biasing method

myker · Post by **myker** » Sun Mar 06, 2005 12:47 am

this 1 ohm resistor biasing method? 1 ohm between pin 8 and ground. what voltage drop are you supposed to be measuring? i just used the reading on the plates multiplied by the reading on pin 5 to get static dissipation...what am i doing wrong?
mike

Flames1950 · Post by **Flames1950** » Sun Mar 06, 2005 1:04 am

When using the one ohm resistor method you just measure across the resistor (one probe on one side, one on the other) with your meter set to the lowest DC range you've got. You should get a reading in millivolts.
The whole thing works by Ohm's Law, where your [bias current = (voltage across the one ohm resistor) divided by (the resistance of the resistor)] and since the resistance is one, you effectively get that your [bias current = your voltage drop across the resistor.]

myker · Post by **myker** » Sun Mar 06, 2005 11:20 pm

thanks flames! i tried it and it worked
many thanks to ya buddy!
mike

martin g · Post by **martin g** » Sun Mar 13, 2005 2:51 pm

Hi what is correct value for 100 watt super lead in mV?

Flames1950 · Post by **Flames1950** » Sun Mar 13, 2005 4:51 pm

martin g wrote:Hi what is correct value for 100 watt super lead in mV?

The correct value is really determined by your plate voltage, and will vary a little bit by your own ear, and what sounds good to you.

martin g · Post by **martin g** » Sun Mar 13, 2005 5:55 pm

Ok but if set the bias to high or to low may I damage tubes? I use 4 * El34 so is there any recommendable value?

tooloud · Post by **tooloud** » Sun Mar 13, 2005 5:59 pm

35 Miliamps across the 1 ohm resister....

Dax-The-Ax · Post by **Dax-The-Ax** » Mon Mar 14, 2005 3:30 pm

Here is a bias chart, if you can read it.
Too loud is right, for around 470 volts, about 30 to 40 Ma. This number can be + or - depending on which way your meter is set.

5150loveeddie · Post by **5150loveeddie** » Mon Mar 14, 2005 4:16 pm

can someone explain the other method (more dangerous but more precise...) I remember Jon talking about it, something with power trannie and something else?????????????

Country Boy Shane · Post by **Country Boy Shane** » Mon Mar 14, 2005 5:19 pm

Order your tombstone...

5150loveeddie · Post by **5150loveeddie** » Mon Mar 14, 2005 6:37 pm

Hey Shane everytime I see your picture I get that funny feeling you had garlic for lunch

Gtr_tech · Post by **Gtr_tech** » Tue Mar 15, 2005 1:45 pm

The method you speak of involves shunting half the OT primary with an ammeter, or a multimeter set up to read *current* (very important). Not recommended for Marshalls or other english amps (low dcR primary, won't give an accurate reading) but fine for Fenders and the like. One lead goes to the center tap of the OT, the other goes to the plate of the output tube (or pair) you're measuring. Plate (+screen) current reads directly on meter. 4 tube amps (parellel pairs) will read current for BOTH parellel tubes, so remember to figure half the reading to get one tube's idle current.

Don't try this unless you're ABSOLUTELY sure you understand what you're doing.
Use a good quality (Fluke, Beckman, etc) meter. Cheap meters are notorious for bogus I readings, not to mention they probably aren't intended to be used with 400v+, even though the scale may go that high.

Remember that when the meter's set up for reading current, there is a very low resistance from one lead to the other (for I sensing) and if you have one lead connected with the chassis off stby, there will be HV on the other probe. Connect leads (preferrably minigrabbers), then hit the stby sw, read, back on stby. DON'T touch the chassis at anytime when handling a connected lead, even on stby (caps can still have a charge, depends on location of stby sw in circuit). HV through your heart can kill it dead.....really dead. Ampeg's and their sky high plate supply still give me the willies.

I still remember the first (and last) time I got jolted with HV. Back in my high school days I was messing with my Traynor 1-12 el84 combo after replacing filter caps, and found the HV winding by mistake. It took me exactly 3 cycles of AC to figure out what happened. Made a believer out of me. Better safe than dead.......

Country Boy Shane · Post by **Country Boy Shane** » Tue Mar 15, 2005 3:42 pm

5150loveeddie wrote:Hey Shane everytime I see your picture I get that funny feeling you had garlic for lunch

Awesome. Right now it's green peppers.

Guest · Post by **Guest** » Wed Mar 16, 2005 1:03 pm

Gtr_tech wrote:The method you speak of involves shunting half the OT primary with an ammeter, or a multimeter set up to read *current* (very important). Not recommended for Marshalls or other english amps (low dcR primary, won't give an accurate reading) but fine for Fenders and the like. One lead goes to the center tap of the OT, the other goes to the plate of the output tube (or pair) you're measuring. Plate (+screen) current reads directly on meter. 4 tube amps (parellel pairs) will read current for BOTH parellel tubes, so remember to figure half the reading to get one tube's idle current.

Don't try this unless you're ABSOLUTELY sure you understand what you're doing.
Use a good quality (Fluke, Beckman, etc) meter. Cheap meters are notorious for bogus I readings, not to mention they probably aren't intended to be used with 400v+, even though the scale may go that high.

Remember that when the meter's set up for reading current, there is a very low resistance from one lead to the other (for I sensing) and if you have one lead connected with the chassis off stby, there will be HV on the other probe. Connect leads (preferrably minigrabbers), then hit the stby sw, read, back on stby. DON'T touch the chassis at anytime when handling a connected lead, even on stby (caps can still have a charge, depends on location of stby sw in circuit). HV through your heart can kill it dead.....really dead. Ampeg's and their sky high plate supply still give me the willies.

I still remember the first (and last) time I got jolted with HV. Back in my high school days I was messing with my Traynor 1-12 el84 combo after replacing filter caps, and found the HV winding by mistake. It took me exactly 3 cycles of AC to figure out what happened. Made a believer out of me. Better safe than dead.......

I have been using the shunt method for a while but now I am concerned.

How inaccurate would my readings be? ( I did not know about the low dcR on the primary) I ordered some 1 ohm resistors from George yesterday since I figured after reading Gtr_tech's post and what Lord Valve says about the shunt method on Marshalls ( the same thing) maybe I shouldn't use it. I just want the most accurate method possible ( whatever that may be for Marshalls).

white room · Post by **white room** » Wed Mar 16, 2005 1:38 pm

Uhhh, that would be me