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Old 7th Aug 2020, 6:25 pm   #21
factory
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Default Re: Roberts Valve Tester

Quote:
Originally Posted by David Simpson View Post
I'm surprised, David, that HP with all their expertise & up-market equipment, never built a valve tester. Something that could've wiped away AVO VCM's & Hickoks.
Good on you for encouraging young-uns to take an interest in valve technology.

Regards, David
In quite a few HP service manuals for older test equipment, they advised to check for weak tubes/valves by substitution rather than using a "tube checker" and that the results they give can be misleading. This may explain way they never offered a valve tester themselves, though they must have had something in the factories to test & select for certain parameters, or maybe they got the valve manufacturers to do this for them?

Here is the paragraph from the HP 460 manual.

David
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Old 7th Aug 2020, 6:40 pm   #22
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Default Re: Roberts Valve Tester

Thanks for your reply, David. My only past experience with HP test equipment was back in the late '60's when the RAF started to replace their CT436's with HP180A's. Several passed over my workbench, mainly for mods, but some for repair Can't remember any valve problems, though.
The Roberts "in-situ testing" design is a novel one, which I hope to improve on by using a trusty AVO8 to initially test a particular valve's circuitry voltages within a radio's circuitry, then switch to a gm assessing circuitry for the valve itself.
I've now got my Roberts Hybrid's panels for metering & switching in place & ready for interconnecting wiring. Will update soon.

Regards, David
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Old 7th Aug 2020, 7:59 pm   #23
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Default Re: Roberts Valve Tester

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Originally Posted by factory View Post
though they must have had something in the factories to test & select for certain parameters, or maybe they got the valve manufacturers to do this for them?
They did! An AVO VCM IV in our plant. Just the one and it sat pretty much unused on a shelf in the test equipment maintenance department.

The production lines were making valved LF-UHF sig gens of several models in my time there. They didn't bother testing valves. If in doubt they tried a new one.

Where valves were selected for performance or enhanced reliability this was done at the valve manufacturer. Some were bought as mil-spec part numbers cited on the specification control drawing, but all were handled by HP part number.

Statistical process control was in use. Suppliers with poor records were avoided. Essentially, we hitched a ride on work done for the military.

David
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Old 8th Aug 2020, 11:46 am   #24
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Default Re: Roberts Valve Tester

Digressing slightly, but I recall some time ago there was a long running thread about CV Valve's specifications. When I worked in 103MU's test eq. section, I vaguely remember that the word down from our "mother" unit at Henlow, was that the CV Valves issued to us were of a higher trustier standard for straight forward substitution. We had our own CT160, as well as repairing/servicing 160's for other units in the middle east, but we didn't use it that much.
Back then, and now many years later, myself & many others would use a trusty AVO8 & probe away at valve pins, wrt to earth, or -ve, or chassis - to determine that a valve was receiving the correct voltages. I gather from Roberts' wee handbook that this was considered a time consuming procedure, & their inclusive method would reduce bench time or customer's house visit time. In fact, Taylor's extended "47" set-up has a multimeter facility. But Roberts & Taylors were just a rudimentary facility, and I'm hoping that a "built-in" AVO8, plus independent Vg monitoring will be much more reliable. We'll see. Much "Suck it & see" is involved. Also, its just a domestic radio portable testing set, so to speak. Definitely not for PA's & the like, with 807's, KT88's, TT22's, QQV06-40's, and so on.
Jesus, has anyone seen the pictures of Denis Tabor's massive old collection of valve testers ? My efforts & knowledge pale into insignificance.
The METRIX 61B & the Tektronix 570 - what machines they are !
Seeing that TEK brought out their 570, made me wonder why their main competitor - HP - didn't produce something ?
Hey, but nowadays, if someone has got very deep pockets, and whose mindset isn't stuck in 1960's analogue ideas, just cough-up mega euros & buy a RoeTest.



Regards, David
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Old 8th Aug 2020, 12:17 pm   #25
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Default Re: Roberts Valve Tester

The Eddystone 358 receiver had a built in emission indicator.

