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Old 13th May 2017, 12:08 am   #21
joebog1
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Default Re: VTVM / Valve Voltmeter articles from magazines in the UK

I have some very excellent full constructional articles on VTVM's from the Australian magazine "Radio and Hobbies". They use standard meter movements and have large easily printable scales that can be copied/scanned/or traced in CAD.
They have full design engineering explanations written in very easily understood form.
There are separate articles for various probes that go with them.
One of the best things about the articles, is that they use standard components still available today.
If MrBungle ( or anybody else) wants copies please PM.
I have scanned them into large PDF's so be prepared for about 50 megs for both meter designs.
The pics are the first page of each article. All up about 30 pages

Joe
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Old 13th May 2017, 7:34 am   #22
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Default Re: VTVM / Valve Voltmeter articles from magazines in the UK

Quote:
Originally Posted by David G4EBT View Post
Using a 10meg OPV meter such as the VTVM such is the subject of the article,
I just spotted a typo. That PE design isn't 10meg Ohms per volt, it's 10meg on all ranges, which is quite common for electronically-assisted voltmeters.

An AVO on a 250v range would look like a 5meg load. On its 1kV range, 20meg - which now beats the VVM.

David
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Old 13th May 2017, 8:23 am   #23
David G4EBT
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Default Re: VTVM / Valve Voltmeter articles from magazines in the UK

Thanks for the update David - good news for AVO fans then!
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Old 22nd May 2017, 4:10 pm   #24
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Default Re: VTVM / Valve Voltmeter articles from magazines in the UK

There seems to be a meter movement for the Heathkit V7-AU on a well known auction site (not me selling it) if anyone is interested.

Resistance ranges on electronic meters such as a VTVM do have advantages over those on an AVO, but linearity isn't one of them.

For either type of meter, the equivalent circuit on resistance ranges is effectively a voltage source (eg the 1.5V battery), a range resistor equal to the centre scale reading of that range, and the unknown resistor all in series. The difference is how the meter is connected - for an AVO, the meter effectively measures the voltage across the range resistor, whilst the VTVM measures the voltage across the unknown resistor.

For an AVO, FSD corresponds to zero ohms for the unknown resistor, i.e. the test leads shorted together giving the full source voltage across the range resistor, and there is a zero adjuster to set this to FSD.

For a VTVM, FSD corresponds to an infinite unknown resistor, i.e. the test leads open circuit giving the full source voltage across the unknown resistor, and again there is an adjuster to set this to FSD.

An unknown resistor equal to the range resistor puts half the source voltage across each resistor, giving half scale deflection for either type of meter.

An unknown resistor of ten times the value of the range resistor will give a reading of 9% FSD on an AVO, and 91% FSD, i.e. 9% below FSD on a VTVM. Resistors a tenth of the range resistor will do the opposite. The scales are the same shape and linearity, just mirror images of each other. Resistors 100:1 different to the range resistor will give a 1% reading or a 1% below FSD reading depending on resistor value and meter type.

Advantages of the VTVM resistance ranges are 1 - open circuit test leads show FSD, encouraging you to switch the meter off the resistance range to a safer voltage range and 2 - poor contacts in the test leads cannot be zeroed out as they can with an AVO, you have to fix the leads, this gives more reliable low resistance measurements. Also most VTVMs have a maximum resistance measurement of 1000Mohms.

Meters such as the Heathkit V7-AU are designed to have a special DC probe with a 1Meg resistor in the probe tip and a screened cable to 1/4" input jack. This is great for low capacitive loading of the circuit under test (most of the capacitance is "blocked" by the 1Meg resistor) or if there are high levels of RF flying around, but a b**** nuisance otherwise. There's a very easy mod on the Heathkit, you just need to connect another 1Meg resistor from the jack socket to the AC/Ohms 4mm socket which then becomes the "universal" input socket, leaving the jack for the special DC probe, or a diode probe, as needed.

Heathkit seemed to have several revisions of the rectifier for AC inputs before settling on the circuit for the V7-A and subsequent models. It is actually peak to peak responding, and has bias to offset the rectifier valve contact potential for better linearity. The "normal" meter scale for AC ranges show the RMS value of the input assuming a sine wave, but there is an additional p-p scale, which can be useful, for example to quickly see how close to the supply rails a transistor amp can get with music playing.

Stuart
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Old 25th May 2017, 9:52 am   #25
MrBungle
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Default Re: VTVM / Valve Voltmeter articles from magazines in the UK

I saw that movement but decided to skip it on the basis that you can get a relatively decent one slightly cheaper.

Decided to skip AC measurements. I'm mainly interested in RF + DC so the plan is to leave enough space in the chassis to change my mind later

Quote:
Originally Posted by David G4EBT View Post
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.
Just a reply to David here. Thanks for the pointer on this. After a very respectable 13 day turnaround, one of them arrived this morning and it's actually rather good for the money!

Click image for larger version

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Rest of the build is progressing very slowly. I've made a box and mounted the transformer and controls in it for the "12v isolation" side of things. The rest of the work hasn't started yet unfortunately due to numerous other things going on at work and at home.

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I've left enough space in the above box to stick a TO220 power transistor on the back, use a slightly higher voltage transformer and turn it into another 12v supply when it has served its purpose as an AC supply.
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Old 25th May 2017, 5:02 pm   #26
MrBungle
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Default Re: VTVM / Valve Voltmeter articles from magazines in the UK

Just a heads up if you buy a Chinese cheap meter. Check out the damping, or lack of!

https://youtu.be/fAIMRgiSMIk

This is driven from a 50uA current source.
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Old 12th Jul 2017, 7:09 am   #27
MrBungle
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Default Re: VTVM / Valve Voltmeter articles from magazines in the UK

Quick update on this project. I got it working last night, well at least deflecting linearly. Having some stability problems with it however. Not sure what they are but even after 5 minutes warm up the needle drifts up and down a lot. I have shorted the input terminals to get rid of any ambient charge.

I did do something stupid which woke me up though. I soldered the smoothing capacitor in the wrong way around and was greeted with a proper bang. Appears that axial capacitors don't have the same venting system as radial ones do. Joy to safety glasses!
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Old 10th Aug 2017, 7:30 pm   #28
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Default Re: VTVM / Valve Voltmeter articles from magazines in the UK

One post moved to a new thread: https://www.vintage-radio.net/forum/...d.php?t=138880
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