Valve tester meter protection?

[resurgance]

Hi there.

I am lucky to have a working meter in my MkII - And am in the process of putting a 29T into my CT160 as per Martin's (Dekatron) great post.
Am not sure if this is deviating too much from post subject - let me know if it is - but I was wondering what the best method of protecting the meter is?

I have seen Karsten and Martin's recommendations of 2x back to back 1N5711 Schottky diodes and a 100 uF 6.3 V bipolar SMD ceramic capacitor OR "a pair of 1N4001 standard Silicon rectifier diodes has been connected in parallel to the 1N5711 Schottky pair"

I was wondering if this applies also to the 440uA meter as well as the 30uA VT160 meter? And I also have a Hickok 539b with a replacement meter from the states that I wish to protect.

In simple terms as it is voltage related in my understanding these protection methods are applicable to any meter in a valve tester, but there is usually more going on than that.

Thanks and let me know if this is really new thread material

Stew

[GMB]

I always add protection to meters on all my equpment, whatever it is. This is a result of bitter childhood experience when meters were the most expensive thing I bought.

So a pair of power rectifiers going each way, either striaght silicon or Schottky ones depending on the meter voltage drop.

The way you decide is simple - get the meter showing FSD with a battery and resistors, then apply the diodes to see if you notice a difference in reading.

For ultimate protection you could add a low value fuse (or very thin bit of wire) in series - checking it has really low resistance so again that it doesn't affect FSD.

[David Simpson]

Stew, its refreshing(in view of recent AVO VCM catastrophes) to see a keen valve testing guy who wishes to take care of his valve testers. I use the Schottky Diode method on mine, & others I've serviced/calibrated for folk. If you haven't access to an adjustable DC mV source, then as GMB says - a wee 1.5V cell plus a decade resistance box is the alternative. A good old AVO8 on the DC 50 uA range to monitor the meter current is ideal, as you benefit from the safety trip if anything goes wrong.
A major caution with AVO VCM's - the full Peak AC Va is on the meter's terminals under "Test" conditions.

Regards, David

[Bazz4CQJ]

Originally Posted by David Simpson

Stew, its refreshing(in view of recent AVO VCM catastrophes) to see a keen valve testing guy who wishes to take care of his valve testers. David

David, can you point me to the “recent AVO VCM catastrophes” to which you refer? I certainly don’t recall any problems with meters in the VCM’s being discussed recently.

The only recent discussion of a faulty VCM on the forum, which is still not working, appeared to relate to a problem on the AF amplifier board (last I heard), but that's another topic.

B

[Station X]

I don't think we should be effectively naming and shaming forum members, which is what links to threads would do.

[Bazz4CQJ]

Fair comment, but applying terms like "catastrophes" seems, as a minimum, a little OTT with respect to both the instruments and their owners (in general).

B

[Radio Wrangler]

They aren't making any more AVO VCMs, and they have a cute charm all of their own, so damaging one or two seems a bit like kicking kittens.

Attractive as they are, they aren't really essential. Any person needs somewhere between zero and one of them.

We seemed to have a plague of guitar people some years ago who'd decided they were a good way to run valves in for a few days. They took out a number of VCMs that they'd paid a lot of money for. There must have been something written somewhere that it would help give them a tone like Stevie Ray Clapton and that soon they would achieve recognition and the millions would start pouring in.

The AVO VCMs have no effective protection against bad combinations of control settings, so they are easily damaged. Yet they seem to be finding their way into the hands of people who do not understand this and set out to learn by trying. There's no protection against steering your bicycle at a wall, but that's more visible and more widely understood to be stupid. You can buy several bicycles and a lot of bandages for the price of a VCM

David

[David Simpson]

Hey David, your Bicycle/Wall analogy is the best I've seen in a long time, regarding the treatment of AVO VCM's.
I do worry about you & Martin getting RSI &/or Carpal Tunnel problems after extensively trying to keep some folk on the straight & narrow in relation to their self induced VCM problems.
Me - for sometime now, rather than see folk acquiring these pensionable but still hellish expensive VCM's as a "must have" item - have been trying to encourage folk to build their own DC Tester or Curve Tracer. Five hundred quid + for a MK3/MK4 or half decent CT160, & a Grand for a 163. Bollox says I, £200 tops for bits & bobs for your own Homebrew V/T, I reckon.

