New DC Valve Tester Project

[David Simpson]

Having thought over the festive season about Pat Pending's venture into sorting out good & bad valves amongst a massive amount of valves. Also my venture into a few hundred earlier last year - have decided to build a simple DC Tester to just eliminate the duff ones on the grounds of u/s heaters &/or low insulation between electrodes. This will save a lot of fannying about repetitively going through the formal procedure with my Mk3 or my big DC Tester, valve by valve.
Something that could test say 4 valves at a time in the more common valve holders :- B7G, B9A, IO, MO, B8A, etc. Just recently acquired the valve holder unit(with Thumbwheel Sw.) from a AVO 2 Panel, I've the bottom Clamshell of a CT446, a spare 444uA Meter out of a Mk2, a Tinsley Res. Bridge with a heap of 0 - 10 rotary switches, a wee digital Metrotest Megger, several new & spare valve holders, and so on.
Ia, mA/V, etc. tests, & valves with B4/B5 bases, UX bases, big BT's & Pentodes, etc. - I'll continue to use the Mk3 & the LeyMarcAvo. But as many know, by just looking at the AVO Thumbwheel Switch test codes in the AVO Valve Manual - many, many common user valves use the same code, or maybe vary by just one digit in the sequence.
The idea is rummaging around in my head just now, haven't got down to drawing a possible circuit diagram, just a rough block-schem. diagram so far. But surely, some, if not all, the above bits & bobs lend themselves to building something useful. I'm a bit reluctant to strip out the Tinsley Res. Bridge, as its in good nick. But have tried to sell it - no takers. The CT446 Chassis I've kept for spares for my working Tr. Tester. The AVO VCM Meter is out of the Mk2 I stripped out a couple of years back.
I'm all for re-using &/or hybridizing old test equipment for use in a new Homebrew venture. Valve testing lends itself to this. Valves aren't getting any cheaper, cost of manufactured VCM's & Testers is becoming prohibitive, as is particularly AVO VCM spares.
Any advice, or criticism, most welcome. I might be soon on the look-out for spare Mazda Octal & B8A valve holders.

Regards, David

[Diabolical Artificer]

Looks like you have the bits to make a decent tester. Having built one myself I'd advise to keep it simple as possible. I'd build either a variable 4A heater voltage or current source as it saves faffing about with lots of wires and tfmr taps. Two 250 - 300mA ish variable 0-300v ish for HT and screen and a simple short tester using a neon and a few discreet components. I'd use cheap digital voltage/current meters instead of MM type meters. You van get them that do both for about a fiver. I prefer to use a patch panel to switches using banana jacks and plugs, again saves on wiring.

If you want a tfmr making let me know. Andy.

[David Simpson]

Thanks Andy for your post. I don't envisage requiring any Va,Vs, Vg power supplies for this simple wee project. As I've already built, a while ago(see my LeyMarcAvo Thread), a whopping big DC Valve Tester(LeyMarcAvo) which will sing & dance up to 500V HT & 150 odd mA Ia when used with its equally big(both built inside empty Marconi TF995A cabinets) external DC PSU(which uses the HT & Heater Transformers out of a VCM Mk2 for HV & MV supplies). Its heater voltages are from a modern multi-channel DC bench PSU.
This project is just really a speedy DC heater continuity/electrode insulation tester, which my big DC tester doesn't do. My DC tester tests under both Static & Dynamic(Signal) conditions, & lends itself to graphing tabulations.
AVO VCMs, like my Mk3, initially test for simple continuity & insulation, before going on to Ia & Gm(under AC 50Hz conditions), etc.
As I said initially, I envisage this project coming in handy should I, or one of my nearby vintage pals, end up with another big box of mixed up valves which need the u/s dross removing speedily. Its time wasting enough sorting through valves dumped in the wrong boxes. Then you get valves with some of the lettering/numbering missing. Some are just downright filthy(dust, cobwebs, nicotine, etc.).

Regards, David

[Diabolical Artificer]

Got you David, so you want something like the Heathkit CC1 valve checker but with multiples of each valve base type? That would mean at least 8 top caps as well. If you could design an intelligent checker with a software program even better, that tell you the mystery valves. That's a lot of work and fiendishly complicated.

Andy.

[David Simpson]

I'm just going to keep things simple, Andy. No fancy programming for unsophisticated me. Just want to use spare items at hand, mostly analogue. If I build something that quickly eliminates duff valves, it'll release time to carry out full testing on my Mk3 or DC tester of the more promising valves out of a large batch.

Regards, David

[Pat Pending]

I've sort of been overtaken by events, with family concerns etc, but my pal Don't has taken over the mantle as it were. I was pleased however that he made up my simple idea for a few valve holders connected to a "patch" panel that could be configured to connect alternate electrodes to an continuity meter whilst running the heater from his bench PSU. Just plug in, leave for a minute or two to warm through to check. It's weeded out quite a lot of "hot short valves" as well as loads with duff heaters. As an aside, he is also noting down the corolation between the mA/V readings (on my valve tester), and the space charge current that I was observing as per my topic. He is going to do a spreadsheet on the "audiophool" types to see if there's any obvious relationship.

