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AdrianH 3rd Jun 2020 7:26 pm

Yes, yet another home made valve tester.
1 Attachment(s)
Not too sure how useful this will be for testing valves but it is occupying time whilst building.

Fitted two meters today, some 4 mm sockets and a few switches.

Red ones on left hand side for heater/filament.
Black same side for Grid.

To the right side two reds for anode and g2 and two blacks for Cathode & g3.

There is not enough space in the chassis for a power supply so I am going to fit the buck style PSU's into a metal box to provide a bit more heat-sinking and a couple of pots to set HT and heater volts, I have yet to sort a linear pot to set a negative Grid bias.

There is no plan to this, I never seem to make any as it is always down to what I can find in my bits and bobs boxes or what I can fabricate as I go along.

One HT output has the 4 or 40mA meter in series so by changing leads I should be able to either check g2 or anode current. Changing grid bias +/- 0.5Volt from spec, I hope should allow me to figure gm.

I am sure there are other things I can do with it, but it is a start. Got to make up the jumper leads as well.


Diabolical Artificer 4th Jun 2020 6:02 am

Re: Yes, yet anoither home made valve tester.
Looks ok to me, all the holes line up, grand job. What's the HT source there your using?


AdrianH 4th Jun 2020 8:52 am

Re: Yes, yet anoither home made valve tester.
It is one of the Ebay units that give 150 to around 400 Volts and DC for heaters from 12 Volts, all switch mode style buck inverters. if you search Ebay for 'Amp/ Preamp/ Filament DC 12V to 150V-420V DC High Voltage Power PSU Board f Tube' These are sub 15 so should be OK for import duties I think if you only get one.

I also have another style I can use which is just HV, these can be found with 'DC-DC 45-390V High Voltage ZVS Boost Converter Step-up Booster Module' again for around 10 each no heater volts on this.

I used then, when I started playing not having suitable transformers to make up a PSU, and I hate the idea of rectified mains.

For the valves I have many which are TV valves with higher volt heaters I will probably use my variable DC power supply for heaters and use one of the above for HT.

I have found in the bits box a NMS1215SC which is 12 Volts DC to isolated 30 Volts out, so I can further regulate with a Zener to give me a grid bias down to -15 Volts. Or two 9 Volt PP3 batteries and a Zener?

I now have to wait for a replacement Voltmeter as I knackered one on the unit last night trying to stop the needle sticking, the meter must have been dropped at some time.


AdrianH 5th Jun 2020 10:49 pm

Re: Yes, yet another home made valve tester.
Well to date the tester has been a complete flop!

I tried to test a small selection of valves and was mightily confused with the responses I got, the first vale trying the triode side of a 30FL1 it seemed OK, moving on the the Tetrode side it gave OK readings until I started to back off the grid bias, in which case it went hard on.

Checked wiring and all seemed OK, try another 30FL1, still the same. EL84 when backing off the grid bias from say -12 Volts strange things happened, last valve to try 6925 same again. OK got out the diode probe and sure enough at a certain point the valves are oscillating.

I have tried 1nF caps down to chassis on the 9 pin valve style connector and 4n7F caps at the end of the wire chain on the octal vale, makes no difference, run the grid wire in a piece of co-ax to shield it, no difference. large smoothing caps on the DC still the same.10K resister as a grid blocker.

I can see why the rectifier type tester are easier to do.

I have two things left to try, rewire all the sockets try to get them a bit neater, rather then flexible wire and some small loops so I could get in to solder, try solid wire short as possible. Or the switch mode power supply which can oscillate on it's own when it wants to apparently, so would need to use a transformer, but that means a HV regulator.

Perhaps with a new day I will try again.


Edit just tried replacing the HT with the one from my milliwatter audio amp which is transformer based around 160 Volts DC. and it still oscillates, so redo the wiring!

