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Old 8th Jul 2024, 6:34 pm   #1
6SN7WGTB
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Default The Valve Tester for the Person with multiple PSUs

I am fortunate enough to have acquired a number of high quality stabilised PSUs: 3 x 30V, 1 x 160V, 1 x 350V.

It occurred to me I could construct a simple valve tester, which I have.

The PSUs are the big 'get out of jail free' card, and the rest is basically a patch box.

I have:
1. A 'gas test' function which bypasses a 1M resistor in series with the control grid
2. A heater strap to cathode switch
3. A 'test' (spring loaded) and 'run' (locked) on the anode and screen feeds
4. A small AF transformer on the control grid line to allow audio testing
5. An option to either measure anode current, or on the parallel binding posts, to use an anode resistor and measure voltage instead using the same shrouded meter sockets.

It's probably easiest just to let you peruse the photographs - hopefully all will make sense. Happy to answer any questions.

One picture shows it in use testing an ECC82, with the Fluke measuring anode current.

It's a pretty basic design but seems to do what I was looking for.
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Old 8th Jul 2024, 6:35 pm   #2
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Default Re: The Valve Tester for the Person with multiple PSUs

As I could not add a sixth attachment, here is a set of curves for that valve.
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Old 8th Jul 2024, 6:41 pm   #3
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Default Re: The Valve Tester for the Person with multiple PSUs

Nice - both in terms of the underlying concept and the neat implementation.

What sort of anode-current can it do? [most of my valve-stuff which needs testing is for power-bottles where you need to test cathode-emission, and that means up to an Amp!]

Personally, I would be adding some ferrite beads on the wiring, because it is the ambition of every amplifier to become an oscillator and that can really mess-up your test readings.
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Old 8th Jul 2024, 7:36 pm   #4
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Default Re: The Valve Tester for the Person with multiple PSUs

Currently, umm pun intended, the 350V PSU is 0.2A only.

Interesting re beads, I have them on the AF section and I've put 1k? in the control and screen grid feeds.

Do you think I need anything else? Would welcome suggestions as I am no expert.
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Old 8th Jul 2024, 8:29 pm   #5
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Default Re: The Valve Tester for the Person with multiple PSUs

Nice job.
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Old 9th Jul 2024, 12:18 am   #6
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Default Re: The Valve Tester for the Person with multiple PSUs

AVO patented a design where they added ferrite beads to the wiring to minimize oscillations, now they had the wiring in loops but it will help even if you don't have loops, you can read patent GB735865A here: https://www.vintage-radio.net/forum/...ad.php?t=86837.

Other testers like the uTracer and Roetest have used ferrite beads to minimize oscillations so it is highly recommended.
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Old 9th Jul 2024, 8:15 am   #7
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Default Re: The Valve Tester for the Person with multiple PSUs

The risk comes when you're testing a valve with high-ish mutual conductance and a response that's still useful into the VHF region. These devices can give sufficient RF gain that the inductances of the wires and the coupling between them can create unintended RF oscillators. Unless you have a fast scope or a spectrum analyser nearby the only clue can be some odd readings on your meters (even plain old moving coil meters) The readings can be simply offset a bit and attributed to the valve's performance, or they could be affected enough to make a good valve look faulty. If you're lucky, they're off enough to look impossible and you smell a rat.

The AVOs have long lengths of wire connected to the valve pins via the selector switches, and also loops in the conections around the sockets. From an RF stability point of view, they are a disaster waiting to happen. No wonder they went for ferrite beads in later models.

Your layout is smaller, neater as a result, and you don't have large loops. You are less likely to run into stability problems, but having beads can be thought of as a wise precaution. It would have been an awful lot easier to fit them when doing that wiring, of course.

An added thought is that you have relatively long wires to your power supplies and to your DVMs. These to connect into your tester and straight to the valve electrodes, so they are available to have some fun with oscillations. I'd apply some decoupling capacitors where your binding posts exit the diecast box and maybe back them up with some beads. A series-shunt-series RF filter is a lot easier to get high rejection than just a shunt element. A series-shunt pair is almost as good.

DVMs and even power supplies can react oddly if they get some RF shoved into them.

