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Old 30th Oct 2016, 11:54 pm   #1
chriswood1900
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Default Building a Valve Characteristic Tester 'Lampemeter'

I recently decided I wanted to get into more detail on valves and have the ability to do curve traces without the hassle of drawing them by hand.
So I did some research and found the Lampemetre range of testers sold by Alain Ducrocq Trading as Radiolec in France as offering good value and had heard positive comments from a couple of forum members.
Alain does several models but only one that is computer controlled via RS232 which needed a HT supply, heater and LT for the board at around 24v AC. So I bought it as my Xmas present via his Ebay shop last December. See:-
http://www.radioelec.com/en/vacuum-t...2_387-828.html
Most of these designs seem power the heater all the time they are on but use pulses to take the measurement’s by passing rectified but unsmoothed AC to the anode keeping the grid biased off except as it pulses through the voltages in the set range. If you need to test pentodes or tetrodes you need to arrange your screen supply, this proved to be the fiddliest part.
I then had to make some design decisions about how to house it, how to select the pinouts, what sockets to use, how to power it etc. For a case I decided I wanted to copy the principle of the AVO CT 160 design which I thought neat and convenient, so found some aluminium cases in a kit of various sizes to play with. I decided that the main valves I wanted to match and see the curves were audio valves limited to double triodes 6SN7, ECC81-83 and output valves like EL84, EL34, 6L6, 6V6, KT66, KT88, 6550. Looking at the AVO characteristics books I realised I could do all this from 2 types of holder International Octal and B9A with only 2 different pin arrangements. This obviated the need for switching if I used just 4 valve bases and one switch to select Anode 1 or 2. A rummage through my junk box produced a transformer which had the following AC outputs 270V, 25V and 6.3V and as it was only powering one valve at a time should have sufficient output.
Looking at the size of the components I laid them out in the cases to judge the likely size and layout choosing the medium size case 380 x 270 x 120 mm if I only housed the tester circuitry and kept the computer elsewhere, which allowed me to change it as I upgraded over time and made the project compact.
I had to find a way of mounting everything, I had some vinyl coated aluminium from another dismantled piece of scrap equipment which would make a nice top panel and bought some standard aluminium angle from B&Q to mount it in the case. I folded the panel at one end to make an L shape and decided to mount the power components on the small fold and the valve holders, switches and meters in the top panel and the PCB on the underside of the panel. I wanted a symmetrical layout so experimented on paper at 1:1 A3 until I had something I was happy with and then transferred the dimensions onto the underside with a felt tip pen. I then drilled pilot holes to allow me to use my Q hole punches and nibbler to create the apertures to mount everything else. Overall pretty successful at the first attempt with just a bit of filling to get everything fitting neatly.
To follow Set Up, Screen supply, Tests.
Chris
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Old 31st Oct 2016, 5:44 am   #2
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Good idea to put it in a briefcase Chris, that way it can be stashed away rather than taking up room on the bench.

Will you be able to test big OP valves at full whack?

Andy.
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Old 31st Oct 2016, 10:05 am   #3
chriswood1900
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Andy
I did choose the layout so that it could be easily moved or put away.
No it can't do full whack it does short burst samples to draw the curves, running a KT88 at full power will have to wait, there was an interesting tester on US Ebay that might do it!! The listing has ended but it is worth a look.
http://www.ebay.co.uk/itm/RARE-DAYST...sAAOSwx2dYDHJd
Seeing the Variacs used for power supplies it should be able to manage some hefty tests.
Chris
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Old 2nd Nov 2016, 7:03 am   #4
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Hells teeth, that Daystron is a beast! That would do the job, but as you have found out, having a hugh VT on a bench takes up room and they don't get much use. I didn't think of this when I made mine, I would do it differently now.

