Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on December 13, 2012, 10:13:08 am
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I have measured many of my 1 ohm resistors with 3 separate meters. Nice meters. I always get 1.4 to 1.6 ohm. Is it just because the meters leads add resistance? Using this for bias, if the reading are correct, it will cause a very low bias setting. I have replaced some with 10ohm and found the bias to be lower that expected, but not near 50% lower. Even putting the resistors in series with another still gives more than 1 ohm in addition to the reading of the other resistor. Even though I just use this as a beginning place for bias and them turn it where it sounds best and check fro red plates, I am curious if others have seen this.
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Is it just because the meters leads add resistance?
Yes.
Didn't you think it odd that all of your 1Ω resistor indicate values 40-60% away from their specified value? On 1-5-10% tolerance parts?
I have measured many of my 1 ohm resistors with 3 separate meters. Nice meters.
Do any of your meters have a Relative function? Touch your probes together while set on resistance, and hit the Rel button (might be marked as a "delta"). Now your meter reads zero resistance instead of the lead resistance.
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Is it just because the meters leads add resistance?
Yes.
Didn't you think it odd that all of your 1Ω resistor indicate values 40-60% away from their specified value? On 1-5-10% tolerance parts?
I have measured many of my 1 ohm resistors with 3 separate meters. Nice meters.
No I did not think ALL my resistors were that far out of tolerance. I have 2 flukes and both
Do any of your meters have a Relative function? Touch your probes together while set on resistance, and hit the Rel button (might be marked as a "delta"). Now your meter reads zero resistance instead of the lead resistance.
No I did not think ALL my resistors were that far out of tolerance. I have 2 flukes and both have the Relative or Delta button in the center. I use them all the time as I have many different types of probes. All 3 meters have this ability. Still I get a high reading, but when I read 10 ohms or higher they seem accurate.
The reason for my question is I am trying to find out if I got a recent order of bad 1 ohm resistors. In the past I have just used 10ohm, but I ordered quite a few 1% tolerance and they seem like they are 1.5 ohm.
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I've always wondered how to do this too. I even stuck the leads of the resistors into the sockets on the meter to eliminate the leads, but still it's hard to get a accurate reading at 1 ohm. I don't have that calibration feature tho. So i just got the most precision 1 ohm resistors i could and if i recall correctly, I tried a few on a socket to see which read the same so at least i could be sure the 2 tubes would be biased the same if not exactly at the current i wanted. At least i remember thinking to do that but it's been so long now i'm not 100% sure i did.
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Measure the meter leads without the resistor - you'll probably find they're 0.4 - 0.6R.
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Measure the meter leads without the resistor - you'll probably find they're 0.4 - 0.6R.
I have many different leads which measure differently, but I can zero the meter. I think I have gotten a batch of mislabeled resistors. This is why I am asking. I guess the best way to check is to check the meter leads and then add the resistor. If they are .5 over the total, then I will know.
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Do any of the resistors have oxidation on the leads (which you could remove by scraping with a razor)? Are you using firm pressure when measuring?
Any resistance measurement in the low-ohm ranges will be affected by surface contact considerations.
>I think I have gotten a batch of mislabeled resistors.
I honestly think that's highly unlikely, unless you bought the resistors on ebay from a chinese or Hong Kong address (God knows what you got in that case). I've never run into mislabeled new parts in ~20 years of amp tinkering. When something didn't seem right/measure right, it always turned out to be my equipment or my use of the equipment.
Also consider that unless you bought a reel of resistors on a single tape, it's highly unlikely that any two resistors in your possession came from the same batch. Maybe not even on then, as I'm not certain when in the production process the resistors are put on a tape.
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Do any of the resistors have oxidation on the leads (which you could remove by scraping with a razor)? Are you using firm pressure when measuring?
Any resistance measurement in the low-ohm ranges will be affected by surface contact considerations.
>I think I have gotten a batch of mislabeled resistors.
I honestly think that's highly unlikely, unless you bought the resistors on ebay from a chinese or Hong Kong address (God knows what you got in that case). I've never run into mislabeled new parts in ~20 years of amp tinkering. When something didn't seem right/measure right, it always turned out to be my equipment or my use of the equipment.