Regards. Andy.

[David Simpson]

Andy, looks like Don has done some pioneering work, duff valve elimination-wise.
Me - an empty Marconi TF995 cabinet just arrived this morning. So homebrew work can commence, The Tinsley Bridge can be reprieved. Shall now start to chose an component layout which is "ergonomic"(I think that's the term). The AVO 2 Panel valve holder unit will certainly fit on top of the cabinet, but I might place the thumbwheel switch assembly elsewhere.

Regards, David

[David Simpson]

Well, a month has passed. Improving weather has meant more outside jobs. But, in-between those, have managed to do some preparation work on the TF995 cabinet and the ex AVO 2Panel valve holder panel. Additional valve holders are fitted into a handy small sheet of 3/16 paxolin.
Cabinet now sprayed to look RAF/Marconi Grey - ish. Valve holder Panels painted black.
Note - (see picture),I keep being forced to gobble up Ferraro Rocher chocolates just to get to use those great handy wee clear plastic boxes.

Regards, David

[Diabolical Artificer]

Good to see your cracking on David, plenty of space inside too. Have you shined the valve holders up? That's attention to detail.

Re Ferrero Roche chocs; I find 1L ice scream tubs usefull for storing resistors (1K2-1K8 etc, easier to root around for value X than the coffee jars I used to use) but I have an ice cream monster to help devour the stuff.

Keep up the good work, Andy.

[David Simpson]

Thanks Andy,
Valve holder panels, old & new now made up & fit fine on top of the cabinet. The new panel holds the following extra valve holders :-
3 x B9A, 3 x B7G, 3 x IO, 2 x MO, 2 x B8B, 2 x B8A. Plus the moved over B9G. I just had to buy the two B8B American Loctals, and the three new IO's were kindly donated by local vintage radio chums, along with some of the other decent 2nd hand ones.
The picture shows my three TF995 Cabinet derived babies. (Lower rear = DC PSU, & on top - the existing DC Valve Tester). Yes, they take up some space, but they are lovely strong aluminium cabinets, and have plenty of spare space internally for future alterations/modifications &/or fault finding.

Regards, David

[Pat Pending]

A fine job you're doing there David. We only had b9g, b7g, octal and Mazda to contend with! Don and myself have come to some conclusions about measuring space charge versus valve health, they did show that my original assumptions were correct, and that near the end of its life the cathode has an almost linear decline into decrepitude, however, as the "as new" measured space charge current varys between examples of the same types, let alone different types, I can't see any convenient way of using it in a practical test.
Regards. Andy

[David Simpson]

Don't know if its much help with your research, Andy, but on a number of occasions with 2nd hand valves when I've been drawing up Ia/Vg graphs - I've noticed wee kinks in the lower Ia regions.

Regards, David

[Pat Pending]

Thanks, although interesting our observations just kind of proved that you need some kind of dynamic test to be sure of quality in second hand valves. There is no such thing as a free lunch!

Regards. Andy.

[Diabolical Artificer]

Can't beat a roomy enclosure David, nothing worse than burning wires with your soldering iron in a packed device. That look "right" too. Nice job. Just been working on a Marconi distortion analyser, loads of room inside and well laid out.

By a dynamic test, do you mean gm Andy? Or something different?

Andy.

[Pat Pending]

As I said in previous posts, we were just trying to gauge cathode health directly from the current that the space charge could drive between cathode and grid, we were hoping to be able to set up a cheap, quick, plug in test based upon the idea, we failed!. It's a text book phenomenon I'd read about years ago, and forgot about, (its something for the designer of valve gear to consider more than a "repairer", like me), but its been very interesting for me to the point of my starting to construct a couple of simple homebrew valve radios.

Regards. Andy.

[David Simpson]

Right enough, Andy, the space charge phenomenon is too variable amongst valves for it to offer any reliable indication of a cathode's health. But any investigation into how valves function, is something I support.
My tester project is on hold just now. I've finished work on a Mullard HSVT, then a lovely old medical induction coil,(complete with a kit of probes & prods. I dread to think which parts of one's anatomy they are used on). Now started on an old battery eliminator. Then our washing machine sprung a leak yesterday.
Hopefully, I'll return to this project soon.

Regards, David

[Skywave]

Originally Posted by Dr Wobble

Can't beat a roomy enclosure David. Nothing worse than burning wires with your soldering iron in a packed device.

Too right! Anyone who has worked on the insides of an Eddystone valved comms. receiver knows that.
But I love them (well, most of them) nevertheless.

Al.

[David Simpson]

I was dead lucky in acquiring a complete spare 995A/2, and the two empty cabinets. Many thanks again to the providers.
I guess part of the attraction to Marconi cabinets stems back to my RAF days. No equipment bench in a Wireless Bay was without one or two Marconi Sig Gens. I have fond memories of a CT394B alongside a CT452A, alongside a 65B.
As for Eddystone, they are certainly loveable as Al says. My 830/4 is one of my prize possessions.

Regards, David