home *** CD-ROM | disk | FTP | other *** search
- Xref: sparky sci.electronics:20705 rec.audio:16664 alt.guitar:10845 rec.music.makers.guitar:923
- Newsgroups: sci.electronics,rec.audio,alt.guitar,rec.music.makers.guitar
- Path: sparky!uunet!zaphod.mps.ohio-state.edu!cs.utexas.edu!csc.ti.com!tilde.csc.ti.com!mksol!strohm
- From: strohm@mksol.dseg.ti.com (john r strohm)
- Subject: Re: Tube amp transformers
- Message-ID: <1992Dec14.212249.28188@mksol.dseg.ti.com>
- Keywords: tubes transformers
- Organization: Texas Instruments, Inc
- References: <Bz4FK9.Fx3@watdragon.uwaterloo.ca> <Bz8xMo.5v7@dcs.glasgow.ac.uk> <1992Dec14.174659.18860@ncsu.edu>
- Date: Mon, 14 Dec 1992 21:22:49 GMT
- Lines: 19
-
- Nathan Stewart was worried about frying his multimeter while working around
- tube circuits with typical 350V - 500V plate supplies.
-
- Not to worry, guy. PROVIDED THAT YOUR MULTIMETER has an appropriate voltage
- range, you are not gonna draw enough current to hurt it. The typical DVM
- has an input resistance of around 10 Megohms. The typical old-style VOM
- used a 50 uA meter movement (20,000 ohms/volt) and the internal voltage
- dividers were sized to allow the meter to draw that current at full-scale
- voltage. If you hit the meter with a higher voltage than the current
- range, you can hurt it, which is why MODERN DVMs say OUCH when their input
- protection circuits clamp.
-
- The usual problem is that your meter draws enough current from the circuit
- to throw it off. You usually won't see this, unless you are working on something
- REALLY touchy. (I had a couple of megohms in series with a big electrolytic
- capacitor a few years ago: I was trying to observe RC time constant in action.
- With a 20,000 ohm/volt meter, on a 3V scale, I was effectively shunting my
- capacitor with a 60K resistor. Made a nice voltage divider. I bought a DVM
- that day, right after I realized what I was doing wrong.)
-