced by about 10% to simulate a depleted cathode and, to avoid actual damage to the cathode, this “life” test should be run for no longer than about three seconds.

But, after I have strapped all grids together with the anode and, with the valve thus running as a rectifier, what have I achieved in measuring the anode current? Valve manufacturers, as far as I can see, did not publish figures for valves tested in the emission mode. Nor will my self built unit have a meter “reassuringly” marked Good/Maybe/Bad, as many commercial valve testers do. So, given that the above two authorities cite the emission test as a most useful one for determining cathode depletion, but are silent on matters of interpretation, how should I interpret such measurements, and why can I not find a published account of same?

Also, while the reputation of the elaborate Sussex is well established, what are forum members’ views and/or experience of the other two simpler units?
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Old 8th Feb 2019, 4:58 pm   #2
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Default Re: Valve Testers

Lots of questions, but to start with two of them, yes, brand new valves do vary considerably in their test results. Also, some brand new valves will "burn in", their emission changing appreciably over the first ~day or so of use, while others change very little.

What can also come as a surprise is that valves often perform very adequately in service even when their test results are quite marginal. That may be an indication of good circuit design which was tolerant of valve deterioration.

If you look at the basic DC test set up to assess a valve, it just requires supplies for the heater, grid and HT, and many people will have exiting PSU's that will provide those voltages. What really makes the transition in to a general valve tester is the addition of various valve sockets and the facility to change all the pin connections. Obviously, that is that is quite a transition!

The extent to which you want to devote time/money on any kind of valve tester really depends so much on the benefit you will get from having it and that is hard to quantify. I suspect that many valve testers get very little use and that includes mine!

B
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Old 8th Feb 2019, 5:25 pm   #3
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Default Re: Valve Testers

I go along with the wise words in a Tektronix 'scope manual. Something like 'the best valve tester is the circuit that that valve is used in'. OK, it almost certainly said 'tube' in the manual, but you get the idea.

I normally check the heater is not open-circuit and look at the getter. If it passes those 2 tests then I'll power up the device with the valve in it and check the electrode voltages. If they're way out, it could be a faulty valve. Or a resistor. Or a capacitor. That's when I start checking more carefully.

I own 4 valve testers, 3 of which I have never used, the other one I last used about 20 years ago. It is not an essential instrument for me.

As to what I own :

The one I use occasionally. My father got me a 'scrap' valve tester as a Christmas present many years ago. It's an AVO VCM4 and works perfectly.

The others : A Mullard High Speed Valve Tester with a box of cards. I bought it (for a lot less than they sell for now) as I am interested in punched card control, etc in relation to my vintage computer interests.

A Taylor something-or-other that a late friend gave me when he was clearing up his house

A Radio Shack (American) emission tester. Got it in a second-hand tool shop becuase it was cheap and I collect odd test gear.
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Old 8th Feb 2019, 7:43 pm   #4
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Default Re: Valve Testers

Thank you for that gentlemen. I’ve done a fair amount of reading on the subject, including this forum and, indeed, previous wisdom from those kind enough to reply to this post. I know that the circuit in question is often the best test bed and that valve substitution will answer a lot of questions, regardless of what a tester indicates. But I think that what’s driving this is my love of test gear and the desire for some independent confirmation of what, for example, I can hear with my ears! Not laboratory grade stuff, but just enough to confirm basic parameters. Hence my fascination with the possibilities of emission tests, even if the gear won’t see a lot of use, just when a new acquisition comes along. Naturally, I’ll start with a range of short circuit, leakage and “gas” tests.
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Old 8th Feb 2019, 8:21 pm   #5
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Default Re: Valve Testers

There is some variation from device to device in new valves, but it is far smaller than the spread of parameters seen in transistors. Transistor circuits need to use special techniques to handle this, but valve circuits were used for decades with much simpler arrangements.

Mutual conductance testers are fairly rare in America but are the norm in the UK. America seems to have survived alright with mostly just simple emission testers.

A valve tester comes into its own when you want to go through a box of old valves and see which ones are worth keeping, which are junk, and you don't have all the radios and circuits needed to run them handy on the bench. Valve testers come into their own when you want to check the match of a pair, maybe for a push-pull amplifier or for the differential amps in a vintage oscilloscope. They can be handy for a double check when something weird is happening and you need some confidence in the devices you're using.

You can do without one without too much fuss, but it's comforting to have one around, just in case.

If, however, you fancy having one because they're interesting, then yes, they are certainly interesting and impressive looking bits of kit and therein lies their problem.