Herald1360 6th Jun 2020 10:52 am

Re: Yes, yet another home made valve tester.
Ferrite beads?

retailer 7th Jun 2020 1:17 am

Re: Yes, yet another home made valve tester.
Sometimes the combination of lead dress, layout and a high slope valve will cause oscillation - the anode current will suddenly go high, hence Herald1360's suggestion of ferrite beads. If you place ferrite beads on the valve socket wires it will help with this issue and possibly eliminate it altogether.
In case you don't know ferrite beads are small hollow pieces of ferrite, come in various sizes, most common ones used on valve testers are around 4-6mm long and have a small hole through the middle - unsolder the wire going the valve socket pin, slip the wire through the hole and resolder, do this to all of the pins, with luck it may cure the problem.

marty_ell 7th Jun 2020 6:58 am

Re: Yes, yet another home made valve tester.
I recently experienced a similar problem with some 954's on my tester.

I had made up an 'acorn' adapter to octal plug with short, direct wiring to g1,a and k, each lead no longer than one inch. The tester's socket has all round ferrite beads right up to its pins.

Even so, all my 954's immediately broke into spurious oscillations from ~600MHz upwards, with symptoms very like AdrianH's. The total cure was adding a ferrite bead threaded on the g1 lead right up at the valve pin, same for the a lead.

To be honest, I hadn't thought a 954 could get up to 600MHz with such short wiring in a lossy situation. We live and learn.

Hope this helps, Marty

AdrianH 7th Jun 2020 9:54 am

Re: Yes, yet another home made valve tester.
3 Attachment(s)
Well I have nothing to loose by trying to get hold of a bag of ferrite beads.

At present I have only the 9 pin valve socket connected, added an aluminium plate as a shield between the adaptor socket and the valve holders.

I have tried co-ax cable on the banana leads grounding with trying to ground the outer sheath. The only time I managed to stop it from oscillating was when I tightly gripped the length of the leads together in my hand!

I have 1nF caps from each banana socket to chassis, 1nF and 10uF across the g2 connection to chassis, 22uF across the HT and even decoupled the heater wires.

The bias is basically now just two PP3 batteries in series to give -18 Volts across a 5K multi-turn linear pot with the +ve connected to the cathode connection which is connected to chassis. The wiper has co-ax to the grid connector with 1nF and 10uF cap to chassis.

I seem to be able to make it worse by adding caps across the vale socket to ground?

Other then that HT comes in 22uF Cap. To the g2 connector and across a volt meter with range switch, also to a mA meter again with range switch to the anode connection, I must do a circuit for myself so I can keep track of things.

The buzz of the rf bug detector is getting really annoying.:wall:

Thanks all for your comments.


David Simpson 7th Jun 2020 10:02 am

Re: Yes, yet another home made valve tester.
Good for you for attempting a homebrew valve tester. I wish more folk would make the effort. Particularly in view of how many hugely expensive AVO testers have gone tits up recently. I've said for some time that a decent homebrew DC analogue tester can be built for under 200 quid. Yep, switching valve electrode connections for up to 9 pins can be a fiddle to work out. So can designing a source of ranges of suitable well smoothed Anode & Screen HT's, Vg, and heater supplies.
But, and its a big but, parasitic oscillation problems need to be thoroughly sorted out. As Herald 1360 says, ferrite beads is a must in all valve electrode leads. You could try using a screened wire for your Vg supply. A spare bit of Mic or Tel screened wire would do, as we're talking zero or min Ig.
The likes of high slope TV valves such as PL500's etc. are very prone to PO. I made up a B9 to B9D Magnoval extension lead with extra ferrite beads on each wire.
If you PM me your address, I'll send you a bucksheesh 6AQ5 or 6AU5 DC Standardised valve to help with your calibration.
I've advised this many times regarding AVO AC operating valve testers, but can equally stand for Homebrew DC Testers - always give a few volts of extra -ve Vg(above the recommended book value) for determining Ia, then slowly reduce to the book value whilst observing Ia rise. On my DC Tester, Va & Vs are supplied via a DP Relay(H/Duty PTT Relay), controlled by a spring operated Toggle switch. If a fault occurs or something doesn't seem right, then you don't have to spend more than half a second to cease testing a suspect valve.

Regards, David

AdrianH 7th Jun 2020 10:59 am

Re: Yes, yet another home made valve tester.
Just ordered 100 ferrite beads to go over the wires, I am assuming it is fill up the wires and not just one bead per wire?

