VTVM / Valve Voltmeter articles from magazines in the UK
 

I am building a VVM/VTVM at the moment as I have failed to win several on ebay now and the FET voltmeter I built while functional has a few flaws and is very boring.

I have a copy of the excellent book on VTVM design and construction from Tube Books[1] and have done lots of analysis and have come up with a design based on the Heathkit V/IM series VVMs. However I'm missing a number of construction ideas and practical approaches to building them nor is my research entirely complete.

Ergo, does anyone know of any articles, particularly from the likes of PW in the UK from the 1950-1960 period which contain projects for VVMs at all?

[1] http://www.tubebooks.org/Books/rider_VTVM.pdf

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

If specific PW articles (with issue dates) can be identified, I am happy to scan them, as long as the number of pages is not too great, as I have a set of PWs complete over that period.

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

There are two designs in "Test Equipment For The Radio Amateur", which I could scan for you if you don't already have them.

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

I've actually got a copy of Test Equipment for the Radio Amateur and have referenced the designs in there.

I've downloaded all PWs from www.americanradiohistory.com - was hoping that there would be some handy references to them. I'm going through them by hand at the moment and there is one found in July 1955.

They all seem to be a relatively similar design so far.

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Just because I was reading it recently, there's a thorough 'article' on building one in E.N. Bradleys 'Radio Servicing Instruments', a little A5 book which I imagine you could pick up cheaply. Based on the US McMurdo 'Vomax' design it says. Valves are an EB91, 2 x 6SN7s and a GZ32. The article has circuit description, proper parts list, chassis layout and drilling points, testing and set up procedures etc etc.

Andrew

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Interesting. Thanks for the reference. I will attempt to obtain a copy of that.

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

These are the designs it has in it. I must try that Wobbulator! It originates from 1953 and mine's a 9th printing which must have meant it was popular back in the day I guess.

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

That certainly looks like a very nice book. I've got a 3JP1 here waiting to be turned into an oscilloscope as well. I was going for a solid state solution originally but I'm having a bit too much fun playing around with valves at the moment :)

Edit: numerous copies located on abebooks.co.uk - will order one tomorrow when my eyes are working better :)

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

I used to have a copy of that book by E N Bradley! Jeepers don't some things take you back?

Wobbulators with reactance valves are not all that satisfactory as their tuning range is very limited. But if making the thing is the primary interest rather than using it....

Back in the day, serious sweepers were made either with motor-driven butterfly capacitors (no earth contact needed to the rotor) or else by swept current into a coil on an iron core saturating a ferrite core of an RF coil positioned in a gap in the iron core.... a saturable reactor arrangement.

If you want a sweeper to use, it's a lot easier to use variable capacitance diodes..

David

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

There was a valve voltmeter & ohmmeter in the April 67 PE. Attached is a scan of the article.

Keith

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Thank you for that. That's a particularly good article and exactly the sort of thing I'm looking for.

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Excellent project - that was well spotted Keith!

The intro outlines why a high input impedance VTVM scores over a conventional analogue meter such as an AVO, which has a lower input impedance.

It cites an example of an anode of a valve where it is known that 200V should be present. When measured with a 1,000 Ohms Per Volt meter (EG, AVO7), due to the loading effect of the meter, it will read just 100V. That will be the actual voltage present due to the meter dragging it down from the expected 200V. A 20,000 OPV meter such as an AVO8 will read a more sensible 195V, and when using service data, as often as not, the Voltages will have been measured with an AVO 8 originally, so would most likely state 195V in this instance. Using a 10meg OPV meter such as the VTVM such is the subject of the article, it would read 198V. The lower the voltage being measure, the greater the loading effect of low impedance maters.

One thing that puzzles me about the design - given that it uses an isolation transformer, a Paxolin panel and wooden cabinet - is why it is earthed. At the conclusion of the article the author states thus: "A word of warning - always bear in mind that one meter lead is earthed, so that any attempt to measure directly across a floating voltage may have dire consequences. A 'floating voltage' is one that has a certain potential difference across - for example, a resistance, both ends of which are above chassis potential". I wonder if that was a 'coded message' between the lines from the author to say 'don't earth it'?