Lawrence.
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Old 8th Aug 2020, 3:13 pm   #26
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Default Re: Roberts Valve Tester

Right enough, Lawrence - that Eddystone 358 looks a really interesting vintage Rx, particularly its Ia metering. I've just got a trusty 730/4.
I've attached some progress(slow) pictures. To aid installation of wiring & future mods &/or fault-finding, I've followed the old resistor mnemonic BBRO---- colour coding. Plus have drawn a simple working diagram to follow with just Pin 1's wiring to follow(brown). First connections made are for "Valve Holders" & I've temp installed "clip-on" ferrites, but worry slightly about their hefty properties affecting RF circuitry when the whole caboodle is tried out on a domestic radio. I've plenty of normal wee beads, plus a big B9 adaptor full of wee beads, if I need an alternative.
Along with the AVO8/6 there are four separate screw-down panels, utilising modern terminal strips instead of hard wiring - again for any mods needed or fault finding. This is, after all, a "suck it & see" "as you go" project, with, no doubt, glitches on the way. I've also taken A4 colour photographs of items as they are fitted, then inked in circuitry numbers & descriptions etc.

Regards, David
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Old 25th Aug 2020, 2:58 pm   #27
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Default Re: Roberts Valve Tester

Quick update. Switch gear all fitted & tested. Bit of circuitry(Delta Vg) still to be finalised. Annoying hiccup - had planned the installation of an AVO8/6 as the multimeter provision, as its nice & light, but discovered that its phooked. As is the standby 8/6. Might attempt stripping out both & getting one functioning, but their double sided layer-cake of pcb's seriously puts me off. Got three earlier AVO 8 versions, and have plumbed for a lovely old but accurate 8/1 as a replacement meantime.

Regards, David
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Old 18th Oct 2020, 11:47 am   #28
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Default Re: Roberts Valve Tester

At last, project is finished, (well almost - might do some mods later on).
Hey - it jolly well works. It relies greatly on the incorporation of an AVO 8 in a re-purposed mode. I've drawn a simplified block diagram & an (easy to follow, I hope), circuit diagram. No way was I going to hand-draw an AVO VCM - like, over - complicated diagram. What I've shown for pin 1's circuitry (Brown) stands for all 9 pins plus Top Cap(pink) colour coded wiring. Construction wiring & looming etc. was made much much easier by using the Resistor Mnemonic.
With what keen homebrew folk have stored in boxes or on shelves - a variant of this could easily be built. Its not rocket science.
Helluva job typing & printing off written report - MS Word didn't like my sequencing of numbered paragraphs, & kept auto-altering my technical terms & paragraph layout.
Many thanks to "captainpugwash" & "pamphonica" (aka David & Jeremy) for help with replacing my long-lost set of Rotring pens, stencils, etc. Much needed to be re-learnt, drawing skills - wise, though.

Regards, David
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Old 18th Oct 2020, 11:52 am   #29
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Default Re: Roberts Valve Tester

More paperwork, Dave
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Old 18th Oct 2020, 12:00 pm   #30
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Even more, Dave
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Old 18th Oct 2020, 1:01 pm   #31
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Default Re: Roberts Valve Tester

Thanks for sharing the project Dave, you are an inspiration to others like myself who like to re-purpose vintage test equipment.
Time I shared my mods to the Mullard High Speed Valve tester.......

Peter
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Old 26th Oct 2020, 6:11 pm   #32
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Default Re: Roberts Valve Tester

Looks brill/fun but kind of complex - though the idea of the original tester was supposed to be that it was simple??
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Old 27th Oct 2020, 8:26 am   #33
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Default Re: Roberts Valve Tester

I, and doubtles others, am very impressed by how you have persevered and what you achieved with this.

Well done

Ian
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Old 27th Oct 2020, 9:40 am   #34
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Default Re: Roberts Valve Tester

I've been watching this project with some interest, and wondering whether the principle of the unit is correct or not. How useful is this approach to valve testing? The judgement of history appears to be "marginal" given that the unit is rare, and competitors didn't rush to emulate the technique (so I understand).