I was considering doing a PCB for the 4 sockets and including decoupling caps and inductors at each valve socket, but with different valves having heaters on various pins, some going over an amp current it started to get a bit daft.

Not sure if you can see the last picture underneath the tester in enough detail, the thin blue/brown twisted cable is connected to the 2 x PP3 batteries to the ends of the 5K pot, there is a 10uF cap across the wires and the output is co-ax going to the grid socket.

Must spell switch correctly next time.

When I have been testing I will always start at -18 Volts and wind up, typically with the EL84 or a PCL 86 when I get an anode current above7 or 8 mA off it goes into oscillation.

I will gladly take a vale off you if I need one but not if I don't? Not sure what you mean by a DC standardized valve and why that should be any different (apart from characteristics) then say a 30FL1, EL84 etc. Unless you mean you can state the characteristics out before sending it down so that I can check for the same?

The analogue meters have been adjusted to be within a couple of % with my digital meters both on volts and mA, I like analogue as it is easier to see what is happening and to know the difference between a gm of around 7mA/V or 6.9mA/V is overkill. But I will pm you but only send a valve if you think it is necessary, and thanks very much for the offer.


David Simpson 7th Jun 2020 11:59 am

Re: Yes, yet another home made valve tester.
Hello again Adrian,
All my 6AQ5 St.Valves come with an A4 Graph of Gm(mA/V), with Vg ranging from 25V down to about 10V DC, and the corresponding rise in Ia from about 8 to 10mA up to about 55 to 60mA DC. So folk can chose an Ia/Vg to suit their calibration procedure.
Quoted manufacturer's spec for valves are for such pure DC conditions. All the well known British valve testers & American "Tube Testers" test valves with AC Sinewave P to P pulses, but quote an "equivalent" RMS(DC - like) value for their electrode voltages. If in spec, valve testers like AVO's can get jolly close to the pure DC "Curve". And the "Sussex" can get even closer. The hugely expensive "RoeTest" is spot on, and is probably the benchmark in the 21st century.
But, and its a big but, AVO's/Taylors/Mullard/Hickoks etc. are now in their twilight pensionable years(60/70 years or more). Some are still horrendously expensive to buy & their original parts replacements are likewise very expensive. If you phook one, you could be out of pocket by 100 or 200 or so for just a meter/Vg pot/Thumbwheel switch/or mains T/F/etc. However, (I keep saying this) a decent handy DC valve tester could be made from reliable 2nd hand items & bits & bobs for about a couple of hundred quid. Folk using their own design & layout to suit their requirements, might take a month or two to get things right.
Ferrite beads - maybe two per lead max to the valve holders.

Regards, David

AdrianH 7th Jun 2020 12:23 pm

Re: Yes, yet another home made valve tester.
OK David thanks for the information.

I will put the unit to one side whilst I await the ferrites, see if I can solve the instability and then add a socket at a time back in circuit.
One question is it a good idea to add caps 1nF at the end of runs or not, I would have thought so but in some instances I have made it worse.

I do not have a lot of valves, only some I bought cheap from Ebay to play and learn with, will never be reselling valves so can not really consider spending hundreds on the Avo's etc, would be a nice to have but for me not essential which is why I tried this project.

I was simply going to plot Ia against Vg for fixed Anode and screen Volts for the valves I have and I have some 'good' EL84's coming slowly from abroad in the hopes of building a Mullard 5-10 amp in the future, so hope it would help find reasonable matching sets.

I never realised that the ECC83 could get to be such an expensive valve, I blame that on all the audiophoolery.

Thanks for the help.