Audio and guitar hobbiests have latched onto them, and they are prepared to spend a LOT of money for the most impressive looking AVO models. No auction listing of an expensive valve is complete without a photo of a (hopefully that) valve plugged into an AVO. So wannabe valve traders are after them too. Oddly, late model Taylors (Made by AVO) the excellent Sussex and microtracer aren't in demand. Maybe they don't improve valves ;-)

If you just fancy some great old test gear, there is a lot by Marconi instruments which offers a lot of fun per pound spent. HP and Tek have some gorgeous stuff. You could fill your house for the prices seen asked for some AVOs.

Collecting classic test equipment is akin to having a fancy for classic sports cars. Valve testers put you in the silly prices zone like Ferrari 250 GTOs and Bugattis. You can find plenty of things both faster and more affordable, but they are seen to not have an intangible charisma.

Have a look at a classic tektronix scope or an HP spectrum analyser and see what's inside it and compare that to the sought-after valve testers.

It's an interesting field, but don't jump in at the expensive end unless you're really sure it's right for you. There's plenty of fun at the less competitive end.

David
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Old 8th Feb 2019, 8:45 pm   #6
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Default Re: Valve Testers

Quote:
... Valve testers come into their own when you want to check the match of a pair, maybe for a push-pull amplifier ...
This is true. It can also be handy to know how well the two separate valves inside a double triode match one another since a lot of push-pull amps use one of these as the phase splitter. But it has to be remembered that the test has to be done at the operating point that the valves will run at in the amp. For example the standard AVO test point for the KT66 is with the valve triode-strapped and Vak=250V, Vgk=-15V, mostly because this is the specified performance point in the M-OV data sheet. But hardly any amps run the KT66 under these conditions. Vak is more often between 340V (e.g. Quad II) and 400+V (e.g. Leak TL/12 Point One).

Cheers,

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Old 9th Feb 2019, 12:26 am   #7
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Default Re: Valve Testers

And thank you gentlemen, too. I should, perhaps, have mentioned that my main interest is Heathkit valved test gear (not the valve tester!) and post war UK valved radios, rather than amplifiers. So the need for an Avo or Mullard tester doesn't arise, especially in view of their expense, complexity and often suspect calibration and condition. I like restoring other test gear and radios, but not valve testers. Now, about those two simple testers I mentioned which weren't the Sussex, and the significance of emission test results. Anyone?
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Old 9th Feb 2019, 1:43 am   #8
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Default Re: Valve Testers

Quote:
So the need for an Avo or Mullard tester doesn't arise, especially in view of their expense, complexity and often suspect calibration and condition.
I don't think that comment about calibration is at all justified. It must be about a year (?) since forum member David Simpson initiated an activity in which he prepared and subsequently circulated quite a number of "standard valves" which he carefully prepared via standard DC methods. My recollection is that the majority of the people who collaborated with him were owners of Avo testers, including myself, and that there was a remarkable standard of agreement in this "round robin" exercise, which gave a lot of confidence that the Avo instruments, including the VCM163, are still giving very good test results despite their age.

My own 163 gave test results on one of David's valves which were well within any reasonable experimental error. That is what I would have expected, as my 163 is set up (i) as per manual and (ii) as verified by my own standard valves, the latter being produced taking considerable effort, and not just casually bought from Jacmusic.

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Old 9th Feb 2019, 4:21 am   #9
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Default Re: Valve Testers

Emission testing gives a clue as to how decrepit a valve is, but not the whole story.
The baking tin tester looks as though it will be much more useful at minimal extra cost.
Some members will gladly test just a few valves for you, but sometimes tested good used valves are cheaper than the outward postage. And both much cheaper than making a valve tester.
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Old 9th Feb 2019, 6:54 am   #10
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Default Re: Valve Testers

I built a simple valve tester when I first got into this hobby, it's basically three variable PSU - two 0-300v and one 0-30v for the heaters. I just tested for emission using the valves mutual conductance graphs, it's big, gets in the way, i hardly use it.

After learning a bit more about things I realised I could do the same test with two bench power supplies and a test rig, a two meters, although i do have a 0-500v PSU. Most mutual characteristics graphs for pentodes/beam tetrodes test at 250/300v on g2/screen, so a simple resistive divider can be used with a HV PSU to set DC conditions.

A simple short test circuit lifted from one of the Heathkit testers could also be added on, the whole made to fit in a small box, with perhaps some sort of gm tester added. What I'm trying to say is rather than have a big bit of test gear you never use, a LV (most of us have these) and HV bench PSU is all you really need with a small test rig with a SC/gm test.

