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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.


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