The AVO EA113 is a gorgeous little electronic multimeter but it is intensely sensitive to any RF fields in the vicinity and doesn't even need to be connected to the RF to be upset. Power supply circuits using the common 723 regulator control chip are also known to be surprisingly sensitive to RF.

In a valve tester, you are trying to asses the gain of the device and biassing it up to the optimum point. If it has any gain at RF, then you're putting it into the best circumstances for oscillation.

It's a case of the obvious way to design a circuit is to design it to do all the things you want it to do. Less obviously, you also need to design it so it can't do all the things you don't want it to do. Ferrite beads and stopper resistors become sensible precautions and are easier to do right at the beginning.

David
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Old 9th Jul 2024, 11:09 am   #8
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Default Re: The Valve Tester for the Person with multiple PSUs

Very helpful, er, feedback. Seem to be stuck with the puns in this thread!

Regarding decoupling caps, what would be the best configuration? The binding posts are as you will have surmised, in series with the anode.

The diecast case is at the moment directly connected to the Cathode connection, and thus to mains earth via the incoming HT -ve (where the strap to earth happens - although this can be lifted).

I do have grid stoppers.
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Old 9th Jul 2024, 12:55 pm   #9
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Default Re: The Valve Tester for the Person with multiple PSUs

There are two advantages to what you've built. Questions about the anode current capability are pretty much irrelevant, you just change to a bigger external supply and the box itself needs no changes so long as any resistors and transformers carrying that current are OK for heat and saturation current.

The second advantage is that a DC tester is pretty easy to calibrate against a known good DMM.

I'd have a good 10 or 100nF RF capacitor to ground and have a bead on the wire from its hot end to the binding post and another bead on the wire to the valve. 3rd order filtering and RF breaks limiting circulating currents.

You may be testing audio valves, but its RF oscillation that you don't want to happen. So the style of construction while obviously having to be appropriate for the intended activity also has to be appropriate to what you want to avoid.

In the solid state world, device Ft has climbed to the point even for jelly-bean parts that there is no longer any DC or low frequency design work. Everything needs an RF guy or it honks at many MHz.

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Old 9th Jul 2024, 1:10 pm   #10
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Default Re: The Valve Tester for the Person with multiple PSUs

Thanks David, I will get about those modifications.

And yes re PSUs - I do have a isotran variac for which I built a plug on (as it has some shrouded output sockets) rectifier and smoothing unit. That would do 0-380V DC at up to 3A and internals are rated for that.

Meanwhile I have applied ferrites to the incoming bundle at the multi-pin socket, and one on each of heater and HT supplies at their PSU ends. May or may not be worthwhile but done.
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Old 9th Jul 2024, 9:44 pm   #11
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Default Re: The Valve Tester for the Person with multiple PSUs

Which particular beads did you use, given they have different RF properties?

(I'm planning on building a valve tester next, so will be doing the same)
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Old 10th Jul 2024, 8:11 am   #12
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Default Re: The Valve Tester for the Person with multiple PSUs

Just what I had to hand - TDK ones for the cable bundles and some ones from RS for the component leads.

The TDK ones I use all around the house on SMPS and LED light flexes.
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Old 10th Jul 2024, 8:23 am   #13
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Default Re: The Valve Tester for the Person with multiple PSUs

Great thanks for the pictures
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Old 10th Jul 2024, 1:15 pm   #14
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Default Re: The Valve Tester for the Person with multiple PSUs

Hi Richard - I don't know how conductive these ferrite beads might be at high voltage, but I wondered about the possibility of shorting out this connector when I saw the photo?

Cheers
Chris
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Old 10th Jul 2024, 1:32 pm   #15
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Default Re: The Valve Tester for the Person with multiple PSUs

Some beads are conductive,some aren’t. Use heat shrink

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Old 10th Jul 2024, 1:48 pm   #16
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Default Re: The Valve Tester for the Person with multiple PSUs

It's a very good spot, so thank you, but they do measure completely open/non-conductive, and are on small AF signal lines.

They are also secured with a dab of glue, and run over insulated wire.

Good to be diligent however - annoyed I didn't test them before use.
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