Reading Bias Through 1 Ohm Resistor

[Rocketrob]

I recently bought some 1 ohm 1% resistors for a bias reading circuit. When I put them on my DMM to check them before installation they read 1.6 ohms. I believe the extra .6 ohm is from the tester's leads. I also think that when checking the bias that this should be considered; that is, if I'm reading 30mA, the actual current is 45mA, and could have serious consequences on the tubes and how the amp performs. Is this thinking correct or am I off base on this? I have not seen it mentioned much in the articles on biasing. Rob.

[loudthud]

What reading do you get when you just short the meter leads together? What reading to you get when you measure two resistors in series? Most meters aren't very accurate at low resistance because of the test leads.

[Rocketrob]

I don't have the parts with me right now. I'll check that later tonight. Thanks for the reply.

[daz]

Thats typical. Every meter i've had is not remotely accurate at such low resistance. I had some precision 1 ohmers too and same thing.

[JHow]

Are DMM any more accurate at reading voltages than resistance, or is there no difference?

[Jazz P Bass]

Lead resistance

Quote:"also think that when checking the bias that this should be considered; that is, if I'm reading 30mA, the actual current is 45mA, and could have serious consequences on the tubes and how the amp performs. Is this thinking correct or am I off base on this? I have not seen it mentioned much in the articles on biasing. Rob. "
I would not worry about the leads when measuring bias voltage.
Unless they are no good!

[Enzo]

When you measure the resistor, your leads are then in series with the resistor, so the meter sees the total of them all. The leads do not add any resistance to the resistor. The resistor is still 1 ohm while in the circuit. The voltage across it will be accurate.

When taking voltage readings, your meter has a input impedance that is very high - something like 10Meg - 10,000,000 ohms. The probes still add that extra .6 ohms. The extra .6 ohms of your meter leads would make that 10,000,000.6 ohms. That will make your readings off by less that 1/100,000th of a percent. I wouldn;t worry.

So when your meter reads 30ma, in reality, the actual current would be more like 30.00000006ma. Or something.

Fortunately, the fractional ohm resistors we use rarely go off value much, they usually either open completely or go WAAAAY off, and your meter is perfectly fine for determining that.

If you really can't stand not knowing, there are meters that can read fractional ohms, some meters have four test leads for this. Two for the actual reading, and two more to check for probe error.

[Rocketrob]

Well,I measured the two 1 ohm resistors in series and got 2.5 ohms; then I clipped the test leads together and yes, there is 0.5 ohm resistance. This would not concern me much except when measuring bias through this 1 ohm resistor. If we are using this method to find the current in mA, it would seem that the difference of 0.5 ohm would make calculating the bias current inaccurate if you did not compensate for it.

[R.G.]

Well,I measured the two 1 ohm resistors in series and got 2.5 ohms; then I clipped the test leads together and yes, there is 0.5 ohm resistance. This would not concern me much except when measuring bias through this 1 ohm resistor. If we are using this method to find the current in mA, it would seem that the difference of 0.5 ohm would make calculating the bias current inaccurate if you did not compensate for it.

You're confusing some concepts.

You don't measure the bias through a 1 ohm resistor. You measure the voltage ACROSS a 1 ohm resistor to estimate the current going through the resistor. While this may seem like pedantry, it's an important distinction.

That's because you want to read the current but not change it. Using a low resistance does not affect the operation of the tube much. And we have very good ways to measure voltages.

Measuring a low resistance is a demanding technical job. Low-ohms meters are specialized equipment. I would not trust any general purpose meter or a DMM to be accurate below 47 ohms. I would trust a 1 ohm, 1% resistor to be more accurate than the meter reading for that reason. It makes "measuring the resistors" an exercise in futility unless you can find a low-ohms meter with four-wire sensing and recent calibration.

As you've seen, measuring 1-ohm resistors has to take into account the resistance of the leads, the (variable) resistance of all the contacts carrying the current the meter puts into the resistor to measure the resistance.

Bottom line - if you bought 1 ohm 1% resistors, give up on testing them with ordinary meters. If you can't get over needing to measure the resistors, you'll have to either find a low-ohms meter, or do something else to measure them.

You could order 100 of them series them, and measure that. Or put a largish current through the resistor, maybe 1A, that you CAN measure accurately with a meter, and then use a second meter to measure the voltage across the resistor. What makes this work is that the current is not changed by the variable resistances of the leads and contacts. The voltage meter can probe directly on the resistor leads and side step the contacts issues. Or you could balance the voltages on two or better four of them in bridge, or some other scheme. But measuring precision 1 ohms with a dmm - not likely to be successful.

[Rocketrob]

When you measure the resistor, your leads are then in series with the resistor, so the meter sees the total of them all. The leads do not add any resistance to the resistor. The resistor is still 1 ohm while in the circuit. The voltage across it will be accurate.

When taking voltage readings, your meter has a input impedance that is very high - something like 10Meg - 10,000,000 ohms. The probes still add that extra .6 ohms. The extra .6 ohms of your meter leads would make that 10,000,000.6 ohms. That will make your readings off by less that 1/100,000th of a percent. I wouldn;t worry.

So when your meter reads 30ma, in reality, the actual current would be more like 30.00000006ma. Or something.

This makes sense to me. Thanks Enzo, and everyone else who has tried to enlighted me here. I'm entering that dangerous place where you think you might know a little something, but you really don't know anything. More dumb questions on the way i'm afraid. Rob

[tubeswell]

I'm entering that dangerous place where you think you might know a little something, but you really don't know anything. More dumb questions on the way i'm afraid. Rob

Welcome to the club

FWIW you could use 2 x 2R 1% in parallel for each "1R", which is a pedantic but theoretically even more accurate setup than any random 1% 1R, if the tolerance of the 1% 1R worries you. (Whilst you might not get much more accuracy with trying to measure 1% 2Rs, you might just get peace of mind knowing that in parallel, they are twice as likely to get somewhere pretty close to 1R in 99 cases out of every hundred.)

[Jazz P Bass]

Bias reading

I really would not get too fussy over the bias reading.
If you are running Class AB, bias "gets rid of" crossover distortion.
If biased too cold, the push/pull circuit has a kink where the tubes trade off.
Bias it hotter (more positive) until, walah, she is gone.
Much more than that you are wasting energy.

[Russ]

How about using 10R 1% resistors. You'd get a very accurate bias reading, and 10 ohms probably won't affect the sound.

[tubeswell]

Bias it hotter (more negative) until, walah, she is gone.

"Hotter" is less-negative bias voltage. "Colder" is more-negative bias voltage. 2CW. I agree about the cross-over distortion bit BTW, but anything from 60% to 85% will work well.

[Enzo]

It is simply not a critical setting. If you intend to have 34ma through your tubes and it becomes 33ma or 35ma, so what?