David Simpson 7th Jun 2020 3:36 pm

Re: Yes, yet another home made valve tester.
Adrian,I think you've got the right idea, homebrew-wise. Testers like the AVO's have a sequence of continuity & insulation procedures before finally testing for Ia/Vg by twiddling bridge balancing pots which indicate mA/V on a handy dial. However, folk with a modest collection of valves can easily do these basic tests by using a multimeter & a megger. Then simply alter Vg by a fixed amount & read off the change in Ia. Bobs your Uncle. And, the bonus is - you're supplying the valve with proper working DC, just as it would experience in a radio or amp chassis. In fact, its been mooted several times over the years by Forum folk that a decent redundant amp chassis can make a good donor for a homebrew DC tester.
Hopefully, once virus restrictions are lifted, you'll be able to pop down the road to Golborne & cheaply acquire any additional bits & bobs at the next BVWS Swapmeet.
Audiophoolery - has a lot to answer for. Expensive ECC, KT**,EL, etc. valves. Plus the urge to cough up mega bucks for a "must have" CT160 or similar tester(then go & phook it by proding inside with snips/screwdrivers & soldering irons !).

Regards, David

Top Cap 7th Jun 2020 5:03 pm

Re: Yes, yet another home made valve tester.
In some cases I found it beneficial to wrap some tinned copper wire around the glass envelope of the valve under test, connected to the circuit Ground.
Keep it slightly loose so you can slide the wire up or down the glass for minimum or if you are lucky, no oscillation.

This may bring back memories to some of the PL81/PL504 era though for a different kind of oscillation :-)

Petedox 7th Jun 2020 5:32 pm

Re: Yes, yet another home made valve tester.
1 Attachment(s)
Hi Adrian, I experienced oscillation and various forms of instability on mine, but after much head scratching found it solely down to lead length and lack of ferrite beads.

In the pic below you can see AVO's patented circuit (taken from the uTracer manual), which shows how each pin is connected in a 'ring', of more or less equal length, back to the switch. I did this and then fitted as many ferrite beads as I could!

Problem finally solved!

AdrianH 7th Jun 2020 5:41 pm

Re: Yes, yet another home made valve tester.
To be honest the best thing that I have read out of these posts, is that oscillation has been encountered by others.

I was beginning to think last night that it was just me, the layout I was using, or that I had missed some really fundamental issue in what I was trying to achieve. So rather then strip it down for valve holders on other projects I will leave it be and see how I get on when the ferrite beads arrives.



David Simpson 8th Jun 2020 10:23 am

Re: Yes, yet another home made valve tester.
Nah, nah, Adrian, keep at it. Don't let PO issues put you off. Another thing you could try is to keep Grid Voltage supply wiring well away from Anode & Screen wiring where possible. To this end you might consider colour coding your circuitry wiring inside the tester. There was an old code sometimes used years ago :-
Heater = Brown,Cathode = Yellow,Grid = Green,Screen = Red,Anode = Blue, & Earth = Black.
As Pete says, AVO's design does much to prevent PO. Apart from effing CT160's, which because of the cramped wiring inside the clamshell case, are prone to bursting into PO with high power &/or high slope valves.
Top Cap's idea is a jolly good hint. Also,B7G & B9A & one or two other types of valve holders often have B/C skirts to fit spring-loaded screening cans.

Regards, David

AdrianH 8th Jun 2020 10:58 am

Re: Yes, yet another home made valve tester.
The holders I have used for the 7 and 9 pin valves can hold cans, and when I get a ex military unit to break down (after playing of course) I may have the sizes available. The metal plate was meant to be a bit of a screen between the external wires and the valve.

Will hopefully be towards the end of the week when the beads arrive and I will have another play time.


daveobuchanan 9th Jun 2020 6:21 am

Re: Yes, yet another home made valve tester.
There are some Russian ECC83 equivalents, which have just 6.3 volt heaters, who's name escapes me, quite reasonable on ebay, and work ok.

AdrianH 9th Jun 2020 7:50 am

Re: Yes, yet another home made valve tester.
Hello Dave, going back briefly to the Mullard 5-10, and ecc83's, I have not bought any yet it will be months off as I learn more on valves and understand more on miller effects, stoppers, etc and the uses of all the valves! Unless I wind my own transformers buying the magnetics is a part I would have to consider, but I found the Baily/Radford article and circuit that uses the ECF82 rather than the ECC83. These do not seem to have fallen foul of the Audiophoolery prices yet.


daveobuchanan 9th Jun 2020 8:19 am

Re: Yes, yet another home made valve tester.
I remember being interested in that. Endless hours of entertainment, good luck.