Regards, David

[Bazz4CQJ]

I literally have no time for anyone who regards VCM’s as commodities, or less still, as “kittens to be kicked”. But the real catastrophe here is that we now seem unable to have a technical discussion about AVO VCM’s without someone bringing up the subject of their “mistreatment” (alleged or otherwise, I don’t know). This is not the first thread with this problem. Please can we go back to technical threads staying technical?

B

As mentioned before a couple of anti parallel diodes is very good protection for meters. Ordinary 1N4001's for meters that need 1/2 a volt or less for FSD or Schottky for 1/5th or less. To be pragmatic ordinary ones will do for any meter, they can take quite a bit of overload for a short while. I suppose that when these VCMs where made low leakage diodes where not readably available. given that the VCM uses (at best) a half wave rectified signal I would go for "ordinary" diodes.

The kicking kittens analogy upset me a bit and left a bad image in my head.

[Radio Wrangler]

I'm pleased to say that there is a large number of assorted animals which come over to greet me as a friend. They know where they're safe.

David

[Skywave]

'Tis a strange phenomenon that 'radio people' - especially many here on this Forum - have a strong affection for the world of felines (myself included, a cat owner). Consequently & unsurprisingly, David's graphic analogy has upset certain members. Having said that, I have no doubt whatsoever that David would never ever do such a thing under any set of circumstances - it was simply a poor anecdote to serve as a illustration, albeit an over-dramatic one.

Having said all that, can we now return to the topic of this thread?

Al. / Apr. 18

[Skywave]

Originally Posted by David Simpson

Me - for sometime now - have been trying to encourage folk to build their own DC Tester or Curve Tracer.
Regards,
David

Including the issue of 'go / no-go', the determination of the condition of a valve with regards to its essential parameters does not lead to a particularly difficult project to design and build. Probably the biggest problem is catering for the large number of different valve sockets and the fact that the various valve electrodes all appear on different pins.
It's a simple co-incidence that right now I am working on the design of a 'd.c. characteristics' (to use a term) valve tester of my own. At present, it has cost me very little, extensive use having been made of salvaged / re-cycled / spare mechanical and electrical/electronic parts, together with existing test equipment I already own to complete the overall 'valve tester' function.

Al. / Apr. 18

[resurgance]

Hi all, thanks to the moderator for shifting this and creating a new post, I think that worked well.

Thankyou also to all who replied - I've got to say I had a lot of fun reading the comments technical or otherwise! I really like the variety and appreciate the different opinions and viewpoints. An important part of being human and will soon become sacred as things get more PC and people are slowly being taught by the internet to become offended at the drop of a hat.

So I would like to expand on the method of arriving at the diode type as although I think I can make a mV source using a regulated DC supply and decade box I wanted to clear some things up. Right now I am still very much absorbing information and am a while off compiling a clear picture in my head.
For example I understand that the waveform of the avo is quite different to the Hickok 539b - is it one half of the rectified DC? So lumpy with gaps...
I am looking at protecting the following (Have got the VT160 sorted)

VT160 FSD = 30uA @ 3250ohms
MkII = 440uA @ 100ohms
539b = 115uA @1500ohms

And there is voltage present at the meter terminals which is the bit I am fuzzy on as this totally relies on the selector switch position right? So what max voltage do I need to allow for, as I have seen the warning about full voltage appearing at the meter terminals, and am wondering if this is accounted for when using a mV source and decade box for testing diode combinations?