I've always used David's technique: "......use a trusty AVO8 & probe away at valve pins, wrt to earth, or -ve.....", which does of course establish pretty quickly what those in-circuit voltages are. I've often found it then useful to repeat the exercise with the valve out of circuit, so I can get an idea whether decouplers (anode and screen) are leaky. Then of course again repeat the exercise with the set switched off and checking for resistor values.

With vintage equipment (that's all valve equipment just about) my experience is that its unusual for the valve to be faulty - or so out of spec its actually producing strange voltages on the pins. What does cause that is nearly always duff Rs and Cs. And valve testers (all of them) are somewhat irrelevant in that instance.

Finally even this tester won't cope with all circuits. Definitely not for valves in PA circuits like 807s etc as David mentioned above. But also definitely not for most RF oscillators in receivers. The strays (and ferrite beads) from all the wiring is going to put any such circuit into some very different state to normal - probably stop it working altogether in fact. And the DC conditions will then be way off, since an oscillator normally has to alter those conditions to get the loop gain down to 1. To be fair, it can be difficult to get the DC conditions for any RF oscillator - since probing them with an AVO, DVM or whatever usually upsets the circuit to some degree.


Richard

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Old 27th Oct 2020, 10:41 am   #35
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Default Re: Roberts Valve Tester

In designing a piece of circuitry, you have to take into account spreads in new device characteristics and ageing.

The former shows up when you get it wrong as poor turn-on yields in manufacturing, or in higher than necessary product cost if you go too far the other way.

Ageing allowance sets how long your product is going to be good for.

Valves have remarkably small spreads of parameters when new, but have slow ageing mechanisms. Transistors have scarily wide spreads of parameters when new and aren't supposed to have wearout mechanisms (With extreme devices running high J {=current density} you get dopant migration)

So a valve tester can give you a good indication of where a valve is on its ageing curve. The rate of progress along that curve depends on how hard it's being run, so the time scale is a bit iffy.

What the valve tester can't do is tell you 'Will this radio work when I shove this valve in it?' Because that answer depends so much on the margins derigned into the set.

We have cases of very old bottles with very low emission still fighting away in signal amplifying stages, and then we have things like the ECC85 in FM receivers that needs to be really on top of its game in order to work. The difference here is not in the valves, it is in the circuits they get used in.

As Richard says above, oscillators have a sharp cutoff in terms of device gain and whether they start up at all. If I waved a magic wand at a radio and aged its valves uniformly. The gains would drop, the emissions would drop. The set would get quieter and the available power would drop and this would keep on progressively until there would be an abrupt stop when the local oscillator stopped.

Oscillators are seriously demanding on their devices.

Something people rarely think about is oscillator amplitude. Having to get a loop gain of absolutely precisely unity can only be done by having some form of level sensing gain variation to make it have a stable operating level. Maybe device non-linearity, maybe the device going into cut-off for a variable fraction of the cycle, maybe even a detector and full-blooded AGC loop. They all exist. But in order for these things to work significantly, there has to be a large enough amount of signal.

For this reason, you won't find oscillators running with low levels.... we just can't design or make them. You find oscillators running at quite meaty signal levels with attenuators stuck in after them whenever small signals are needed.

Tons has been written on oscillator frequency control, but damn all on amplitude control.

Anyway, this is why valve testers are not essential items for radio-fixing people. They don't really tell us what we want to know. Amongst some audio people they have become iconic, they probably meet the dictionary definition of 'fetish'.

If you're interested in valve functions and conditions, then doing what David Simpson has done, building a rig where you can measure a device and plot its characteristic curves is the way to go. Getting a single number or pass/fail or "strong!" is very disappointing when the real answer is an entire graph.

This Roberts tester is a sort of halfway house. A quick way with adaptors to measure a valve's operating condition as used in the target application.

As you say, they didn't really catch on. They would be uncomfortably big for the travelling repairman. For use in a shop, they sort of relied on you having an example of the set handy that had no other known faults... It just didn't fit. I picked one up for next to nothing at an auction and saw it as a box with a good selection of different valve holders. I thought "Shame to see this go in a skip, I know who'll enjoy it" so I got it up to him on my next foray Northwards, and as a result of a thread on here, he's acquired a second one. Good entertainment at a time when we all need entertaining.

