Valve Tester Prices?

[GrimJosef]

Originally Posted by David Simpson

... Just quoting "AVO Tested" and some figure for Gm doesn't fully relate to an o/p valve's broad working capabilities ...

This is very true. Quite often the best hum performance in a push-pull audio amp is delivered by matching the valves for Ia, so the action of the output transformer nulls out the 100Hz ripple on the HT rail. More than once I've had customers complain of hum despite having bought 'AVO matched' valves from a retailer. They're surprised to discover that two valves which happen to have the same Ia at the AVO test point (say, Va=Vs=250V, Vg =-15V for the KT66) don't have the same Ia at the amp's actual quiescent operating point (say Va=Vs=400V, Vg=-37.5V for the Leak TL/12 Point One).

I'm a big fan of DC testing. The main drawback with it is that at higher values of Ia the increased anode dissipation changes the temperature of all of the electrodes inside the valve. This, in turn, changes Ia. If we are actually operating with quasi-static Ia, e.g. as in a DC servo controller, then the operating conditions will require us to take this into account and it's important that we know about it. But if we are running an audio amp, where the signal is oscillatory on a timescale much faster than the thermal equilibriation time of the metalwork then, in the small-signal (Class A) regime at least, the metalwork temperature won't change as we drive Vg up and down and the DC measured gm may therefore not tell us accurately what the audio operational gm will be.

Perhaps the best summary is to say a) the real world is usually more complex than any simplified model imposed on us by a choice of tester technology and b) in the end the best way to test a valve for use in a particular piece of equipment is to see how it works in that particular piece of equipment.

Cheers,

GJ

[Radio Wrangler]

Indeed! most power devices are tested with quick bursts of high-dissipation conditions in order to measure their characteristics towards rated voltages and currents.

There are different intents in testing a device

1) Does it have any obvious show-stopper defects (Open heater, gone to air, gate oxide puncture....)?

2) Does it comply with its spec for gain and transfer function/ bias condition?

3) Does it exhibit the specified transfer function at operational voltages and currents?

4) Can it reach and withstand its rated peak voltage and current?

5) Does it meet specs for endurance at high power?

Classic valve testers do the first two of these.

The user of a piece of equipment using that device wants the lot!

Endurance tests only make sense on a sample of production. Life-tested ones are as desirable as a box of tested matches.

The whole valve testing malarky is pointless unless you have a clear idea of your purpose. And almost all testers are far inferior to the intended application.

One R&D engineer at HP went to the manager of the QA department holding a single diode in his hand and asked him if he could measure its MTBF (it wasn't me or anyone else seen on here...)

David

[ukcol]

Originally Posted by Radio Wrangler

............(it wasn't me or anyone else seen on here...)

Thank goodness for that.

[ukcol]

On the subject of prices, a Mullard HSVT went cheaply at Harpenden on Sunday. I wasn't paying particular attention but I think it went for £50. It was described as "working" and as a "late model complete with 2 boxes of cards" (presumably an E7600/4).

I am a fan of the Mullard HSVT in spite of their limitations. I have one myself and they are very useful for weeding out rubbish from a "tray of valves".

[David Simpson]

What I've done with my DC Tester rig(which normally is just used for the static tests) is an add-on dynamic testing rig which can test for Gain G under just Class A conditions. At its heart is a thumping big ex Lab Decade Resistance Box which can provide Rl for quite a few low & medium power valves. But not for the big thumpers, KT88's, EL37's, etc., sadly. Provision is made for monitoring i/p & o/p AF signals on a scope,(distortion, ripple, etc.), and for rms voltage measurement. A lot of fannying about to set it up, but I reckon to get within 5% of Illiffe or Barnards book's tabulations.
For the big valves, your idea, GJ, would be best - using a possibly redundant big amp chassis.

Regards, David

[GrimJosef]

I think one possibility might be to put an output transformer into the valve's anode circuit, but 'the wrong way round' so to speak i.e. with the low impedance winding connected between anode and HT and the high impedance one with the AC monitor attached. This way the load impedance on the valve would be negligible, so it really would be gm (at constant Vak) that would be measured, yet the output signal would be large enough for easy, relatively low-noise measurement. The monitoring gear wouldn't have to be floated at the HT voltage either. I suspect the arrangement would need to be calibrated though.

