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David Simpson 28th Dec 2009 5:09 pm

Re: Homebrew Valve Tester
Mike, I would echo Michael's suggestion regarding a kit. And in fact, as an old analogue guy who should really update his working knowledge on modern electronics, I would willingly cough up some dosh for a kit + working instructions. Would be a very interesting project for 2010.
What a difference between the Ferranti145 my nose is presently stuck inside, or the Black Box on the shelf awaiting some TLC.
I would rather money be paid to a retired chap like Mike, than to some far-eastern business producing kits sold in a magazine or on the internet.

Regards, David

igranic 28th Dec 2009 10:15 pm

Re: Homebrew Valve Tester

Thank you for posting the pictures and for the additional information. My apologies for not responding earlier; festive duties called!

Your endeavors have produced a really professional and functional looking tester, and the three digital meters seem to give all the essential functionality of a commercially produced VCM.

I would like to add my congratulations, and hope you feel able to carry this project forward for the benefit of yourself and other enthusiasts.


G8UWM-MildMartin 29th Dec 2009 2:12 am

Re: Homebrew Valve Tester
That's an extremely elegant design.
And a stroke of genius to make it compatible with the AVO VCM settings!

If I hadn't acquired an AVO VCM at a very reasonable price decades ago, I'd definitely want one.
And I'd still be tempted to build one if a kit came out...

I'd like additional filament/heater voltages of 1.4-117V, though.
I could wind a vaguely-suitable transformer from my Louth kits, same tappings as the AVO, but could you (or anyone else) perhaps design a switch-mode regulator to suit, please?

Congratulations again,

Ed_Dinning 29th Dec 2009 9:45 am

Re: Homebrew Valve Tester
Hi Martin, bit of a wide range for an SMPS, also given the current requirements fos some rectifiers. Linear would do it but double up as a room heater.
If you want SMPS it would be better to go for about 3 or 4 ranges from transformer taps and control of volt between them.


swordholder 29th Dec 2009 9:55 am

Re: Homebrew Valve Tester
Hi All,
Thanks to everone who has posted a comment. My intention is to post the PCB details shortly.
I will not do a kit of parts, most are readily available from Rapid Electronics, and I do not have the resources to either finance or store them. The mains transformer is possibly one fly in the ointment, although there must still be many of the "replacement type" transformers around in junk boxes which is where mine came from. No particularly difficult to get parts are used.
My main interest is Audio (Vintage and modern), the tester meets all my requirements. I dont want to get into modifying the design, I'll leave that to others. It should be fairly simple to accomodate different heater voltages, simply add another filament transformer and use that instead of the 6v winding on the existing transformer.
I don't want to get into producing PCB on a large scale, as I'm not now able to easily drill the holes to the accuracy I would expect to receive myself. Anno Domini strikes.
I will submit the design to the BVWS with a full write up to see if there is any interest in publishing it. If any of the BVWS team read this perhaps they could send me a PM and let me know what they want.Thanks to everone who has shown interest, the response has been quite overwhelming.

swordholder 29th Dec 2009 2:11 pm

Re: Homebrew Valve Tester
2 Attachment(s)
Hi All,
Here is the PCB tracking and layout attachments.
The eagle eyed amongst you may notice some small differences between the layout and the photos. This is because I modified the original board, mainly adding a capacitor across the anode current sensing resistor and allowing a larger GM sensing resistor.
I havent built up the current board, although I have checked and re-checked the tracking I would recomend that you check for any errors yourself before etching. From past experience it is easy to overlook an error no matter how many times you look.
You will see that the LT mains transformer has no tracking connected to the pads. It seems that each manufacturer has their own pin connections, although most work on the same grid pitch. I have tried to make this as universal as possible, so connect links from the 240v input to the relevant pads on the transformer, likewise the secondary.

Omegaman 1st Jan 2010 3:14 pm

Re: Homebrew Valve Tester
Hi Mike,

Your valve tester is a fantastic project and if you have no objections, I would like to build one for myself.
I wonder if you could clarify a few points on the circuit diagram for me. Please excuse my ignorance if these points are obvious and I have missed them!!

ZD11 & ZD12. Are these the same 15v types as in the anode voltage reg.?

In the anode voltage reg. should there not be a resistor to ground from the junction of ZD9 cathode and R5 to provide a ground reference?

What FSD/meter types did you use for the 3 panel meters? I gather the grid volts meter is DC volts but I'm not sure about the other 2.

What value is C10 (Across the grid volts pot)?

What type is D6?