David
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Old 27th Oct 2020, 11:04 am   #36
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Default Re: Roberts Valve Tester

David,

yes, all interesting points, as always!

I will just respond to your comment about oscillators (and I know its probably off topic, but you can always move this if you think it appropriate), where you said:

"Having to get a loop gain of absolutely precisely unity can only be done by having some form of level sensing gain variation to make it have a stable operating level."

Yes, I've heard of oscillators where a control loop actually controls loop gain to become unity. I wasn't referring to those - my understanding of all oscillators is that they automatically have a loop gain of one when stable. If they didn't then the output amplitude would simply go on increasing ad infinitum, and obviously something gives before that happens - running out of rail voltage, breakdown of components - or in the usual case, the active device (valve, transistor, whatever) is driven into non-linearity so that its gain falls to the point where the overall loop gain (including losses in feedback components) falls to one.

Your later point where you said "But in order for these things to work significantly, there has to be a large enough amount of signal." is of course absolutely correct. There has to be enough small signal gain - and the correct phase shift - around the loop within the oscillator for the thing to start up at all. After all, the oscillator only starts because it is amplifying circuit noise, and that signal builds and builds......until we enter the non-linearity required to stabilise the output.

And if we then get back to this particular valve testing scheme, we introduce long leads and ferrites into an oscillator. The effect is going to be hard to predict of course. If it oscillates at all - it won't be at the design frequency due to a myriad of strays. The ferrites might well damp down the intended oscillations - which is of course their intended purpose in all other non-oscillating circuits.

Richard
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Old 27th Oct 2020, 12:20 pm   #37
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Thanks guys for excellent responses. If anything, that was my intention - to get folk to think outside the box regarding valve testing & the predominant use of AVO VCM (AC functioning) principals.
Its no secret that I believe that DC testing of valves is the most accurate. However, I'm also encouraged by the fact that many VCM folk can achieve very close Gm curves when their valve testers are well calibrated. Then on the other hand, a number of folk suggest that Testers/VCM's are an expensive fad, and the most reliable testing can be done when a valve is in it's host chassis.
As I've said several times in this thread, and I agree with what Richard says, Roberts( or actually "London Sound Labs") ideas were flawed & obviously the other established Valve Tester/VCM manufacturers realised that their equipment wern't under threat. Hence LSL Ltd "went under" within 10 years. However, still, the principal of initial circuitry testing for faults & the subsequent testing of the associated valve - without removing the chassis from the cabinet, is a novel one. So, me thought it was a worthwhile project to see how far I could get with it. I'll continue to test other valves & their circuitry, and might well remove the ferrite beading if I discover frequency/stability issues with RF/LO/Mixer circuitry. Another step I'll take is to draw up some comparative Gm Graphs against results from my MK3 & DC tester.
Yep, complicated switching, and I'm still thinking of using my spare AVO thumbwheel switch, &/or some use of relays to reduce the duplication of "WRT" switching in particular. Mind you, has anyone seen pictures of Denis Tabor's 100+ valve tester collection ? Some of those beasts(three times the size of the V-T20 & probably 10 times the weight!) have a much more comlexity of switching & metering.