AdrianH 10th Jun 2020 4:42 pm

Re: Yes, yet another home made valve tester.
1 Attachment(s)
OK so I guess this could be considered over the top! A batch of 100 ferrite beads arrived today and i have now ordered another 100.

I thought I had used all the 100 up until I found one on the floor.:wall:

I have just tested an old EL84 without issues, 230 Va -8 Vg1 with Ia at 35mA so a bit low from the charted 40mA. change to -9 Vg1 gives 28mA so at that working point gm at 7mA/V.

Think I could still use that at on something. Not tested anything using the Octal socket yet as that has no beads on the lines but I will give the old 6P25 another go and see if it causes issues or not, but it will still be getting beads on the leads if it works or not.

Picture attached of present underside wires.


AdrianH 10th Jun 2020 7:07 pm

Re: Yes, yet another home made valve tester.
Well the 6P25 in the octal socket is still oscillation, so awaiting more beads to continue.


AdrianH 16th Jun 2020 4:06 pm

Re: Yes, yet another home made valve tester.
3 Attachment(s)
Another close to hundred beads in place to the octal socket and also wires back to the 9 banana sockets are in place.
The 6P25 will still oscillate, other 7 pin and 9 pin or an EL41 valve I have do not to cause an issue. It could well be down to the fact the valve is missing quite a bit of the silvered paint on the sides of the valve, so will give up with this valve for a while and either try and get some of the conductive paint or await another similar valve.

The anode and screen voltages are tied together from one HV buck converter so for the time being I can not yet test a valve sent to me from David Simpson. The buck converter also has a negative line so I am using that to provide the bias volts with a resistor, Electrolytic and a couple of Zeners.


retailer 17th Jun 2020 1:20 am

Re: Yes, yet another home made valve tester.
That's an awful lot of beads, most of the setups I've seen have just the 1 bead close to the socket, the 6P25 valve is a high slope valve but not that high, 8.8 mA/V - an El84 is 10.5 mA/V, is it only the octal socket that causes problems, I see you have all of the wires from the octal socket going back to the banana sockets, why not try 3 wires from the 7 pin, 3 wires from the 9 pin and 3 wires from the octal - hope you get my drift.

AdrianH 17th Jun 2020 9:41 am

Re: Yes, yet another home made valve tester.
It may be overkill on the beads granted. I need to get some socket savers for it and see if I can make some adaptors for odd valves, but will do that as and when the requirement crops up.


David Simpson 17th Jun 2020 1:21 pm

Re: Yes, yet another home made valve tester.
Adrian, it might be worth your while, and avoid duplication of replies - to ask the Moderators to join your other current thread in "General Vintage Technology" to this one. And, less confusing for us lot out here.
Just wondering - have you tried testing the 6AU5 DC Standardised Valve I sent you last week? Being just a bog standard IO valve base, it should be easy enough to set up in your prototype rig. Hopefully you should get ball-park-ish results near to 40mA & 4mA/V at roughly -20V Vg.
As Retailer says - that a hewer of amount of ferrite beads you've used. One, or maybe 2 per lead is all you need.

Regards, David

Bazz4CQJ 17th Jun 2020 1:59 pm

Re: Yes, yet another home made valve tester.
Sometime ago I built a little unit to do DC characterisation of ECC81's, which is my preferred valve for use as a standard for my AVO163. It soon became apparent with the MkI version that it would sometimes spring in to oscillation, but it settled down with maybe 6 or 10 beads and maybe some decoupling caps, all built in to a diecast box. I think I may have used some feedthro caps on some of the feeds from the external PSU and external meters. It seemed to me that valves with a Gm of more than 4 or 5 were particularly prone to oscillate, as might be expected.


AdrianH 17th Jun 2020 2:02 pm

Re: Yes, yet another home made valve tester.
Hello David;

I will try and answer all your questions in this post under the Homemade section and ask mods to close the other.

At present the small buck converter provides a variable voltage from +50 to +350 Volts DC, it needs to source around 15mA to be a stable output. There is a further 22uF HV cap in the tester, it may not be a perfect DC at present not seeing any flicker in the meter over 15mA draw.

if the likely anode current is small I load it with an external resistor which is not metered so I only measure anode or grid current.