Maybe - even back in those less safety conscious days - the magazine insisted that all mains powered designs must be earthed, even when there are no exposed metal parts? Fact is, that there will be many examples where it will be necessary to check floating voltages - across a resistor between a reservoir/smoothing cap combination for example so we can check the Voltage drop and calculate the current flowing. It would be interesting to see what would happen if one side was earthed by the meter.

As to measuring resistors, I guess most of us these days use a digital meter, but the VTVM scores highly over say an AVO for measuring resistance, in that the scale is much more linear. For example, 10 Ohms at centre scale, close to 100 Ohms at FSD, and so forth. All in all, a nicely designed project with full constructional details. Very tempting to allow nostalgia to rule, and to have a furtle in the spares box for all the bits to build it! The two pole 12-way Yaxley type switch could be a problem, but the two-pole six way switch could be a modern 'Lorlin' one.

It would benefit from a good sized meter, but given that a new 10cm x 8cm (4" x 3.25") 50 uA meter from China on ebay costs only £4.36 post free, that's no obstacle. A paper scale could be temporarily fitted to calibrate the Volts and Ohms ranges, then a new scale created in Photoshop and fixed to the reverse side of the meter scale-plate.

Shame about the ECC82/12AU7 twin triode, given that we're into guitar amp territory these days, so if one isn't to hand, could be anything from £15.00 - £60, especially if it's been sprinkled with fairy dust and has 'presence'. :-)

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

I was going to build that meter back in 67 and had priced up most of the components, which is how I remembered the article. I might have a go at building it, when I can find some time!

Keith

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

I found a cheap source of the 12AU7: http://uk.rs-online.com/web/p/double...alves/6784105/ ... £6.98 each inc VAT and delivery which is doesn't hurt too badly. I bought one, a couple of bases and two transformers to run back to back. So far so good with the experiments:

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Before I forget, I decided to standardise on an intermediate voltage so I don't have to bother with mains while experimenting so I'm building a 12V AC supply which will drive one of the secondary windings on another identical transformer to step up to isolated 12/115/230V outputs inside the device. This is a temporary step and I will convert this into a 9V DC battery eliminator power supply afterwards and put another transformer in the final device. Not the most efficient solution but good enough and works out slightly cheaper than locating a proper transformer for the job. The 12AU7 allows the heaters to be wired in series so the 12v secondary is good for that and the other windings can be the HT.

Metalwork nearly done and it only drew blood once! Limited tooling consisting of a aviation shear, nibbler and drill unfortunately. Bending was done by hand with some mole grips and some bits of steel I had lying around which is why it is wobbly as anything.

Attachment 142454

Also found RS sell proper old fashioned tag strips! I clearly wasn't paying attention when I ordered it as I got ten and I only wanted one. Sure I will use them though:

Attachment 142455

I have also stolen a real breadboard from SWMBO so this can be prototyped properly :)

Incidentally my VVM will be floating (or I'll blow something up) but the chassis will be earthed!

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Annoyingly the Sifam one I have with similar size is knackered by the looks as it gets stuck if you peg it at FSD. The needle is straight so I think this is a faulty bearing. It's very old and scratched to bits and well used in what I extracted it from so I'm not that bothered.

I just did a search and found those on eBay and for that money, that's a no brainer. One ordered! I'll use my fluke DMM until it arrives in 200uA range :)

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Hello Mr. Bungle.
Nice to see someone about to build.
I have got a copy of the (Ryders VTVM book), a very good read if you need it
its all about VTVMs. size 22.1MB pdf.

Also have copy of Rhys Samual (The VTVM) size is 29 MB pdf this has got
all about how the one Keith put on for you works/alignment on page 186,
you can use it with sn7,s

Both to big to post here so if you send me a PM of your Email I will try to send you a backup.
gezza123.

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Thanks for the offer. Much appreciated. PM on the way shortly.

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Hi

Not to mention the 'extra special' cryogenically treated versions either! :)

That does look a very interesting piece of equipment to build. I like the fact that it can measure up to 100 MOhm. I think I have all of the components lying around so will have a go at building this when I can catch up on my outstanding commitments and projects.

Thanks to Keith for uploading the project details.

Regards
Symon

Re: VTVM / Valve Voltmeter articles from magazines in the UK
 

Well I did a simple search the EN Bradley book as well as other books can be found here:
http://www.vintage-radio.info/books
Apologies if anyone has posted that already.