Regards, David
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Old 27th Oct 2020, 5:28 pm   #38
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Regarding para 3 above. To those of us with a "bent" towards valve testing & valve testers, I would recommend a shufti at the past article (in the Bulletin, I think, back in the 90's ?), which someone brought to my attention a couple of years back, by Denis Tabor on his huge collection of valve testers( now dispersed). This V-T 20 of mine(only 7Kg's & approx 24" x 12" x 5.5") pales into insignificance weight/size - wise, compared to the likes of the Everett Edgcumbe from the late 30's, the Metrix 61B from the 1960's, & the Weston 686 from about the same time - both with about twice the number of meters & switches, and no doubt 10 times the weight.
Once the Covid19 restrictions are past, plans are afoot to loan this tester to someone with a large collection of domestic radios - for evaluation. To that end, I've a "wanted" thread post out for a 2nd hand suitably robust leather suitcase or padded aluminium instrument case, an inch or two bigger all round. For safe transportation.
Its weight would be even less if, as originally planned, I could've fitted an AVO MK8/6 or 7, but both the 8/6's I have are more or less duff. So a good old(but heavy) 8/1 is doing sterling duty.
What I want from this long banging-on thread is not plaudits & praise, but to encourage other homebrew enthusiasts to delve into valve tester projects & re-purposing the likes of AVO 8's, or Fluke DMM's, even a clapped out Taylor 47 cabinet, or whatever. As much as I like AVO VCM's & CT160's, they are, lets face it, hellish expensive to acquire &/or repair. The Sussex is a great idea, and there are some jolly good Curve Tracer projects on the go out there. All I've done is take a couple of clapped out late 1940's items & cheaply made something that seems to work jolly well in the 2020's.

Regards, David
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Old 27th Oct 2020, 6:05 pm   #39
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Default Re: Roberts Valve Tester

In order to oscillate, a circuit has to be unstable, by definition.

In order for that oscillation to continue at constant amplitude, then that instability has to be at a stable level. This is inevitably funny. Douglas Adams wrote: "We demand rigidly defined areas of doubt and uncertainty!" shouted by a militant philosopher.

You need a little more gain than unity for an oscillation to build up - you can view it as circuit noise being recirculated, getting amplified and filtered with a little bit more added on each trip round the circuit. Left alone, this circuit will hit the supply rails.

The critical part of the design is to make the circuit gaim be reduced by larger signal than you want. In this way you establish a zone of self-maintaining stable amplitude. If the signal grows too much, the gain reduces and the loop becomes less than self-sustaining so the amplitude falls. If the amplitude falls too much, the gain creeps up.

The gain reduction can be achieved in a minimalist circuit by either having the active device drive itself into cutoff for an increasing fraction of a cycle, or by having the device drive itself into non-linear compression.

I wrote an LTSpice simulation of an 'unrolled' oscillator where instead of going round and round the same circuit, a signal propagated along a string of hundreds of the circuits (with the loop broken to give input and output)

To make it work, I added a small noise generator to each stage, and I used a non-linear model so that you can probe down the length of the circuit and see noise get processed into something starting to look sinusoidal and the amplitude increasing to a level set by the non-linearity and then getting no further. Essentially you get a snapshot of the earliest nanoseconds of a normal oscillator played out in stationary time in the first several stages.

It's also a demo of just how powerful our home computers now are. We've lost sight of this as web browsers, word processors etc have become so monumentally inefficient that they've eaten up all the new processing power as it came along.

The clown in the back of my head would like to point out that when running the sim, it's all seuenced by an oscillator on the motherboard......

David
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Old 27th Oct 2020, 7:10 pm   #40
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Default Re: Roberts Valve Tester

As can be seen in Post 30, on page 7 of my report - the old Roberts VT's were soley distributed by Kerrys - who were a big noise in Radio/TV servicing back in the day. The idea being, in the post-war years, that the "man in a van" would pop out to Mrs Smith or Mr Jones, following a report of a radio fault, and take along his "Roberts" to do a quick test. As opposed to the time consuming removal of the chassis from the cabinet & delving inside with his ancient AVO 7 or old Taylor MM. Quite frankly, I dont think the repair mannie was much bothered if his Roberts upset the RF circuitry by introducing "third quadrant" instability. Having a clamshell tester must've looked the bee's knees to the householder, thus they coughed up the LSD for new valves easy enough. Much like going into a radio shop & seeing "the engineer" wolloping the Mullard HSVT's handle up & down.
From our prospective now in the 21st century - a bit of gimmickry.
However, since it was David (RW) who gave me the 1st Roberts last year, I'll be dilligent when I get more time to do more testing in the winter months with the likes of triode hexodes, or even the one or two octodes I have somewhere. Might even think of fitting a couple of BNC connectors so that 'scope monitoring of signals can take place.

Regards, David
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