There is also a negative rail output as well, this is not regulated as such but follows the positive rail, so when the positive rail is at +50 Volts the negative rail will be around -60 Volts, +150 and -162Volts etc. So the negative rail is used to provide a negative grid bias with a series resistor and Zener diodes.

I am only pushing the Buck converter at 40mA to 50 mA positive rail at present and it will happily sit at the for some hours. I have another of the same type that I plan to add to provide a independent g2 supply rail. I also have some of the better units meant for valve pre-amps with filament supplies built in, but the down side is they only have one positive rail and it starts at 150 Volts.

There are two banana sockets on the tester supplied from the single (at present HT rail,) one is directly from the rail and the other goes through the current meter. The scales are 4 and 40mA. So depending on which way I connect the terminals I can measure g2 or anode current and will typically do anode first, then screen. The buck shuts down with excessive draw.

So at present when it comes to testing valves that go over the meter range (40mA at present) I have been looking at the charts and checking against the lower current ranges of the vales, such as a few EL84's I have.

Of course the supply current is not that great for testing some power valves yet, and the plan is in the future to build a mains HV PSU. I guess I could see if the transformer for the Sussex and the PSU boards are still available, or my preferred option is to attempt rewinding my own.

No your valve is still sat in it's cardboard box yet until I add the separate rail.

When it came to ferrites for me it was a case of making sure, rather than adding two, undoing adding another etc. for the extra cost it was not an issue.

I have just received another batch of 2nd hand valves from Ebay that I can play with, if I damage any of these there is no great loss to anyone, the package even has valves in there which look to be new and would be spares for my Pye P76F and Vidor CN430 radios. So happy on that score.

But and this is a big but! I doubt if I will ever be a collector of valuable radios and/or valves, I plan to build a Radord design audio amp, a receiver, test the valves in my AR77 and generally try to build circuits as I go. The buying, refurbishing and then selling on which I saw on the Facebook side of this site does not appeal to me.

Avo meters. I have a model 8 mk7 sat next to me, it is duff, current ranges work but the voltage ranges do not, yet! It came from where I worked many years ago in a clear out as it was faulty and everyone went to DVM's including me. It may get fixed or offered on the site. I think the plastic tracks have a break in them and yes I could fix it

Hope that answers a few of your queries they have reminded me to add a 100mA fuse in line of the HT.


AdrianH 17th Jun 2020 2:11 pm

Re: Yes, yet another home made valve tester.
Hello Bazz just seen your post.

With the lockdown I had little chance to get anywhere to select the die cast boxes, that would have been my preferred option. But I also had some aluminium sheet off-cuts in the garage, so decided to guillotine, bend and TIG weld up in to a couple of small chassis for my small amp and this tester. I now have just enough left to add closers to the bottom of them both.

On the beads as I was adding then I just left approx 1/4 inch gap between each one where I could, there are also 1nF and 4n7 decouplers that can be seen in the pictures., Time will tell, the above box leads do not have beads on them that will be next, perhaps another 100?

I am using co-ax at present and have left tails that could be connected to chassis. it may or may not be needed with this tester, I think perhaps to much in a small volume.


David Simpson 17th Jun 2020 2:23 pm

Re: Yes, yet another home made valve tester.
Adrian, you're certainly soldiering on. Hopefully, with Boris starting lifting more social restrictions, you'll soon be able to get along to Golborne & other BVWS venues to rake about & get decent stuff for your project. Also, if some friendly Forum folk are fellow Lancastrians, or Mancunians or Scousers, you might be able to barter or scrounge or whatever near to Bury.
I'll make one final suggestion - if your proposed metering & switchgear is all going to be on one panel, make a drawing template with some strong cardboard, then transpose that(once it suites you, ergonomically) onto some thin plywood, then drill, cut, file, whatever & then mount everything. Maybe eventually you'll acquire a nice big sheet of satinized aluminium which can be safely cut & drilled using the plywood as an accurate template.

