Biasing the 1ohm resistor way.

[nee]

I have no idea why the audiophiles use 10 ohms. I think that's what's in an SVT.

Yes, I was wondering about 10 ohm resistors. If the 1 ohm method is just a convenient way of applying Ohm's law, then using a 10 ohm would simply require adjusting the decimal point. What effect would a 10 ohm have on the actual operation of the amp? Or a 100 ohm for that matter?

[drz400]

Greetings!

Just wondering about the margin of error when biasing using a 1ohm resistor.

I have a bunch of 1ohm resistors, and it's difficult to get a reading of just how close to 1ohm they are on my DMM. Measuring them by themselves I always get readings in the region of 2.5ohms, which I guess is because of the DMM's (lack of) sensitivity down there.

I then measured a 20 ohm resistor and got a stable reading of 20.7ohms, and put each of the 1ohms in series, to get readings of between 21.6 and 21.9 ohms. My theory was to select the 1ohms that gave a reading as close as possible to 21.7ohms when in series with the 20ohm resistor.

So my question - if the 1ohm is actually 1.1 ohms, or 0.9 ohms, how will this affect the readings when biasing?

Thanks!

You need to set the ohm meter to 0 out the probes and use the high accuracy on something like a Fluke 87. It is difficult to get a true reading of a 1 ohm, just get 1% parts and assume they are correct

On thing about transformer shunt method I have found besides it being dangerous if you slip. As the transformer gets hotter the DC resistance changes quite a bit. When you change cathode current with EL34s you need to factor in the screen current as it has been said before. If you really want to figure it out then you can break the plate connection and put the current meter in series at leas to see how that compares to the other readings. I still like to do cathode current and double check with a look on the scope.

[Structo]

What brand and tolerance do most of you guys use.
Where do you buy them? Mouser? Got a part number?
In my D'Lite I can't really make out the brand.

When I was wrapping the leads around the test jack post I broke some of the ceramic material off around the lead.
Do you think that would harm the resistor.

[drz400]

What brand and tolerance do most of you guys use.
Where do you buy them? Mouser? Got a part number?
In my D'Lite I can't really make out the brand.

When I was wrapping the leads around the test jack post I broke some of the ceramic material off around the lead.
Do you think that would harm the resistor.

This is what I have seen Dumble use
71-RS2B-1.0
www.mouser.com

[Structo]

Just checked with Mouser and their Vishay/Dale 3 watt 1 ohm 1% is out of stock.
Lead time 15 weeks.

Both the 2 watt and 3 watt 1 Ohm 1% Vishay/Dale ROHs are out of stock.
Part # 71-CPF31R0000FKE14


I found this one which is not ROHS compliant but it's in stock.

71-CPF3-F-1

http://www.mouser.com/Search/ProductDet ... 1-CPF3-F-1

[Structo]

_______

[Structo]

OK,

Do these 1 ohm 1% biasing resistors need to be wire wound?

When I was searching Mouser it didn't dawn on me until I saw Drz400's post with that number.

Will Metal Film work or does it have to be wire wound for safety?

[Tonegeek]

mouser # for one ohm/ 2 watt: 71-CPF2-F-1/R
Been in my amp for 2 years, no problems so far.

BTW there was a convincing argument backed up by some math that 'ayan' made a while back for just using 5% resistors. IIRC you will only be off by 2-3 mv if the resistor is off 5%.

[drz400]

OK,

Do these 1 ohm 1% biasing resistors need to be wire wound?

When I was searching Mouser it didn't dawn on me until I saw Drz400's post with that number.

Will Metal Film work or does it have to be wire wound for safety?

They dont have to be wirewound, at 1ohm non of that is going to matter. It is easier to find this wattage and precision with a wirewound. These are not Rohs but who cares, they work great and Dumble uses them as well. They are in stock at Mouser
http://www.mouser.com/Search/Refine.asp ... 1-RS2B-1.0
These are also just fine
71-CPF2-F-1/R

[Firestorm]

Now that we've gotten into a precision resistor discussion, I feel compelled to point out that it doesn't really have to be all that close. In most cases, you can bias a Class AB push-pull amp anywhere between 45% and 70% (unless the power amp input is very hot or you always play with the volume on "8."). I always use the by-the-numbers bias as a starting point and adjust by ear from there. It usually winds up sounding better at less than 70% (unless it's a Class A or pseudo-Class A amp, which is a different beast altogether).

[Structo]

So what will using a 10 ohm resistor bring to the table?

Does it provide greater accuracy because of the decimal shift?

[mhuss]

Yes, ten ohms gives better accuracy using an 'average DVM,' as you're reading ~ 1/3 volt vs. 10 times less. This value does not rob much more power/voltage either. Remember, a typical 100 watt OT measures around 20 ohms (DC) per half-secondary

--mark

[C Moore]

I was just contemplating substituting a 100 ohm resistor in place of the standard 1 ohm. After reading this thread, I guess I was really going overboard.
So, for setting the bias of a fixed/adjustable amp, is the 1 ohm resistor the chosen method for most of you guys?

[drz400]

I was just contemplating substituting a 100 ohm resistor in place of the standard 1 ohm. After reading this thread, I guess I was really going overboard.
So, for setting the bias of a fixed/adjustable amp, is the 1 ohm resistor the chosen method for most of you guys?

The 1% 1ohms are like 50 cents each and that way you get a direct reading of DCV = A

[Wayne]

I was just contemplating substituting a 100 ohm resistor in place of the standard 1 ohm. After reading this thread, I guess I was really going overboard.
So, for setting the bias of a fixed/adjustable amp, is the 1 ohm resistor the chosen method for most of you guys?

I'm thinking you would probably be able to hear 100 ohms unless you bypassed it with a capacitor, but that might have a sonic penalty as well.

After reading this thread, I'm considering trying out 10R/1W in something.

W