I intend to wind a transformer to give all voltages required from one unit and also to use a multipole switch to select each of the common types of valve I use rather than the 9 switches like the AVO. My VCM163 will be resigned to the shelf now!!

Kind Regards


swordholder 1st Jan 2010 10:45 pm

Re: Homebrew Valve Tester
Thanks for the comments, very much appreciated.
All the 15v zeners are the same (If I remember correctly 1W rating)
There is no bleed resistor in the anode circuit as the load resistor is the valve itself, no HT is applied until the fuctions switch is in the test position.
The meters I used are
200v DC for the Grid voltage
200mV DC for the anode current
200mV AC for the GM meter
I bought these from
Note these have to be run from separate 6v AC supplies or they will be damaged.
C10 is 100uF 63v
D6 is 1N4148
Hope this answers all your questions
Kind regards and a Happy New Year


glowinganode 1st Jan 2010 11:15 pm

Re: Homebrew Valve Tester
Mike, thanks for taking the time to publish this design.
I wonder if you might clarify / confirm a few points, as I'm not sure if I understand this circuit totally.
The Anode current meter is a DC Voltmeter, with a 1 ohm shunt the display reads directly in in Amps. This seems slightly insensitive.
The GM meter is an AC Voltmeter, with a 10 ohm shunt. Each displayed unit equates to 100mA of superimposed AC. If a 5mA/V valve were being tested, then to get a reading of 5.0 on the display would require 500mA's of superimposed AC, and 100 Vac signal on the grid. This seems too high. Also should the voltage across the shunt be AC coupled to the meter as it has DC component too.
The DVM modules I've used in the past require a DC auxilary supply of around 12 Vdc, assuming yours are the same, where do you obtain this from as both the above meters are at Anode potential.
I'd be inclined to decouple the Screen and Anode supplies after the S/R's to reduce any tendancy of parasitic oscillations, and to reduce the output impedance of the Anode supply appearing as additional Anode load which might cause inaccurate GM readings.
I am currently building a Valve tester and would like to incorperate parts of your design as you have obviously put a lot of thought and effort into yours.
I've used a small audio output transformer in reverse to measure the Anode AC component, as this gives a reasonable signal level from a low apparent impedance, removes the DC component, and isolates it from the HT so the metering circuit can be at ground level. I've experimented with a band pass filter (2KHz - GM test frequency), to reduce the effect of ripple and the amount of HT smoothing required.
Many thanks in advance, hopefully your advice will help resume a stalled project.

P.S. I've just read your reply above which you must have posted at the same time as me. I assume the meters are scale 0 - 200mA, and 0 -20mA/V, with 1 Vac on the grid (you might find this is a little high for some valves). It's quite easy to modify these DVM modules to get the required D.P. to illuminate with a suitably placed resistor.

swordholder 2nd Jan 2010 9:33 am

Re: Homebrew Valve Tester
I'll try to answer your points. The anode current meter is indeed a shunted voltmeter, a resitor of 1 ohm will generate 1mV when 1mA is passed through it. The 200mV module (actually 199.9 mV) will actually read to a resolution of 0.1Mv , current can be read to 0.1Ma . Tested an ECC83 and read anode current of 1.8Ma.
I use approx 100mV of audio on the grid, Similarly, the AC Mv meter reads to 0.1Mv and the ECC83 reads 1.7 mA/V.
The modlules I used will operate from 6v AC so admittedly the transformer winding is at HT, but so far no peoblems, and I dont Anticipate any. The modules are well insulated from the panel. Go to the website and have a look at the pictures.
I did think about a transformer in the anode like AVO do with the VCM163, but decided repeatability would be a problem, and I'm not too sure how a small transformer woiuld react to constant DC. Would it saturate the core? plus of course a resistor is cheaper.
Do you run your tester from DC like mine? It may be possible with your design that the transformer is picking up hum from the mains transformer giving false readings.
In my design, the GM meter which is running all the time reads zero for 99% of the time with an occasional flick to 000.1, which may be the zeroing of the meter itself. Nothing to get worked up over.
Hope this answers your questions

Omegaman 2nd Jan 2010 4:54 pm

Re: Homebrew Valve Tester
Hi Mike,

Many thanks for your reply and the clarification of those points.

I'm going to order some meters next week and get on with it!

If it's ok to do so, I would like to show my efforts later on in the thread when I complete my project. I don't know if that follows the etiquette of the forum?

It would be nice to see other members versions of the project too.

