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AdrianH 3rd Jun 2020 8:26 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 7:02 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 9:52 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 11:49 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 11:52 am

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

retailer 7th Jun 2020 2:17 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 7:58 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 10:54 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 11:02 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 11:59 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 12:59 pm

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 1:23 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 4:36 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 6:03 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 6:32 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 6:41 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 11:23 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 11:58 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 7:21 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 8:50 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.


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