Here's another simple tester - http://www.valvewizard.co.uk/valvetester.html but it only tests a small selection of valves like the one in your first link, though the valve heaven one looks simpler. Out of the two simple testers, this looks better in that it's easy to build, costs less, there's also less to go wrong and it's fixable yourself.

Andy.
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Old 9th Feb 2019, 10:22 am   #11
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Default Re: Valve Testers

A straight-forward set of DC tests will allow you to plot the characteristic curves of a device. All mutual conductance is, is the slope of a chosen line at a chosen point. "See it all!" as a tabloid would say.

Now if you want a classic valve tester, how about Tektronix' curve tracer?

For ordinary sorting of valves, set fixing etc, many people have survived with the most basic testers. Plenty of people sneer at them nowadays, but they still work. Not comprehensive, but useful.

I picked up a Taylor 45D though for 20 quid back in the nineties. I rarely use it, but I find it good for sorting old valves.

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Old 9th Feb 2019, 11:40 am   #12
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Default Re: Valve Testers

Attached is an extract from the Radiation Laboratory book Components about valve characteristics spread.
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Old 9th Feb 2019, 11:54 pm   #13
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Default Re: Valve Testers

Again, gentlemen, thank you.

Bazz4CQJ – I’m sorry if I appeared to be slighting the grand old ladies of valve testers (If money and space were no object, and I could possibly justify its existence, then an AVO CT160, being bridge based, would certainly be my ideal) but I’d hoped from the context that it might have been discernable I meant calibration “as found” and, from the original post, that I was looking for an altogether simpler, less expensive and time consuming approach than refurbishing such excellent instruments as AVOs. This is particularly so in consideration of the often miserable “untested” condition and “I don’t know much about these things” comments frequently seen in adverts for these optimistically priced offerings. I regard those possessing the will and enthusiasm necessary to take on such considerable restoration effort with much admiration. But then, what is so terribly wrong with those wanting to “casually” purchase valve testers from Jac.music? Horses for courses, surely?

G8UWM-MildMartin – Right, “Emission testing….not the whole story”, hence my interest in the Baking Tin and DuoVac. Thanks for your Baking Tin comment and also for letting me know that we have such a generous community with some members willing to test valves for others. That’s good to know.

Diabolical Artificer – Thank you, this is the first hint as to how to use emission readings I’ve received. I’m not sure that relating them to mutual conductance is the ideal approach, but it seems to work for you, so I’ll give it a try. Also, I’m grateful for an account of your learning curve, since these are precisely the pitfalls I’m trying to avoid. Regarding your Heathkit mention, it must be synchronicity because I’ve just bought a multi way rotary switch with a shorting facility to use in the short circuit and leakage part of a tester à la Radiotron Figure 3.4, with some neon modifications from one of Mr. S’s more basic models! The neon circuit in the Baking Tin model, which I’m perhaps veering towards, is not as indicative as this might be. Thanks also for the valvewizard link. I’d not included it in my list, since it will take a bit of re-design to work on a wider variety of valves and the Baking Tin, particularly with some minor mods, seemed more versatile. Your suggested approach of a pared down tester with my LV and HV supplies on tap is very appealing.

Radio Wrangler – Thank you for you comments about basic testers, which is what I’m trying to do, using my own LV and HV supplies.

Woodchips – Yes, thank you, exactly!

As ever, I would welcome further comments specifically about how the results of emission tests, carried out as described, should be interpreted.
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Old 10th Feb 2019, 8:11 am   #14
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Default Re: Valve Testers

Quote:
But then, what is so terribly wrong with those wanting to “casually” purchase valve testers from Jac.music? Horses for courses, surely?
There is one thing wrong. Valve testers are not designed to protect themselves and it is all too easy to set things up to burn out the coil in the meter or to overheat the transformers. Nowadays people expect electronics to not be damaged by simply setting the normal operating controls. They conveniently forget that they can kill themselves in their car just by a wrong movement on the steering wheel.

So we have the occasional appearance on here of people who bought expensive valve testers but didn't know how to drive one. Some start with too sensitive ranges and ruin irreplaceable meters, a few think they can be used to burn-in valves for several hours. These people are largely interested in audio power valves, where the risks to the tester are greatest.

As a consequence, the pool of good AVOs is shrinking, demand is still there, and this keeps the prices pumped up.

Perhaps what's needed is a guide to how not to destroy that valve tester you just paid thousands for.... but where should it be placed?

