rzenc
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« on: March 12, 2012, 05:53:44 pm » |
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I want to build my own VTVM...If you only could have one Voltmeter AC/DC tube based, which you guys would build?
It's a project for collegue - extra credits -
Found some schema online but wanna hear from you before this ride starts...
Many thanks in advance!!
Best Regards
R.
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PRR
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« Reply #1 on: March 12, 2012, 09:46:39 pm » |
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A full DC/AC/Ohms 1.5V/5V/15V/50V/150V/500V meter is a VERY complex job, with elaborate multi-deck switching, not to mention meter scale and many funny-value precise resistors.
Back in 1940s-1950s, geeks DIY-ed a lot of gear, but VTVMs were bought whole or as kits.
For a college project: The tube-pair, 1mA meter, and 1/10/100/1000 attenuator. That gives you a functional 1V-1000V DC meter, should be good for credit, and quite useful in projects.
I know the warm glow of tubes, but I'd lean to using JFETs because they can be battery powered and avoid the fuse-blowing issues of wall-power VTVM. (Yeah, a TL061 opamp might be easier, but I assume you are supposed to do some non-trivial design work.)
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jazbo8
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« Reply #2 on: March 13, 2012, 05:26:09 am » |
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I think you should be able to build this very basic VTVM http://dl.dropbox.com/u/1326040/DIY%20VTVM.jpg. It utilizes pretty standard parts. No resistance measurement though... Also for AC measurement, you need to add some caps and a diode to rectify the signal. Jaz |
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HotBluePlates
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« Reply #3 on: March 13, 2012, 01:56:12 pm » |
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The RCA VoltOhmist WV-77 is fairly simple. However, as PRR pointed out, the cost of the multi-deck rotary switch is prohibitive. It's probably cheaper to buy a VTVM off ebay than it is to buy that one switch new or used!
Elaborate VTVMs widen the cost of building/buying very much wider. The Hewlett-Packard VTVMs I have contain extremely high quality rotary switches, regulated power supplies, lots of precision parts... You can buy an old one from folks who don't care about them for $30-40, and you'd probably spend over $400 to acquire the parts to build it yourself.
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From Principles of Electron Tubes: "... the phase of the output voltage is retarded."
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rzenc
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« Reply #5 on: March 13, 2012, 08:31:58 pm » |
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Many thanks for the answers!!! I searched for some more schems but things were getting wierd with a hell lot of transformers and other almost impossible things to get...DL posted a 'simple' schematic from RCA and althought it hurts my eyes trying to get thru it, I believe it's doable... Concerns: 1- switches.... I think they are 12 positions/ 04 decks ???? Shorting or non-shorting? If they are indeed 12p/04d, I might have find the switches... NKK HS16-4S (if shorting type) Data sheet: http://www.nkkswitches.com/pdf/hs_ps_ts.pdf... 2 - meter... No idea yet on what to use... AFAICT, Sifam made/makes amazing meters..however I don't know if the make this type of meter...I saw a lot of Sifam on Manley gear. Is there something you guys would recommend?? PRR mentioned a 1mA meter movement. Any brands you are aware of? I'm sure there will be other concerns but for the moment I think they are the most relevant in order to make it come to life. jasbo8: I will try this circuit out as a start-up exercise. Many thanks!!!! Do you think a meter from an el cheapo analog meter would work? HBP: It's an extra credit activity...gotta do something...my professors know nothing or next to it about tubes...most the other guys are building Remote Controlled miniatures of cars..some folks will build a auto lightning system for hydrogrow...I will try something that I would like to have... DL: I will download the book and pdf's... Get in contact soon... Many thanks for tour help guys! Best Regards!! R. |
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PRR
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« Reply #6 on: March 13, 2012, 11:24:54 pm » |
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> Shorting or non-shorting?
No.
Look at the short and long contact arrows and the high/low rotor cutouts. This is a very custom switch.