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Old 3rd Nov 2016, 9:46 pm   #5
chriswood1900
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Set Up, Screen supply, Tests.
First off the software ran but I could not get any sensible readings doing the static tests. So I emailed Alain and he responded that evening saying there is a problem with some USB serial adapters which uses the Prologic chip set. So a quick rummage through the computer junk box produced another using the FTDI chip set. Things started working and the initial tests proved successful with triodes with neat curves which made sense. The software is in French and the error messages flash up in French and my language skills are extremely rusty it took a while to work out with the help of Google what needed changing to setup correctly.
When I started testing Tetrodes and Pentodes it was less successful. With strange shaped curves with steps in and little resemblance to the data books.
I emailed Alain again with copies of the traces and he pointed out that what I was seeing was an oscillation possibly due to the screen supply or the need for more filtering. I had already used a lot of ferrites on the main grid and anode wires at each socket and experimented with different wiring layouts but this brought no change.
I decided to monitor what was happening with a scope and found the G2 supply was dropping markedly as the cathode current rose indicating a big dip in the G2 supply. This was the LR8 shutting down as the current requirements exceeded is capability. So I redesigned the supply keeping the LR8 as a regulator but adding a transistor to carry the bulk of the current. This improved things a lot but we still had some rogue oscillations at the beginning of the curves with Tetrodes as now Pentodes seemed OK. Experiments with different screen supplies or adding a small capacitor eliminated the oscillations but affected the curves so I still needed to find a permanent solution.
The attachements show the internal layout, screen supplies and the Tetrode curve which was a problem.
Chris
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Old 4th Nov 2016, 3:26 pm   #6
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

When using the LR8 you need to make sure that the minimum output current is at least 0.5mA as it will otherwise not work in its stable region. If you draw too little current it becomes susceptible to external noise/interference as well as internal effects. I've seen this problem in some designs and also asked Supertex about it, before Microchip bought them, and they told me that the LR8 was designed to work with a fixed output voltage, possibly with some small adjustment capability but not in a fully variable design if the minimum output current level was not met. They said that you needed to load it to the minimum output current to have a stable output voltage and stable workings of the internal protection and regulation circuitry. I've experienced these problems with other designs when the output wasn't loaded enough. Most designs I've seen use 1mA as loading current.

Loading the output with 0.5mA is problematic if you want the full swing from 1.2V up to 440V as that requires a 2.4k resistor for the 1.2V setting, but it needs a power rating of 100W and cooling - 440V across 1.2k is some 81W. So using the LR8 in a variable power supply is somewhat of a challenge. If you just want it to be at 440V the resistor only needs to have a power rating of some 0.5W (0.22W) for the 880k resistor. Don't forget the voltage rating of those resistors too.

You might not experience any instability but if you do you can check if the LR8 is stable or not or if it is oscillating due to external interference.
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Old 4th Nov 2016, 8:41 pm   #7
chriswood1900
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Martin
Thanks for the pointers, the design which I cobbled together followed a design from the web with some small changes from the data-sheet, so far it seems stable and works well over the 20- 300v range.
I will have a look at the calculations and keep an eye on it.
Thanks
Chris
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Old 5th Nov 2016, 6:43 am   #8
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Chris I've built and used this simple circuit without problems on my homebrew tester, here is the same circuit but better thought out http://sound.whsites.net/project151.htm and see pic two. Havnt built the second but thre first t's more stable than using a regulator IME.

I built a variable HV supply and could never getting it working properly - see here -https://www.vintage-radio.net/forum/showthread.php?t=128470 . the srceen grid can go into negative resistance on big tetrodes; more here - http://www.cpii.com/docs/related/22/C&F2Web.pdf

Andy.
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Old 10th Nov 2016, 12:18 pm   #9
chriswood1900
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Finally
Another email to Alain reminded me that I should have included a capacitor on the transformer HT supply which he had included but which I thought was just a spare this brought about a permanent solution and the ability to do a range of curves. The curve tracer has proved a useful addition to my other testers being able to rapidly draw up curves of the limited range of principally audio valves using the 4 sockets I included.
I then decided to add the facility to do a few more valves via some adapters namely the EF86 and EF80/85/183/184 this proved easy using some cheap Octal bases order from China along with some B9A bases from the junk box and some appropriate wiring up of the pins so they fitted in the Octal socket for Pentodes, this also produced curves matching the data sheets and I enjoyed comparing NOS Mullard’s and Brimar’s with the more modern Chinese or Eastern European output. The NOS were very close to the data book but the newer ones are more variable.
Altogether an enjoyable and useful education as a it helped my knowledge of valves and their testing.
At the moment things are working OK with the screen supply so I will leave alone but thanks for the input on alternatives, I may well seek to improve things later.
To finish I have added some photos of the finished item and some curves obtained from valves I have tested.
I hope others have found this useful and please PM me if you want more info.
Regards
Chris
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Old 10th Nov 2016, 4:43 pm   #10
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Nice work Chris, well done. It looks very proffesional.