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Old 10th Feb 2019, 10:48 am   #15
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Default Re: Valve Testers

Thanks, Radio Wrangler, but my point was aimed at Bazz4CQJ who wrongly thought I was taking a pop at AVOs and such like. Not at all, but he did seem to be dividing the world into two types, those who slog, persevere and restore and those who “casually” buy, with the implication that only the first was worthy.

Losing anything of value is sad, and there are surely enough people out there who are ignorant of, or if knowledgable indifferent to, the value of things. And while I, like you, regret to see the ruination of such things, it is difficult to see how, without gross curtailment of freedoms, valve testers might be protected from over enthusiastic idiots unwilling to learn how to walk before they attempt to run.

But I have a feeling that, however many weeping owners have appeared on the forum seeking advice as the result of doing something silly to their valve testers, the greater number of losses has been caused by their former commercial owners. We have all heard tales of, indeed seen, skips brimming with valuable redundant equipment, simply because no one had the will or imagination to think of a better alternative to it being buried as landfill. Perhaps that makes preserving what does exist even more important, but I still can’t think of acceptable ways of insulating ourselves from the actions of idiots.
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Old 10th Feb 2019, 10:54 am   #16
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Default Re: Valve Testers

Quote:
Originally Posted by Radio Wrangler View Post
Mutual conductance testers are fairly rare in America but are the norm in the UK. America seems to have survived alright with mostly just simple emission testers.
All of the American valve testers I have seen and used are mutual conductance testers. That is the main parameter measured. Obviously any with good-bad meters are just useless toys.

The Ones made by Hickok (eg the famous TV-7, TV-10 and TV-3) are the most famous. They devised a brilliant dynamic mutual conductance test with voltages sourced from the windings of a single precision power transformer. Once the line voltage is set, all the voltages in the unit fall into calibration (normally).

Also, the genius of their idea is that a calibration valve (CT = calibration tube) is not required. It is not necessary to have a valve with a known transconductance to calibrate them. All that is required is that the meter bridge circuit is correctly set. This is done with a simple transformer & resistor.

I developed an additional method to do it with a calibration tool. The article is here, which explains how the wonderful Hickok transconductance testers work:

http://worldphaco.com/uploads/Hickok...d_the_ACS..pdf

While there are a myriad of tests that could be performed on a valve, the single most important one is the transconductance. Most valve fault mechanisms will degrade this value.

I wish this were true for CRT's, they are truly a case where the only way to assess their performance is in the actual circuit to inspect the brightness, contrast & focus. This is why I call CRT testers totally "useless" and valve testers "sometimes helpful".
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Old 10th Feb 2019, 11:29 am   #17
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Default Re: Valve Testers

As has been said, valve testers don't protect themselves, or the valve.

I have experimented with some Roband Varex power supplies. These have adjustable current limiting which will work if the PSUs are stacked several high to get 350V at just 30mA. Hence getting the wrong connection and being blinded by an exploding heater won't happen. I am impressed by these PSUs, well worth looking out for.

For emission tests, try the article Analysis of Rectifier Operation in volume 2 of the RCA books Electron Tubes, available for download. Only place, so far, that explains why modern power valves have sparks and flashed when run at HT voltages the original valves worked ok.

As far as I can see, the only real use for valve testers is the rapid changing of connections for different valves. The tests are easily done with bench PSU's, DVM's and a megger. Just that the number and type of bases quickly becomes a pain. Ok, so mutual conductance needs an AC signal, but I still haven't worked out exactly why compared with a simple DC offset. Do wonder if it is only because an AC amplifer is far more stable than a DC version back 50 or more years. Something no longer valid with semiconductors.
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Old 10th Feb 2019, 12:07 pm   #18
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Default Re: Valve Testers

Sorry upfront for a rather long post and lots of 'ifs'.

Not sure how far you are into the valve part of the hobby or what stock you have?
Or what you think you need at this stage.

If you want to test a valve from a set, that's normally a waste of time, the set is as good a tester as you need. In 50 years I cannot remember using a tester when working on a set.
If you have a box of random valves you want to test, it is OK if you know their type but soon becomes tedious. And if you do not know what they are the tester is useless.
If you want to measure against data curves for fun, OK, but you know there is a wide variation in new stock, so how does that help? If the original circuit design is good 10% plus or minus will not matter.
If you want to find matching halves of dual triodes or pair output valves it can be useful.
If you have that 'itch' it does not matter what we all say ;-)

My tester history is – did not have one for 30 years, never had a problem that need one. Then I had access to a tester (10 year long term loan) checked most of my audio types then it became tiresome. It sat in the garage most of the time. Realised for most repairs it was not needed and gave it back.
Then 5 years or so ago, I 'won' a box of audio valves and wondered about testing them... I built my own simple version of the Tintray tester, using junk box stuff apart from the meters. It is very simple but that is the point and it works well enough as long as you operate it carefully.

