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Old 19th March 2011, 03:35 PM   #1
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Default VTVM substitute?

So, I've got my service manual for my Conn 427 Caprice tube organ, and it requires a VTVM for some of the calibrations. Is there any substitute for a VTVM?

Or, if there isn't, can somebody point out an audio VTVM with db measurements that's less than $50?

Thanks,
Austin
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Old 19th March 2011, 03:39 PM   #2
Ron E is offline Ron E  United States
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VTVM has high input impedance as compared to a typical analog voltmeter. I believe just about any DMM now will have a high enough input impedance. I could be wrong, though
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Old 19th March 2011, 04:01 PM   #3
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Any good jfet input vom should be ok aswell.

A high impedence meter is required so that it doesn't drag down the tube circuit drawing to much current causing either a false reading or a change in operating conditions around the tube circuit being tested. jer
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Old 19th March 2011, 04:22 PM   #4
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Quote:
Originally Posted by geraldfryjr View Post
Any good jfet input vom should be ok aswell.

A high impedence meter is required so that it doesn't drag down the tube circuit drawing to much current causing either a false reading or a change in operating conditions around the tube circuit being tested. jer
Fjet input vom?
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Old 19th March 2011, 04:30 PM   #5
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The standard digital meter should be fine, it has a high impeadance input.

Sal
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Old 19th March 2011, 05:21 PM   #6
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Quote:
Originally Posted by Sal Brisindi View Post
The standard digital meter should be fine, it has a high impeadance input.

Sal
All righty then. Guess it's time to go shopping!

Thanks all
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Old 19th March 2011, 07:32 PM   #7
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My Sears Craftsman DVM is insensitive to AC frequencies except 50-60 Hz. I've proved this with my organ, where I had 8 V pp coming out at 360 hz and was reading zero on the Sears. I've heard other dual slope integrating DVM's are the same. Meters advertised as "RMS" may have more frequencies they respond to. Old style VOM's with 100000 ohms/per volt may be useful. I did AC debug with my Simpson 260 on the 2 VAC scale before I blew it up. Now I use a $40 B&K 2120 scope, but the 10x probes are $55 each from mouser. Watch, the $55 probes are only rated to 300 Volts.
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Old 19th March 2011, 08:15 PM   #8
Ron E is offline Ron E  United States
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I have 3 cheap DVM's (including an Extech "True RMS" meter) and they are accurate from ~40-1000Hz, anything above or below that and they roll off, giving low readings. I just bought an old Fluke bench meter that reads true RMS to 50kHz.
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Old 19th March 2011, 09:11 PM   #9
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Cheap modern DMM's are not well suited for your application,especially if you need to measure full audio bandwidth AC voltages : DMM's usually have very limited AC frequency response and no dB scales. For this kind of application old AC VTVM's are hard to beat and dirt cheap on the second hand market. There are plenty of models available but I would stay away from kit form VTVM's like HEATH,KNIGHT,EICO,etc... which suffer from doubtful building quality and calibration. Try to pick up any old HP (Hewlett Packard) 400 series (D,H,L,E...) AC VTVM which are a pleasure to use,stable and very robust industry standards. As demand is very low you should not pay more than $ 30.00 for a good condition HP400 on the used market.
For DC voltage measurments only any DMM with a 10Megohm (or more) input impedance will fit the bill and will not impose an excessive load on your (tube) circuit.
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Old 19th March 2011, 11:59 PM   #10
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It's a fact that lots of good builds were, and still are, done with 20000 Ω/V. VOMs. Those instruments rarely cause problems in making DC measurements of tubed circuitry. OTOH, cheap 1000 Ω/V. meters are unacceptable in any sort of electronic work.
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