Never Trust a Cheap Multimeter

[daddyo]

About a week ago, I biased my DRRI with a bias probe and a $10 dmm to 24 ma. But, last night, after reading a bunch of threads I decided to try 18-20 ma. When I tried to change it, the bias started at 37ma, then morphed to 120ma without touching anything. Turning the bias pot did nothing, then the meter started doing a weird cycle of random digits. I plugged my guitar in just to test it and the amp played great !?! I turned off the amp and today bought a decent Amprobe 35XP-A for a bit under $100. Now the thing reads fine and my bias is set to 19.3ma
Moral – never spend thousands on guitar gear and then cheap out on a $10 meter
Duh

[VacuumVoodoo]

Moral – never spend thousands on guitar gear and then cheap out on a $10 meter
Duh

Buy cheap, pay twice:D

[Trout]

Buy cheap, pay twice:D


That is actually true to a certain extent. But basically the majority of
el-cheapo units are all the same electronically. Some cost $6.00 and some cost $99.00 yet they are still the same meter.

A DIYaudio guy down in Florida has a really good article on meters and proper use.
I bought a dozen el-cheapos just use for diagnostics, If I blow one, no loss!
I fried a $200.00 fluke go bad from a severe oscillation once so it is a good idea to have the cheapo's.

Anyway, this is a good read on meters.

Safe Meter Use (http://www.tubelab.com/MeterUse.htm)

Trout

[John Phillips]

I bought a dozen el-cheapos just use for diagnostics, If I blow one, no loss!
I fried a $200.00 fluke go bad from a severe oscillation once so it is a good idea to have the cheapo's.
That's why I use cheapos too often. If you do something wrong and it turns into a firework - which can happen, even if you know what you're doing... :( - at least it's only a few dollars down the pan.

The main thing to watch is that the voltage ratings on some of them are maybe a bit optimistic - I fried one a few weeks ago when I didn't do anything wrong - it just arced internally for no very apparent reason while measuring only about 350V on a 500V scale. (Not in a user-hazardous way BTW.)

When I tried to change it, the bias started at 37ma, then morphed to 120ma without touching anything. Turning the bias pot did nothing, then the meter started doing a weird cycle of random digits. I plugged my guitar in just to test it and the amp played great !?!It sounds like the meter made the amp unstable and it started to self-oscillate. This can actually happen with any meter, not just a cheapo. It's more common to get it when metering on the OT primary connections than the tube cathodes, but it's a potential problem more or less any time you connect some trailing wires to an amp circuit.

Don't automatically assume a cheapo is no good, you just need to know its limitations.

The accuracy usually isn't that hot either, but it really doesn't matter for guitar-amp work.

[donnyjaguar]

I'm glad John answered this as I was going to say the same thing yesterday but ran out of time. So I guess add this as a +1 to his comments. Having worked in radio for many years its is a common problem with RF getting into a VOM or DMM, or the test lead introducing reactance to the circuit. Its also common in high-impedance circuits (like vacuum tubes) although admittedly not normally when measuring at the cathode. Guessing any NFB is exacerbating the issue. That said, get yourself a decent meter anyway. Its a one-time purchase and will pay itself back in equipment that it doesn't destroy. :)

[daddyo]

Maybe the meter just fried because, although not indicated in my first post, I shut the amp off, had some dinner, then tried it again and the same results. Would an oscillation or RF signal hang around that long?

[TopBooster]

It sounds like the meter made the amp unstable and it started to self-oscillate. This can actually happen with any meter, not just a cheapo. It's more common to get it when metering on the OT primary connections than the tube cathodes, but it's a potential problem more or less any time you connect some trailing wires to an amp circuit.


+1, I recently discovered that it was my cathode current bias probe (into a cheapo DMM) that was causing strange oscillations in a few amps, causing the idle current to shift by large amounts (20mA or so). I removed the probe and the amps became stable.

Unfortunately, I have yet to get a nice meter... :(

[John Phillips]

Maybe the meter just fried because, although not indicated in my first post, I shut the amp off, had some dinner, then tried it again and the same results. Would an oscillation or RF signal hang around that long?It doesn't have to. As soon as you re-introduce the meter it will start again.

[JJGross]

I've noticed the goofy oscillations on my ElCheapo Grande DMM are only there if I use the "Auto range" feature. When I manually select the range it doesn't do that at all. I also noticed it drifts in AR but doesn't in manual mode. It also came with really crappy batteries in it new and was a lot more stable and accurate once I put good high quality (insert brand name here) alkaline batteries in it.