Some of it could be done with ordinary 1-of-11 (NON-shorting) switching.... and Allied can get those switches.... $65 special-order.
Sifam's meters are sexy but mostly crude IMHO.
An H-P class signal generator is very useful, uses fewer magic parts, and there's no $4.99 replacement for a signal generator.
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Shrapnel
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« Reply #7 on: March 13, 2012, 11:34:11 pm » |
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It's drawn different in RC-23.
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-Later!
-Bill.
"All the great speakers were bad speakers at first" - Ralph Waldo Emerson
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stingray_65
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« Reply #8 on: March 14, 2012, 12:14:39 am » |
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I've scrapped out quite a few VTVM's. I even have one of those RCA's mentioned above just laying around. If you need parts, let me know. got a box full of multi layered rotary's most with the components left on. Any one mention the Heathkit IM10? http://www.mzentertainment.com/technical_archive/heathkit_IM-10_vtvm.gifRemember the goal is to pass the class, not build the best VTVM. |
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My mind is aglow with whirling, transient nodes of thought careening through a cosmic vapor of invention (H. Lamarr)
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rzenc
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« Reply #9 on: March 14, 2012, 10:09:14 am » |
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This is a very custom switch.Some of it could be done with ordinary 1-of-11 (NON-shorting) switching.... and Allied can get those switches.... $65 special-order. Cool, so how can I proceed to order such switch? I would also like to know how can we manage to make our own switch...  Sifam's meters are sexy but mostly crude IMHO. Which brand would you reccomend over Sifam? It's drawn different in RC-23.Many thanks for this information..Will compare both... Ray: I pm'ed you... So many many thanks!! To be continued... Best Regards R.
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HotBluePlates
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« Reply #10 on: March 14, 2012, 12:00:33 pm » |
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This is a very custom switch.Some of it could be done with ordinary 1-of-11 (NON-shorting) switching.... and Allied can get those switches.... $65 special-order.
Cool, so how can I proceed to order such switch?
Don't. Find someone with a "parts" VTVM, and steal the switch. Ray's offer is your best bet. The diagrams shown before are at least 95% the same as the VoltOhmyst I mentioned (I didn't check every part value, but the topology is exactly the same). Since I bought my WV-77 for $10-15 (can't remember now), ordering a switch for $65 doesn't make sense. An H-P class signal generator is very useful, uses fewer magic parts, and there's no $4.99 replacement for a signal generator.
The 2- or 3-gang tuning cap is the one magic part. Well, plus the OT and the light bulb. |
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From Principles of Electron Tubes: "... the phase of the output voltage is retarded."
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Bassmanster
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Rarrrawweth!
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« Reply #11 on: March 14, 2012, 12:20:59 pm » |
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My casual use VOM is from an Archerkit I got on clearance in '92. You won't see that again...
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I will be swift. And merciful.
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PRR
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« Reply #12 on: March 14, 2012, 09:27:57 pm » |
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> The 2- or 3-gang tuning cap is the one magic part. Well, plus the OT and the light bulb.
Ah, yes, the cap is a Problem.
Find dual-1Meg Audio (Rev-Audio is better but harder) and range-switch the caps.
The OT does not have to be as good as H-P's. The early 200 is SE out, looks like a Champ power stage. Use a hi-fi 5K:16 10W OT, take NFB to the driver, it'll be real flat 50Hz-15KHz and not far out at 30Hz or 30KHz.
The "magic lamp" was apparently a dead-standard 7 Watt 230V night-light, a large Christmas Lamp.
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billcreller
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« Reply #13 on: March 23, 2012, 01:41:11 am » |
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I still have a cheapie VTVM I bought in the late 50s It has a 12ax7 in it, which the manual says has to be a good tube without gas !!  |
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I'm still tryin' to figure this out........
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PRR
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« Reply #14 on: March 23, 2012, 08:11:58 pm » |
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> has a 12ax7 in it, which the manual says has to be a good tube without gas !!
