Tube Amps / Music Electronics
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|4/19/2000 9:39 PM|
||Buying a multimeter: need your recommendation|
Im going to be replacing my old cheapo Radio Shack analog multimeter shortly. I'd like to get some opinions on features to look for in a new multimeter. I'm just starting to get into amp repair.
Do you prefer digital or analog?
How about digital with an analog LCD scale at the bottom?
I assume the meter should be able to go to 600VDC (or higher).
Capacitance measurement seems to be a nice feature for checking out caps. Do you do this in your multimeter, or measure this with a separate device?
Is high-end stuff (like Fluke) the way to go, or does basic RadioShack or Sears Craftsman stuff work okay for you?
|4/19/2000 10:05 PM|
Digitial would be the best, that analog LCD would be optional.
Not something I use alot if really at all. I have matched some caps for the PI, but alot would say that is a waste of time. YMMV here...
I first picked up a Radio Shack meter when I started just in case I blew it up. Went thru a bunch of fuses till I remember to take it off of ohms when I put it to the high voltage. I still grab it or use it in conjuction with MY REALLY COOL FLUKE METER. I would say whatever works for you and you wallet. I have no complaints with my RS meter--gutiar amps aren't rocket science.
just my thoughts
|4/19/2000 10:25 PM|
There's a lot of good Multi-meters available. But you won't go wromg with a FLUKE. I've used one for years at home and at work. They're built well and I've been very satisfied with all of the different models I've used over the years.
|4/19/2000 10:25 PM|
A Fluke 79 mark III is the way to go if you want top of the line. It costs 200 bucks though. Worth every penny IMHO.
Get a meter that has at least 1000 VDC. this will allow you to measure 600VAC.
I use the cap meter all the time -- not the most accurate, but I can never tell what the codes on the caps are supposed to mean.
I think radio smack sells a ~70$ digital meter that has the basic functions you will need.
I like digital MUCH better than analog.
|4/19/2000 10:31 PM|
Radio Shack MMs are fine. People knock them, but you can't beat them, FOR THE PRICE. It's kinda funny. If you need a MM right NOW, you'll have to pay. But if you can wait, you can alway find a good deal on one.
Last week a friend called me up and asked me if I wanted a nice Fluke for $20 (legal). I've already got 2, so I said no.
I found this site through R.G. Keen: http://www.bgmicro.com/. They are selling Vellman multimeters for $16. (On page 1 of the catalog) I think they're reconditioned. But, that's a pretty good deal.
Just some thoughts. Hope that helps.
|4/20/2000 5:03 AM|
I had picked up a $80 Radio Shack DMM to use around the house and it has a lot of problems measuring the B+ on the plates. The display will blink and go into a completely different mode. Everything else seems to work just fine but for some reason it can't handle the B+ in a guitar amp.
After mentioning this here before a few other people chimed in with the exact same complaint... Perhaps there is some AC component in the DC that makes it go haywire.
For the money, Wavetech makes some really nice meters with capacitance and inductance ranges. And I do use the capacitance meter all of the time. I believe that they now have a 2 or 3 year warranty with any WaveTech dealer swapping out your DMM across the counter if problems develop.
As for the analog display along the bottom of the LCD screen- I had that on one meter and it really did not prove to be as useful as a real analog meter ...
|4/20/2000 7:48 PM|
||Measuring that plate B+|
Measuring the B+ on power tube plates is not too easy sometimes because even if everything is fine before you probe the socket or connect a clip lead, once you add the probe to the circuit it can act as an aerial or part of a tuned circuit and oscillate. Some meters can still give fairly accurate B+ along with an indicated error condition, others indicate only the error, others may not have indicated error but could still be wrong.
Plus if any oscillations are parasitic enough to steal significant power, that can also reduce the B+ voltage below that of the idle condition that you might be trying to measure it under.
This can happen with ocilloscopes too for those whose input range accomodates plate voltage, if you are accustomed to adding an alligator clip extension lead to your probe tip (hey its only audio frequencies we WANT to measure) and clipping it to the circuit node desired for waveform observation, everything can be fine in all other parts of the amp until you try it on the plate (grid sometimes too, it is the input after all). You get the same effect as with the DMM but you can see the trash on the screen if you are using the right settings. Removing the unshielded clip extension and using the probe tip as designed really works so some analogy can be used to make the DMM workable.
A DMM probe that has an alligator clip instead of a plain point can be used to hold a 100K or bigger resistor (with properly trimmed leads) and the resistor is touched to the plate connection. Since so little current is drawn by the meter, there is virtually no voltage drop over the resistor and the reading is as accurate as usual. Any potential signals within or extraneous to the amp that might be picked up by the unshielded lead wire and input to the tube (frustratingly through its output pin) are now greatly attenuated by the added resistor though.
Hope this makes sense . . .
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