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How to bias.....

5/5/2003 12:54 AM
Prive	How to bias..... Please dirct me to an article like, how to bias your tube amp in 5 minutes or something like that. Thanks.

5/5/2003 11:22 AM
pef	It's easy, you need a Dmm and a couple of 1ohm resistors. First, replace all the cathode wires with 1ohm resitor. (1 per tube) All pots must be to 0 (minimum) Connect you Dmm across one of the resistor and the this Dmm at mV caliber, then power on your amp, stand by on after 30 sec. Then you can read directly the bias current... ( ohms law), what you read is what you have ! To know the current to apply to the tubes ( bias ) make this calculation; (Wa.max / plate voltage) * 0.7 Wa. max = maximum anode power dissipation ( el34=25w , 6l6= 30w etc...) Plate voltage = the voltage directly measured on the pin 3 of the power tube 0.7 is for the security. Always run a tube at 70% of maw power. Take a first reading, bias... wait 10 minutes, take a second reading, re-bias... Make this 3 times. Take aware to read the plate voltage each time ( this one decrease with the bias adjustment) and recalculate the bias current. More questions ? Pef.

5/5/2003 11:22 AM

pef

It's easy, you need a Dmm and a couple of 1ohm resistors.

First, replace all the cathode wires with 1ohm resitor. (1 per tube)

All pots must be to 0 (minimum)
Connect you Dmm across one of the resistor and the this Dmm at mV caliber, then power on your amp, stand by on after 30 sec.

Then you can read directly the bias current... ( ohms law), what you read is what you have !

To know the current to apply to the tubes ( bias ) make this calculation;

(Wa.max / plate voltage) * 0.7

Wa. max = maximum anode power dissipation ( el34=25w , 6l6= 30w etc...)
Plate voltage = the voltage directly measured on the pin 3 of the power tube

0.7 is for the security. Always run a tube at 70% of maw power.

Take a first reading, bias...
wait 10 minutes,
take a second reading, re-bias...

Make this 3 times.

Take aware to read the plate voltage each time ( this one decrease with the bias adjustment) and recalculate the bias current.

More questions ?

Pef.

5/5/2003 7:08 PM
hasserl	Try this site: http://www.people.cornell.edu/pages/mt24/Amp/mm/mm_main.html There are a couple of articles on measuring, calculating and setting the bias. HTH Hasse

5/6/2003 1:36 AM
steveR	CAUTION NOTE: If you've never messed around with a tube amp before beware that there are voltages involved that can kill you, or at the very least knock you across the room on your ass whimpering. I recommend using a DMM with alligator clip leads and making a measurement connection with the amp off. Turn the amp on, take the measurement, turn it off, move the lead for the next measurement. I know this takes longer and can be kind of a pain but it's worth a lot in peace of mind and helps me be more methodical and consistent. If you are confortable working with these voltages and know where to probe, then there's always the suggestion of "live" probing with your non-probe hand in a pocket (so you don't accidentally touch something with the other hand and complete a circuit through your heart!) I don't have a meter that will measure current so this is how I how I have been measuring bias lately: Measure the plate voltages by connecting the red(+) lead to one of the plate pins (coming from the OPT) and connecting the black (-) lead to ground. Set the meter to measure DC volts and turn the amp on. Note the highest value (after about 20 - 30 seconds). Shut the amp off and move the red lead to the other plate pin. Power up and measure the other side. Make a note of these values. Measure the DC resistance of each half of the output transformer by putting the red (+) lead on the center tap (probably connected to the junction of some diodes coming off of the power transformer or the cathode of a tube rectifier) and put the other lead on the plate pin of one of the power tubes. This should be done with the amp OFF. Set the meter to read resistance. Make a note of the reistance and then move the lead on the plate pin to the other power tube plate pin and measue that value. They should be pretty close but are rarely identical. This value is roughly the impedance of each half of the OPT. Set the meter to measue DC volts leaving the leads connected to the center tap and plate. Turn the amp on and read the voltage measurement. Do this for the other side. Divide this value by the resistance value measured for this side of the transformer and you get the current. Multiply the current by the voltage measured for that tube and you get the power dissipation. As Pef said this should ideally be somewhere around 70 percent of the maximum rated value for the tube. How you set this value will be determined by the bias method design of the particular amp. If it's cathode biased and the bias is way off you will have to replace the cathode resistor with another value. If the amp is fixed or grid biased and there is a bias adjust pot you will adjust this to get in range. Repeat the steps above after adjusting the bias. HTH steveR

5/6/2003 1:36 AM

steveR

CAUTION NOTE: If you've never messed around with a tube amp before beware that there are voltages involved that can kill you, or at the very least knock you across the room on your ass whimpering.

I recommend using a DMM with alligator clip leads and making a measurement connection with the amp off. Turn the amp on, take the measurement, turn it off, move the lead for the next measurement. I know this takes longer and can be kind of a pain but it's worth a lot in peace of mind and helps me be more methodical and consistent.

If you are confortable working with these voltages and know where to probe, then there's always the suggestion of "live" probing with your non-probe hand in a pocket (so you don't accidentally touch something with the other hand and complete a circuit through your heart!)

I don't have a meter that will measure current so this is how I how I have been measuring bias lately:

Measure the plate voltages by connecting the red(+) lead to one of the plate pins (coming from the OPT) and connecting the black (-) lead to ground. Set the meter to measure DC volts and turn the amp on. Note the highest value (after about 20 - 30 seconds). Shut the amp off and move the red lead to the other plate pin. Power up and measure the other side. Make a note of these values.

Measure the DC resistance of each half of the output transformer by putting the red (+) lead on the center tap (probably connected to the junction of some diodes coming off of the power transformer or the cathode of a tube rectifier) and put the other lead on the plate pin of one of the power tubes. This should be done with the amp OFF. Set the meter to read resistance. Make a note of the reistance and then move the lead on the plate pin to the other power tube plate pin and measue that value. They should be pretty close but are rarely identical. This value is roughly the impedance of each half of the OPT.

Set the meter to measue DC volts leaving the leads connected to the center tap and plate. Turn the amp on and read the voltage measurement. Do this for the other side.

Divide this value by the resistance value measured for this side of the transformer and you get the current.

Multiply the current by the voltage measured for that tube and you get the power dissipation. As Pef said this should ideally be somewhere around 70 percent of the maximum rated value for the tube.

How you set this value will be determined by the bias method design of the particular amp. If it's cathode biased and the bias is way off you will have to replace the cathode resistor with another value. If the amp is fixed or grid biased and there is a bias adjust pot you will adjust this to get in range.

Repeat the steps above after adjusting the bias.

HTH

steveR

5/6/2003 3:53 AM
Richie{~}==:::	what amp is it? Sometimes it depends on the amp..sometimes they are easy..and others are not easy to get to. Some are circut board amps.. and Some amps may be cathode biased which is different. So it would help to know what amp it is, and i'm sure people could explain it alot better. Richie

5/6/2003 6:10 AM
Prive	Ok, thanks a lot, my amp is a Marshall 100W super lead, with a Metaltronix mod in the preamp. Power stage standard with 4 EL-34. It has a bias preset in the PCB. Thanks again, saludos, Marcelo.

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