Regards, David

AdrianH 17th Jun 2020 2:38 pm

Re: Yes, yet another home made valve tester.
1 Attachment(s)
If it was not for the Corvid problem I doubt so much time would have been put into these projects, but I have to be doing something with my time and gaming or watching TV for hours is just not me.

I have some CAD/CAM hobby gear with CNC plasma and milling machines, so generally small stuff.

If the lock down eases enough for decent travel and good weather holds then I will take a pause from this for a few weeks as I get out into my main mode of fun, see the picture below. That was home made as well hence why I have the mechanical tools.

I am looking forward to some of the BVWS meets, perhaps shake a few hands of the helpful members on here. I also owe some money to the charity box;D


AdrianH 19th Jun 2020 1:09 am

Re: Yes, yet another home made valve tester.
2 Attachment(s)
I really must stop doing this stuff in the early hours, but!

I added a second buck converter so I could run an independent Screen/G2 supply. This was set to 150 Volts. Anode at 250 and heaters at 6.1 to match Davids results for a 6UA5GT.

I went through a simple test of the valve to a maximum of 40 mA and found it matched quite well. So I am very pleased with the results, OK a bit of a faff to set up if one was changing valves all the time, but should help me no end.

No signs of oscillation with the 6AU5GT, it is a lowish gm valve, but encouraging all the same.

Thought it easier to take a screen shot of the results in my spreadsheet, this is basically just comparing Davids results with my own.
The columns are:-
DC which I am guessing is a meter in line with the anode current, then a Mk3 tester then my own. I stopped at 40 mA and made a note of the grid volts.
The second picture is just a photo of the setup at a g1 of -27 volts.

Now time for sleep.


David Simpson 19th Jun 2020 9:47 am

Re: Yes, yet another home made valve tester.
Those are really good results. Right enough, the first 6AU5 tabulation is pure DC, the second is my MK3(AC), and the empty column is for your results. I always include my MK3, partly because I can regularly check its spec, but more so to see that any wee "kinks" are duplicated. These offsets from a pure Xsquared Gm graph happen many times with many valves. As I've said before, a maths teacher would want a student to draw an averaged-out pure curve, but a test equipment technician wants to see kinks, warts & all. For DC testing, I'm constantly monitoring Vh & keep tweaking to exactly 6.3V, or whatever. Along with any tweaking of Va & Vs to exact book values. AVO VCM's, most now of pensionable age, can exhibit lowering AC Vh's across H+ & H- with valves that draw 1 to 2 to 3A of Ih. Yep, AVO Manual's calibration spec. might show a generous % of allowable Va, Vs & Vh, and indeed for example, show higher switch selection voltages off-load. But then, for many folk, the very nature of the RMS - ish values given by AVO, can be jolly confusing.
I was extremely lucky with my DC tester construction in having an old AVO MK1/2 as a donor & thus was able to use their valve holder panel complete with its 9 way thumbwheel switch. However, long 9 way/9 contact wafer switches,(Yaxley type), can be sourced & made up. RS used to supply construction kits. A lot better than fannying about with swapping banana plugs, leads, & sockets.

Regards, David

AdrianH 19th Jun 2020 10:31 am

Re: Yes, yet another home made valve tester.
David and others thanks for your help and comments.

The last change I will do to the tester before leaving it for a while and eventually sorting out a mains powered supply for it, will be to change the current meter toggle switch to a double throw centre off. Then I will be able to have 4, 40 and 80 mA ranges which I think will be enough for me. I have plans to rewire a transformer for the job and I have been following the 'High voltage regulator circuit.' thread by Diabolical Artificer for when I get to complete that side.

A long awaited parcel has been collected today, a Cold War TR1998, the intentions is to use for parts, holders, chokes, hardware etc, (sacrilege I know), but that is not to say I will not try to check it out first.
I would suspect like many old units the caps will be down by now. a couple of valves are missing/broken and the aluminium is corroded/oxidized in places, but should help me build up a useful box of parts.


