Biasing the 1ohm resistor way.

[nee]

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!

[j-po]

The solution is to get good quality 1% resitors. Now the problem is do you trust that same dmm to measure the millivolts over the resistor...

[nee]

The solution is to get good quality 1% resitors. Now the problem is do you trust that same dmm to measure the millivolts over the resistor...

Good point.

[Firestorm]

The solution is to get good quality 1% resitors.

+1 to that. But you need a good DMM -- a Fluke -- also remember that when you read cathode current, you're also seeing screen current in addition to plate current.

[Phil_S]

Set your meter for the lowest range. Put the two probes together. What is the reading?

There is always some internal resistance. Great meters (there are three kinds of great meters: Fluke, Fluke, and Fluke) take this into account and should give you an accurate reading.

Since the 1 ohm method includes screen current, which many people ignore as nominal, chances are that your bias will be set a bit colder than what you think. This is more safe than sorry.

If the shunt method makes you uncomfortable (like it does to me), measure the internal resistance of the OT secondary. Use that number and measure the voltage drop across the OT to calculate actual plate current. This should be very accurate, however, it is probably more trouble than it's worth.

Just get high tolerance 1 ohm resistors (1%), and go with what you read. As a practical matter, this is probably good for most players and amps. There is no need to obsess about its precision.

[Structo]

How much does the screen current affect the bias current reading across the 1 ohm resistor?

Is there a percentage?

Or does the plate voltage make a big difference in that you can say for all power tubes the screen current affect the bias reading by so much?

In other words, how much does the added screen current add to the cathode current you are measuring?

For example I set the bias on my D'Lite that has two Winged =C= 6L6's in it at 41ma. This amp has 435v on the plates.

How much of the 41ma is screen current?

[txbluesboy]

The best thing to do would probably be measure the voltage drop across the screen resistors of the amp in question. Use ohms law to calculate screen current, the subtract this from the value read across the 1 ohm cathode resistors. However this might be overkill.

[David Root]

Don't forget the power rating. I used to use 1/4W until I saw a post here, I think it was from drz400, where he had 1/2Ws blow out on him because of a failure elsewhere, and it was a critical time situation. Takes time to pull out the chassis and replace 'em. So he recommended min. 3W. I have been using Mills 5W ever since. Mills because they are small enough to bridge comfortably between meter probe sockets set normally apart, not because I like to spend money.

[jaysg]

Most audiophile designs I've seen use cathode resistors - 1 or 10 ohms. If you're really into it, get 0.1% resistors instead of 1%. What's missing is that you might want to dial in the 70% figure, let the amp stabilize and/or play it a while, recheck, then tweak around that point while playing, to see if it sounds just a bit better.

[Structo]

Who makes them and where would you buy the 1 ohm .1% resistors?

Would using a 10 ohm resistor add any accuracy to the reading?

[nee]

Set your meter for the lowest range. Put the two probes together. What is the reading?

It's an auto-ranging meter. When I put the probes together it reads 0.000ohms. 1ohm resistors consistently read in the 2.5ohm region. (This is using various types and makes of resistor, so they're not all faulty.) But, as I said in my original post, if I add a 20ohm resistor to the circuit the 1ohms turn out to be around 1.0 to 1.2ohms.

[Phil_S]

Can you manually select the low range? What brand/model meter are you using? Ever been calibrated? Down at that level, you really have a sensitivity problem in some meters. If you are picky, you need a Fluke that's been calibrated.

[novosibir]

How much does the screen current affect the bias current reading across the 1 ohm resistor?

This differs a lot from tube to tube, between types and also between brands of the same type - impossible to say something in general!

Just a few examples from my measurements:

100W OT w/ Zpp = 1.75K; Screen R's = 1.0K

EL34 RFT: B+ = 470V; Ip = 35mA; Is = 4.5-7.5mA
EL34 RFT: B+ = 467V; Ip = 38mA; Is = 5.5-9.0mA

EL34 Mullard: B+ = 470V; Ip = 35mA; Is = 3.2-5.8mA
EL34 Mullard: B+ = 467V; Ip = 38mA; Is = 3.8-7.5mA

100W OT w/ Zpp = 2.2K; Screen R's = 470

6L6GC Sylvania: B+ = 465V; Ip = 40mA; Is = 1.2-2.4mA
6L6GC Sylvania: B+ = 462V; Ip = 45mA; Is = 1.7-3.0mA

6L6GC GE: B+ = 465V; Ip = 40mA; Is = 0.9-1.8mA
6L6GC GE: B+ = 462V; Ip = 45mA; Is = 1.2-2.2mA

Larry

[billyz]

Not sure I trust this method. In theory it should work but in practice I have found discrepancy. I just worked over a modern Marshall with the 1 ohm resistors to bias with. Could not get a good sound when the el34's measured as suggested in the marshall manual. Finally I used the Transformer shunt method and found the 1 ohm resistor test points way off and uneven too.

After I set the bias by the transformer shunt method the amp sounded good.

I am using several FLuke models mostly 87's and they all read the same.

[jaysg]

Who makes them and where would you buy the 1 ohm .1% resistors?

Would using a 10 ohm resistor add any accuracy to the reading?

I was going to say mouser...the first one I found was $74 a pop...no good. These are 0.5%.

http://www.mouser.com/Search/Refine.asp ... 4294609806

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