BTW, are the curves on 6L6 and 6550 really that wobbly or is it an optical illusion? The graphs don't give a g2 value as far as I can see. Can you specify how many control grid curves are drawn on the graph or select just one?

As you say it's a nice addition to a normal pass/fail tester.

Andy.
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Old 10th Nov 2016, 6:19 pm   #11
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Andy, Thanks for the compliment, the wobbly lines are down to the resolution I used although the reolution is limited. The G2 value is set on the tester manually, I should note them down in the comments box.
The number of curves are set at 1 volt intervals given a start and stop value. So on an ECC81 you might use O to -8V whereas a on 6550 0 to -30. I have added a screen shot of the settings so you can see.
Chris
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Old 10th Nov 2016, 11:51 pm   #12
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Quote:
Originally Posted by Dr Wobble View Post
Chris I've built and used this simple circuit without problems on my homebrew tester,....
Andy, yep -- I built exactly this one a couple of months ago! Neat little thing, minimal component count, too.
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Old 22nd Nov 2016, 2:31 pm   #13
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Chris, a great project & great results. Makes me wish that my digital & software electronics knowledge was up to date. Instead of being stuck in the analogue valve testing & graphing era.

Regards, David
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Old 22nd Nov 2016, 5:56 pm   #14
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Damn it; this will do no good at all to the residual value of AVO VCM's
Fabulous piece of kit.

Chris, one of the plots is labelled as a 6L6 "calibration valve"; is that one of the ones sold by the same outfit as the kit?

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Old 27th Nov 2016, 6:53 pm   #15
chriswood1900
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

David and Bazz thanks for your kind words. Sorry for the delayed response but I have been away a few days. The 6L6 was bought on eBay and was a NOS RCA Metal one so I'm not sure about the ones sold be Alain.
David the project did not require great digital skills beyond sorting out loading the software and getting the USB to work so try it!
Chris

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Old 15th Nov 2020, 5:49 pm   #16
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Thread reopened by request.
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Old 15th Nov 2020, 6:09 pm   #17
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

I'd just like to link to this thread where I describe my build of Alain's curve tracer.
https://www.vintage-radio.net/forum/...d.php?t=168827

Peter
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Old 15th Nov 2020, 7:21 pm   #18
chriswood1900
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Paul and Peter thanks.
I asked to reopen this thread to support Joseph in Madrid building his Lampemetre.
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Old 15th Nov 2020, 8:43 pm   #19
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Hello good afternoon
I am building this plotter these days and I asked Chris (thank you very much), some reasonable doubts since I am not a specialist in curves or curve data and it costs me a bit to compose everything for HT supply in (g2).

In fact, I have had the kit / project in the drawer for 2 years and it is time to use it.

also very kindly the fabricant
Alain DUCROCQ has answered some questions, thank you Alain from here.

Well, it remains for me to determine where a capacitor is connected to filter that I do not see documented, and the certainty of where I connect the output to the oscilloscope, ( I not see terminals in the site or drills ? ).
which I want to implement as well, in addition to the rs232 output ... but there is still a lot of work to get to that point .

I find the software very basic although I guess enough,
thanks.
Joseph.
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Old 18th Nov 2020, 12:51 pm   #20
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Default Re: Building a Valve Characteristic Tester 'Lampemeter'

Quote:
Originally Posted by Electronpusher0 View Post
I'd just like to link to this thread where I describe my build of Alain's curve tracer.
https://www.vintage-radio.net/forum/...d.php?t=168827

Peter
Good project, and good luck.

I have one model of this tester AVO.......but my AVO live in thid days 8- \
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