Kind regards and happy new year!


glowinganode 2nd Jan 2010 8:55 pm

Re: Homebrew Valve Tester

Originally Posted by swordholder (Post 296164)
I'm not too sure how a small transformer woiuld react to constant DC. Would it saturate the core? plus of course a resistor is cheaper.
Do you run your tester from DC like mine? It may be possible with your design that the transformer is picking up hum from the mains transformer giving false readings.

Thanks for your reply. In response to your above comments, the transformer I used was from a 2W single ended audio amp, so it's designed with an air gap to cope with the standing DC. Since I'm using the secondary in the Anode circuit, the amp*turns is nowhere near enough to cause saturation. In a test setup, I varied the DC current from zero to 500mA with no noticable variation of amplitude or waveform of the AC signal. This setup was done on the bench, running off the bench PSU so it wasn't picking up stray magnetic field from a nearby tx. In fact I didn't actually have a problem with HT ripple since my bench PSU is very good. I wanted to simplify the power supply in the tester, without all the complication of a stabilised circuit. I just though it might be a problem, so I considered alternative solutions.
Thanks again for your time.

swordholder 3rd Jan 2010 9:38 am

Re: Homebrew Valve Tester
I'm sure that there will be no objections to posting your tester on the forum. I certainly don't have any, it will be interesting to see other peoples efforts. My layout was dictated by an available case.
When you order the meters, be a bit cheeky and make an offer for them, one one occasion I got one for 6, buying 3 you may get them even cheaper, after all every penny counts!!
The last one I ordered came in 7 days, superb service.
Good luck with the build

qualityten 7th Jan 2010 6:40 pm

Re: Homebrew Valve Tester
As somebody who is very much a learner, and who would like to own a valve tester, this project interests me too.

Howard, may I request that you record your build experience in as much detail as possible to aid those who may be inspired to follow. I agree with others who have said that this looks to be an important resource for forum members. My thanks too to Mike for sharing his expertise with us.


Omegaman 9th Jan 2010 4:41 pm

Re: Homebrew Valve Tester
Hi Everybody,

Mike, thanks for your permission to use your design.
I'll try to get some meters ordered very soon. I need an extra 2, so will try the "offer" method to see if I can save a bit! I intend to add meters to monitor cathode & screen current too.

David, I would love to attempt to document a build and post it here. I hadn't thought of that. The only problem is, I can be a bit busy at times and may not get to post as often as I would like.

A question for the Mods.
Not wishing to hijack Mikes thread, would it be the correct thing to do to start my own when the time comes, or is it ok to add to this thread thereby keeping all the info together. If I start a new one, which category should I use? Success stories or Homebrew?



Omegaman 9th Jan 2010 5:34 pm

Re: Homebrew Valve Tester

I forgot this in my last post.

You mentioned in one of your earlier posts that you have some setup instructions and voltage readings from your unit.
Any chance of you being able to post these?



swordholder 9th Jan 2010 7:37 pm

Re: Homebrew Valve Tester
I think that most people (including me) will be interested in any mods you carry out, perhaps the Moderator will indicate regarding starting a new thread or continuing with this one.
Regarding the setting up, I'm not too sure where I stand on that as I have sent the design to the BVWS Bulletin and this will appear in the spring issue.
Perhaps someone from BVWS can advise me on that.

Regards to all


Station X 9th Jan 2010 9:30 pm

Re: Homebrew Valve Tester
I would suggest that any member building one of these testers and wanting to tell us about progress should start their own individual thread in the Homebrew section.

Of course if you finish one, without updating us on progress, post in Success Stories.

This current thread can be used for seeking information from Mike aka Swordholder.

georgesgiralt 9th Jan 2010 9:55 pm

Re: Homebrew Valve Tester
Hi !
I may be a bit late on this, but i was far away from home ....
May I suggest something ? Mike can't produce a kit for it's tube tester. OK.
But if someone will gather all parts and propose a kit, I'll be glad to pay for it, including a contribution to the designer. This may pay for a subscription to one or both organizations Mike can't afford and maybe buy some candy ;-)
It will help us get the difficult pieces : the trannies.
If you look at the price a decent valve tester reach these days, I'll be glad to have a very fine kit for a little less than that price and help a fellow enthusiast in the same time ....
What do you think ?

Ed_Dinning 9th Jan 2010 10:01 pm

Re: Homebrew Valve Tester
Hi Gents, if there is sufficient interest I can probably arrange to get transformers wound and vacuum impregnated. If I have an idea of the quantities I may get a decent price from my local winder.


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