Also consider that unless you bought a reel of resistors on a single tape, it's highly unlikely that any two resistors in your possession came from the same batch. Maybe not even on then, as I'm not certain when in the production process the resistors are put on a tape.
I did buy a reel, but I will try what you mentioned. I have never had a problem with any others. These are from Mouser and I usually get great stuff from them. I am sure it is something I am doing. I got in a large order of resistors and was checking them as I put them in drawers. These are dale 1 ohm, 1%, 2 watt. I would think they should be good. I usually use steel wool to clean the leads and make them shiny. I use clip on probes, the kind with a great bite and hold them up off the bench. Like I said, it is probably something I am doing.
I'll take your word and experience and figure out what I am doing to make the additional resistance. Doesn't take much with 1 ohm.
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These are dale 1 ohm, 1%, 2 watt.
In that case, I'd just trust them to be 1Ω and move along. No need to check them IMO, especially if you're gonna just use them for bias checks in a guitar amp.
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An easy weekend project: http://diyaudioprojects.blogspot.com/2008/11/simple-low-resistance-measurement.html (http://diyaudioprojects.blogspot.com/2008/11/simple-low-resistance-measurement.html), using the 4-wire (Kelvin) resistance measurement method.
Jaz
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> my 1 ohm resistors .... get 1.4 to 1.6 ohm
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That Fluke is honest about internal and lead resistance.
And as HBP says, contact-pressure is critical in low-R tests. If I press the probe tip it says 0.3; the photo says 0.4 because I had both hands on the camera.
As mentioned, fancy meters can also do a REL reading, the difference between two conditions, one of which can be shorted-leads.
There ARE digital meters which "cheat". If reading is 1 ohm or less, they round-down to zero. This avoids embarrassing questions about why it does not read zero on a short.
Here's a different check. You say you trust 100 ohm resistors. And you may be trusting the DC Volts readings. Put 1r and 100r in series across a 5V or 12V supply.
Read the *actual* supply voltage with the 100+1 connected.
Read the voltage across the 1r.
Do math.
1 ohm of 101 ohms is 0.0099099 of supplied voltage.
Suppose your "5V" were really 5.000000V.
So the 1r should read 0.04950V.
If + or - 10% it would say 0.0544V or 0.0454V.
Rounded to a milliVolt (most meters won't do better; some not this well), then 0.05V is right-on, 0.055V or 0.045V is 10% off, 0.052V or 0.047V are 5% off.
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This may not be your problem, but I am going to throw my $0.02 in here... I used to own a number
of flukes, and I had them calibrated yearly, they used to send the detailed spec sheets with them.
If you read the detailed specs, the fluke DMMs have different accuracies for lower resistances.
In the single digit range, a Fluke 87V, for example, is +/- 2.2% accurate, but when you go up in
resistance it goes down to +/- 1.2%. When you get into the double digit mega range, it gets
worse (geez, how accurate do you need to be there? Go get/build a wheatstone bridge)...
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Go to the tone lizard web site. Checkout the Idiots Guide to VTVM's. Scroll down to 1 ohm bias resistors.
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Or just trust they're 1Ω, near enough, unless they measure 3-4Ω.
If you really need to measure/match them, use PRR's method as it's the next best thing to a bridge (and most bridges will use a similar measuring technique to compare to a known resistance).
Or use 10Ω resistors. They did that in the old days because 30mA through a 1Ω resistor will only be 30mV, which is way down at the bottom end of a standard consumer VTVM. So you'd need to buy an expensive meter (like an HP 412A) to measure in the millivolt range accurately.
Instead, you could use a cheap RCA VoltOhmyst on the 1.5v range with a 10Ω resistor, cause now the voltage is 300mV and is getting into a more accurate portion of the analog scale and is easier to read.
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Yes, and those 10 ohm bias sense resistors also did double duty as "fuses" for the power tubes.
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If you pick their power rating low enough.
100mA through 10Ω is only 1v, and 0.1w dissipated. Since that's probably failure current in many output stages, 1/8w may be a little small while 1/4w is almost too big to act as a fuse.
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Or put all your 1 ohm resistors in series, measure the resistance of the total, and divide by the number of resistors. Chances are the answer will come out close to 1 ohm.