I have come into this late in the piece - started in my early years, but really only came on strong in my mid 40s. I have a few really nice ham radios and linear amps that I started to slowly rebuild as my knowledge increased and as I was really green I wanted to ensure that my 'base platform' of valves was in spec so at least I could start with something that 'should work' I actually built a Roetester and apart from some flaws it is a really nice bit of kit EXCEPT that you need a PC to run it. Part of the appeal of old radios is the tactile experience and no operating system.
Next came an 'accidental' acquisition of a Hickok 539b in a bunch of boxes I picked up sight unseen. What a lovely machine inside. Almost same with the Avo MkII. - More recently I purchased the VT160 without a meter simply as I had seen inside an Avo and wanted another one. I get a real kick out of the build quality of these lovely machines and an even bigger kick subtlety improving them and getting them going nicely.

The 539b had an OC meter but fortunately a place in the US was able to supply a brand new Simpson movement even with the correct scale as a drop in for a very reasonable price.
The MkII was perfect and tested just fine after last being used and calibrated in 1987 at the NZRAF. what a great machine.
The VT160 was without a meter but I've put a new Simpson 29T in thanks to Martin (Dekatron) and I am VERY fortunate to have a very good condition and blemish free negative bias pot. I have measured all of the resistors and will replace a few.

Bit of background above getting off topic but thought some context would help after reading the replies

73
Stew Thorp
ZL2VA

[David Simpson]

To recap, and to comply with Bazz's "keep things technical" advice - 'twould seem best for Stew to go down the Schottky diodes fitment route.
"Kicking Pussies, driving bicycles into walls, and folk acquiring weak wrists" is just amusing & light-hearted banter. Long may it continue. Nobody is meaning any harm or disrespect, I'm sure.
I've said it many times, & I'll say it again - it's time that CT160's & VCM 163's had all their threads & queries shoehorned into one dedicated sub-forum or super "sticky" thread. Akin to "Sussex", AVO Multimeters", "Mullard HSVT Cards", & so on. Make it a lot easier for folk to cross-reference everything(& I mean "everything down to the minutest circuitry detail & possible fault remedies") available in "Search".

Regards, David

David (RW) I didn't suspect for one nano second that you would kick a kitten. It was one of those things that brought into my mind the awful things some people do.

Back on topic, VCMs and other valve testers were, as it is said, "a thing of the day". Valves were expensive, repairs expensive too, well worth paying sixpence to have valves tested to eliminate the cost of a new one. Those testers are worth repairing but for the joy of having it working, not a lot of real use IMHO.

[Station X]

Can we get back to the subject in the thread title please.

[GMB]

I think I can make a mV source

You are trying to make a current source, not a voltage source - but don't bother...

From the details you have given, all those meters will be fine with Schottky power diodes in parallel with the meter terminals. The voltages inside the circuits are not relevent. What we are doing to protect the meter is to provide a shunt that only starts working after the meter pointer has hit the stop.

But regular Silicon power diodes will be fine too, but the meter pointer will hit the stop a little harder.

Small signal diodes may themselves burn out so best use something beefy.

But to be sure you got it right you can fire it up and arrange a suitably large reading on the meter, then try the effect of the parallel diodes. You should see no difference at all.

I always do this because you just never know what devious problem might occur to damage the meter (this was my bitter experience when very young). Looking at the circuit is to assume it is all working fine, but by definition it isn't fine when the meter is being burned out! And meters can be hard to find/adapt so best not to destroy them in the first place and the protection diodes will go a long way to do that.

[Radio Wrangler]

You know the rough voltage the diodes will clamp at 300mV for Schottkys, 650mV for plain silicon. The resistance of the meter, or at least of it with its padder resistor is well defined, and so you can calculate the worst case current in the movement.

As GMB has said, a before and after test will give you confidence that normal operation is unaffected.

David

[resurgance]

OK thank you for the replies I will do some testing!

Cheers, Stew ZL2VA