David Simpson 19th Jun 2020 1:59 pm

Re: Yes, yet another home made valve tester.
1 Attachment(s)
Adrian, your initial Ia & Vg tabulations are just great. Smack in-between my DC tester's & my MK3's. See attachment. Kinks & all ! Perhaps there is a Forum guy or two, living near Blackburn, who has a valve tester which might benefit from a shot at the 6AU5. As its one of the last I had. In fact I suspect that this time next year, you'll be doling out your own DC St. valves to folk at Golborne.

Regards, David

AdrianH 19th Jun 2020 2:34 pm

Re: Yes, yet another home made valve tester.
OK thanks David. I was planning to chart a couple of graphs in the spreadsheet at some point, but need to see if the bucks converters will do the 80 mA, they should for a time.

I would say that providing the meters are accurate and that can be checked against a couple of DVM's then anyone could really build a DC tester if I could.

The valve could be passed around to others in the area, no problem, not aware to any in the vicinity., but sure there will be some around.


stuarth 25th Jun 2020 4:10 pm

Re: Yes, yet another home made valve tester.
Didn't AVO have an oscillation suppressing circuit where the wiring for each pin number (eg all the pin 1s to each valve holder) were wired in a complete ring, with a 100~200R resistor somewhere in the ring?

I believe the intention was that for the DC or low frequency AC used for the test, each valve holder had a low impedance connection from the feed point, either clockwise or anticlockwise round the ring depending on where the resistor was, but high frequencies would see the ring as an inductor with a nice energy absorbing resistor in the circuit to damp any tendency for oscillations to build.

Don't know how effective it was, or even whether if found its way into any of their VCMs.


AdrianH 25th Jun 2020 4:30 pm

Re: Yes, yet another home made valve tester.
Hello Stuart.

I think Petedox eluded to that in post 15 as mentioned in one of the utracer manuals.

That is basically what I did in the end, but with lots and lots of ferrite beads. But due to the fact I went back and started from the 9 pin main socket to the B9a socket, the 7 pin is a spur from this otherwise the Noval B9A, B8A and octal sockets all form a ring.


stuarth 25th Jun 2020 5:56 pm

Re: Yes, yet another home made valve tester.
The circuit in post 15 shows a ring with 2 inductors. To avoid excessive voltage drop, those inductors must have a low resistance. Adding essentially lossless reactive components such as inductors or capacitors does not necessarily stop oscillations, what you need to do is absorb the unwanted HF. Hence the (single) resistor in the loop. Any valveholder has a low impedance LF connection through one side of the loop, but HF sees the whole loop including the damping resistor.

As I said, I don't know if they used it, or how effective it might be.

You might get a similar effect by using a capacitor with a 200R damping resistor (you
can buy a capacitor + resistor as a single component for such things a switch click suppression) to decouple each pin, but if the resistor in the loop works, it would obviously be cheaper, you wouldn't need all those high voltage capacitors.


g4wim_tim 27th Jun 2020 8:26 pm

Re: Yes, yet another home made valve tester.
Interesting design for a valve tester in the latest issue of Signal from VMARS.

Page 17, Uses an AC signal and relies on the 180 degree phase shift through the valve to measure gm directly.

AdrianH 27th Jun 2020 8:36 pm

Re: Yes, yet another home made valve tester.
Hi Tim,
VMARS is one I am not subscribed to.
BVWS and the valve Museum are enough for now. The tester is basically done for me, the main interest was plotting or verifying the grid anode current curves and a bit of a learning exercise.

I managed to verify it against Davids Simpsons test valve, so think it is good enough for what I require it for at this stage.



AdrianH 30th Jun 2020 8:09 pm

Re: Yes, yet another home made valve tester.
1 Attachment(s)
Today I managed to get a switch that had three positions so I could do 4, 40 and 80 mA ranges.

So I used David's standard valve again and started to plot going up to 80 mA range, but after 45mA of anode current the valve would break into oscillation again, so a few hours have been spent trying to figure out why.

Then on a whim I removed the 0.047uF caps I had on the Octal pins, so now only the 1nF caps on the Banana sockets and the ferrites on the leads of course. Instant karma, no signs of instability over the full current meter range.

So for reference I did a full run entered the figures in the spreadsheet and provided plots, the valve has been getting a few hours work today, can emissions improve slightly with use?

Now to test some of my valves!


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