So, could I manage without one, yes. Do I use one very often, seldom. If I saw a cheap Taylor 45C at the local auction would I buy it, probably not to use, maybe to sell on...

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Old 10th Feb 2019, 6:35 pm   #19
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Default Re: Valve Testers

Gentlemen, thank you all again for your kind responses.

Argus25 – As you will know, Good/Bad meters are not the sole preserve of American emission test meters, since they feature on many high quality instruments, like the AVOs. In fact, in both sources I mentioned, one Australian and the other American, an “English Scale” is exactly that! It is interesting that the 1954 Radiotronic book requires emission testing of valves in both service and laboratory instruments, in addition of course to a whole host of other tests. I don’t pretend to know what kind of an authority Gary Steinbaugh is – he certainly knows a good deal more than I do – but if I may quote him again :-

“….the most common (gradual) tube failure is cathode depletion, which occurs when the electron-producing oxide coating is stripped off by excessive current, poisoned by the inevitable undesired chemicals within the tube envelope, etc. Characteristics like gM, being determined by electron geometries, should remain constant as the tube ages. If a tube were properly made, and it tested good when new, all that is needed to assess its present state in a proper emission tester.”

He says this on page 47 in the chapter devoted to discussing service testers and goes on to offer his own design of one. But he does not explain how to interpret the results. The main thrust of his book, as I said, is towards laboratory grade instruments, including mutual conductance tests.

Your article on calibration of the TV-10 etc. is dazzling in its comprehensiveness, but such rare and expensive beasts as Hickoks are, I’m afraid, not for me.


woodchips – Agreed, the Robex Varand units, which I’d not come across before, look very desirable and built like the proverbial. I was a little mystified as to why you referred me to the RCA volume until, that is, I came upon Section 4, Current Overload and Sputter, where there is a most vivid description of some of the more arresting sights on rectifier failure! Some people use DC, and still call it a transconductance test.


snowman al – Well, to be frank, I don’t have much of a stock of valves at all, but you’re right, it is a question of having an itch. Perhaps this is mildy silly of me, especially considering the wide range of tolerances encountered but, when I have a piece of equipment in front of me, I have this unquenchable urge to measure things where they are measurable. Childhood insecurity, no doubt. Your version of the Baking Tin tester looks splendid, and this is probably the way I’ll go.


Any more accounts of interpreting emission tests out there?
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Old 10th Feb 2019, 6:50 pm   #20
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Default Re: Valve Testers

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Also, the genius of their idea is that a calibration valve (CT = calibration tube) is not required. It is not necessary to have a valve with a known transconductance to calibrate them. All that is required is that the meter bridge circuit is correctly set. This is done with a simple transformer & resistor.
Firstly, I am never comfortable about using the word "calibration" in the context of valve testers. For me, that word infers traceability back to National Standards, and that's not possible. What we are really talking about here is Standard or Standardised valves.

I have to admit to not being at all familiar with the Hickok testers, but obviously, they have come up with what looks like an attractive technique. Among the Avo testers, the VCM163 also uses a system which does not need a standard valve, either for servicing or day to day use. The servicing set-up procedure does involve a final verification of the instrument using a standard valve (originally supplied by Avo, but now no longer available), but only after all other adjustments have already been made.

In everyday use, the 163 has a self-test "Cal" check, which should show up any fault condition. With any test equipment of this age, it good to try and ensure that it's giving good results and so over the years, I've spent some time producing Standard valves, by basic DC methods and at various times have used ECL80's, EF91's and ECC81's. I have always been very pleased at the outcome of these tests. David Simpson's much appreciated efforts on this forum further confirmed the quality of the test results, both from mine and a good number of other Avo testers.

Furthermore, now that we have computers and Excel (not around when the 163 was designed), you can actually do another kind of self-test on the 163 in just a few seconds, just by using the variable grid voltage and recording the measured Ia, and plot the line in Excel. Again, I've always found that the Gm values measured in this way agree very closely with the Gm value that the 163 directly reads on the Gm meter courtesy of the 15kHz signal.

I used to spend a lot of my professional time ensuring that the chemistry section I worked in could fully satisfy the MoD's ISO9000 assessors for test methods and equipment. If we'd had an Avo163 in the section, that would have been one of the easier instruments to deal with.

B
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Last edited by Bazz4CQJ; 10th Feb 2019 at 6:58 pm.
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