Cheers,

GJ

[David Simpson]

Just go for it GJ. This could be a new venture for you, amp-wise. A new life for an otherwise redundant amp chassis. You could flog them, reasonably cheaply, hopefully, to aspiring valve testing enthusiasts like Chris, who might baulk at the horrendous prices being asked for well known testers. Those enthusiasts, I'm sure, could fit extra switchable valve holders, and a panel of DC meters for Va, Vs, Ia, Vg, & so on. I keep repeating(a symptom of being over 3 score years & ten) that all the extra gubbins could be sourced from the multitude of swapmeets, auctions, ARS junk sales, etc. which proliferate dahn sarf.
Jesus, I even know of one vintage chum who has just recently wombled out of a council skip - two complete audio amp chassis'. They even had EL34's in them still !

Regards, David

[GrimJosef]

Originally Posted by David Simpson

... This could be a new venture for you, amp-wise. A new life for an otherwise redundant amp chassis. You could flog them, reasonably cheaply, hopefully, to aspiring valve testing enthusiasts like Chris ...

Nice thought, but if I flogged them I'd have to CE mark them first (there's a law about that) which rather puts the kibosh on anything 'reasonably cheap' also being profitable .

Cheers,

GJ

[David Simpson]

If a working Mullard HSVT went for £50 at Harpenden then it's the bargain of the century. I hope it went to Peter(aka Electronpusher0), or someone who is keen to follow his fine example of modifying one to read Gm. As per his current thread.

Regards, David

[Herald1360]

Originally Posted by ukcol

Thank goodness for that.

Alternatively the engineer could just have had a suitably warped sense of humour.......

Set of long waits kind of thing.

[Herald1360]

Originally Posted by GrimJosef

Nice thought, but if I flogged them I'd have to CE mark them first (there's a law about that) which rather puts the kibosh on anything 'reasonably cheap' also being profitable .

Cheers,

GJ

Do kits of bits need to be CE marked?

[GrimJosef]

I'm not sure that the status of kits as 'electrical equipment' has ever been tested in court. I'm certainly aware of electronic kits (that's a kit of parts to build a particular item, not a multi-purpose kit) which have been sold without CE marking.

Cheers,

GJ

[dseymo1]

I don't see how kits could be CE marked, as compliance would depend upon the standard of construction.

[GMB]

Kits can't be CE marked per se, but it is not as simple as that.

I think there may be an expectation that a kit could be assembled to meet the relevant regulations, so a kit supplier might need to be able to show that he at least built the product and the result does comply.

Individual parts are generally not CE markable unless they really do have a life of their own. Also a kit that is not complete may be in a better position, e.g. a PCB and a parts list to source by yourself - and especially if there is a whole lot of discretion for the builder, i.e. that he must design some of it himself (more than just a box).

Some part-like devices have CE marks in spite of being sub-assemblies because if they didn't then no serious users would buy them, and sometimes bits of the CE declaration can be up-cycled, e.g. with deliberate radiators which are rather hard to do otherwise.

But at the end of the day CE marking is a self certified process and if someone only made a few kits there is no chance of a problem unless it was an overt safety issue. But having voltages more than about 50V is going in to deep regulatory waters (water and electricity are always dangerous together )

[David Simpson]

GJ, I wasn't thinking of fully CE compliant SOTA Amp chassis'. Just something in half decent nick which has been on your junk shelf for ages, and would lend itself to someone else tarting it up & modifying it for valve testing. Or an old redundant amp which someone has traded-in for one of your new bespoke ones.
I'm not into the Amp scene, but my son found a CRATE GT80 on Gumtree for £20 a while ago. (Dad, Dad, Dad - can you get it going !) That's just a hybrid from 25 years ago & not suitable for a valve tester project. But I'm sure there must be heaps of old valve amps lying around somewhere. So,£20, £40, £60 or whatever ? I still reckon that £200 tops for all the bit & bobs of 2nd hand parts & equipment to build a DC Tester.

Regards, David

[GrimJosef]

Just to be clear David, I don't actually manufacture any amps, I just (try to ) mend broken ones. So all the junk on my shelves is my own and I'm hoping it will all end up working just as soon as I've obtained the necessary round tuits .

If you're sure that there are heaps of old valve amps lying around somewhere then we need to talk. I'd really like to know where that somewhere is ! For the last couple of decades or so demand has exceeded supply for old valve amps and even tatty old rubbish has sold online for a pretty penny. You might build yourself a tester from scratch for a fair bit less.

Cheers,

GJ