Usually a 12AU7 or 12AT7. No voltage gain is needed; the low-Mu tubes conduct better.
Gas: set to lowest DCV. Short input, set Zero. Now open input. Any shift is grid current (mostly gas).
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rzenc
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« Reply #15 on: March 29, 2012, 03:19:29 pm » |
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So I got the parts Ray sent me today!!! Many thanks man!!  |
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rzenc
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« Reply #16 on: March 29, 2012, 03:31:40 pm » |
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So...continuing the task for VTVM project...
I think the rotaries Ray sent me will be good to go with RCA VTVM...
Now to the project itself...
Parts...
Which R's and C's would you guys use?
TIA!!
Best Regards,
R.
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Willabe
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« Reply #17 on: March 29, 2012, 04:11:01 pm » |
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Yup! Rays a generous man. He's been good to me too! Brad |
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stingray_65
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« Reply #18 on: March 29, 2012, 04:55:38 pm » |
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I thought that Knight looked pretty good for it's age.
I've looked a bit further, there MAY be an old battery inside her, I hope not. I never thought to check before I sent it.
Old batteries can gunk up a bunch of stuff.
Any how, have fun with your project, I hope something there is useful to you.
Ray
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My mind is aglow with whirling, transient nodes of thought careening through a cosmic vapor of invention (H. Lamarr)
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DummyLoad
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« Reply #19 on: March 31, 2012, 11:43:50 am » |
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Yup! Rays a generous man. He's been good to me too! Brad that he is... he's a good soul that one... :-) --pete |
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rzenc
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« Reply #20 on: April 10, 2012, 08:36:11 am » |
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Howdy?!
It's time to start collecting parts - resistors and capacitors and potentiometers... for this project.
I would like to hear from you before I start ordering parts...
I found a KG620 manual/building instructions with complete schematic and parts list....too big to post
Except for a few 1W resistors, most seems to be 1/2W;1%;Carbon Comps.
What you think may suit it better? Metalfilm or wirewound?? thinking about MF RN70E... WW NS..
TIA
Best Regards
R.
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PRR
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« Reply #21 on: April 10, 2012, 07:55:02 pm » |
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I'd use 1% Carbon-Film for all the small precise resistors.
If you were selling this to the Army to service combat gear in the middle east, Metal or wire would be real nice just to be SURE it didn't flake-out at a critical moment. But for audio shop use, Carbon Comp was good enough for years/decades, and carbon Film is better.
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rzenc
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« Reply #22 on: April 11, 2012, 08:33:42 am » |
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carbon Film is better.
You got me curious.. why carbon film would be better for such app?
Best Regards
R.
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John
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« Reply #23 on: April 11, 2012, 03:13:56 pm » |
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Carbon comp tends to drift in values over time, and also with heat. Carbon film tends to be more stable. Or so I've read.
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Tapping into the inner tube.
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Fresh_Start
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« Reply #24 on: April 11, 2012, 06:15:24 pm » |
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Yup! Rays a generous man. He's been good to me too! Brad +1 on Brad's comments. Ray - thanks again for the variac! Cheers, Chip |
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We have proven once again no plan survives contact with the enemy, or in this case, with the amp. Plan to be wrong about something. |
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rzenc
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« Reply #25 on: April 18, 2012, 03:49:48 pm » |
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1mA meter
Do you guys have a suggestion for this piece?
This is the last needed part to start my quest on VTVM's....
TIA
Best Regards
R.
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PRR
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« Reply #27 on: April 20, 2012, 12:29:30 am » |
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I think the KG620 is scaled for a 200 micro-Amp movement.
A 4-inch 200uA meter is very unusual. I would look for one.... on an old VTVM.
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« Last Edit: April 20, 2012, 12:31:53 am by PRR »
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rzenc
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« Reply #28 on: April 20, 2012, 04:18:54 pm » |
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I think the KG620 is scaled for a 200 micro-Amp movement.
Yeah...checked on meter and there is F.S. 200uA low riight corner... |
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