Simple cathode-resistor biasing question

[tweedeluxe]

Hi, I just added an external bias pot and test points on two amps, using 1ohm cathode resistors.

I don't usually use the "1-ohm cathode resistor method." When I have used it, it's been with some 10% resistors I found at RS. I didn't think it was too odd that the 1ohm resistors always measured at least 40% high, but with these two amps that I added external test points to, I bought a bunch of wirewound, 3watt, 1% tolerance resistors. The weird thing is, they all measure between 1.3 and 1.5 ohms too. Now I'm thinking that maybe my $35 DMMs just aren't accurate at measuring small resistances, which means that every time I've used this method I've overbiased the tubes.

So, my question is, which do I trust: the 1% tolerance stamp or my $35 DMM?

Thanks

[Johnhenry]

I don't trust value's, I alway's test before installing, and i would never use a 35 dollar meter to rely upon for amp building,
Just my 2 cent's,
FWIW !
John

[Alexo]

Short your meter probes together and read the resistance there. It may very well be .3 or .4 ohms, whatever it is, deduct it from what you measure off the resistors.

[tweedeluxe]

Thanks for the reply John. Unfortunately, I haven't yet been able to convince my wife that a quality DMM is a worthwile investment . I just do this as a hobby and to help friends out from time to time.

I always test first too, because I don't trust printed values either, but in this case I feel inclined to trust the 1% marking more than my meter since I'm consistently measuring 40% high on these low-value resistors. In my other pursuits I've never used anything lower than 100 ohms, which I've been able to measure accurately with my cheap DMMs.

Does anyone know if meters, especially cheap ones, are especially innacurate at measuring low resistances?

Thanks

[tweedeluxe]

ahh great Idea thanks Alexo!

[Johnhenry]

I'm retired now but, i still have guy's that call me out that i've worked for the last tweenty year's or so, i'll give you my call yesterday, he has a Fandango 100 watter, he say's it won't play clean anymore, I go to his house to check it and a Blue's Deville, the fandango run's four EL34's, i don't remember what milliamps they were set at, but Rivera like's you to set the M/A's at the output jack, anyway the roadie had bought a new cheap meter, he asked if i would show him how to use it, I showed him after i had biased the amp that my meter, a Fluke showed 37.8 VDC on pin 5, we put his meter to it and it showed 32.5 VDC, I told him to pitch it in the trash, i'm not trying to be mean, just honest, if a cheapie meter can be that far off, I don't need one,
again, this is just my 2 cent's, and everybody has to learn by thier own mean's,
John

[Alexo]

I'm retired now but, i still have guy's that call me out that i've worked for the last tweenty year's or so, i'll give you my call yesterday, he has a Fandango 100 watter, he say's it won't play clean anymore, I go to his house to check it and a Blue's Deville, the fandango run's four EL34's, i don't remember what milliamps they were set at, but Rivera like's you to set the M/A's at the output jack, anyway the roadie had bought a new cheap meter, he asked if i would show him how to use it, I showed him after i had biased the amp that my meter, a Fluke showed 37.8 VDC on pin 5, we put his meter to it and it showed 32.5 VDC, I told him to pitch it in the trash, i'm not trying to be mean, just honest, if a cheapie meter can be that far off, I don't need one,
again, this is just my 2 cent's, and everybody has to learn by thier own mean's,
John

Yikes! Guess I finally have something I want for Christmas now.

[drz400]

Hi, I just added an external bias pot and test points on two amps, using 1ohm cathode resistors.

I don't usually use the "1-ohm cathode resistor method." When I have used it, it's been with some 10% resistors I found at RS. I didn't think it was too odd that the 1ohm resistors always measured at least 40% high, but with these two amps that I added external test points to, I bought a bunch of wirewound, 3watt, 1% tolerance resistors. The weird thing is, they all measure between 1.3 and 1.5 ohms too. Now I'm thinking that maybe my $35 DMMs just aren't accurate at measuring small resistances, which means that every time I've used this method I've overbiased the tubes.

So, my question is, which do I trust: the 1% tolerance stamp or my $35 DMM?

Thanks

You cant trust the meter on such a small value
IF it is a Fluke 87 and you zero out the probes that is a different but you are talking about a $370 meter, not $35
If it makes you feel any better about the investment I have had my Fluke for 15 years and have done some horrible things to it, it still works great

[rfgordon]

It is hard to find two true 1 ohm resistors, even from a big batch, if they are the garden variety wire wounds.

If you really want accurate ones, go with the 5 watt Kiwame resistors.

[mhuss]

...or if your pockets are not so deep, 1% 1W resistors work well.

--mark

[Johnhenry]

Very good advice from Mhuss and DrZ, Long ago i had a few PA amp's come back on me, seem's i was not getting close enough on the bais resistor's on the o/p transistor's, the guy selling me the transistor's told me a lotta people were having the same problem, he told me for a few buck's extra he would sell me matched transisitor's with a set of presision matched resistor's, after buying the set i quit getting any of them back. you live and you learn !
John

[tweedeluxe]

Thanks for all the great replies guys; you all more than answered my question.

Guess I'll be putting the money I've been saving for my first 'Wreck build toward a Fluke instead , which is gonna be rough but ultimately worth if they are that much more accurate. Good to hear that they're durable as well.

[Tubetwang]

It is hard to find two true 1 ohm resistors, even from a big batch, if they are the garden variety wire wounds.

If you really want accurate ones, go with the 5 watt Kiwame resistors.

Question for In-House Resistors Guru...

I could'nt find any 1 ohm resistors for my Express bias test point...and ordered some Mills MRA-5 1ohm 1% non-inductive wire round 5 watts resistors...will these fit the bill?

My other option was either 1 ohm 10w Lynk metal oxide or 1 ohm cement type...

Where can we buy 5 watts Kiwame resistors?

Thanks!

[rfgordon]

The Kiwames I get are from partsconnexion.com in Canada. There's also a US vendor, but I can't remember the name. The Japanese company limits their sales to only one vendor per country, in order to keep the prices up. However, Partsconnexion does have several good sales each year. Great folks to deal with.

[roberto]

With a so low value resistor, many errors came out of the basket.
The first two tips coming in my mind are: test resistence value as near as possible to resistor's chassis, and test resistence value of one pin of the resistor. A little bit of superficial Oxide can change values. Pay attention on it!