Question about biasing an amp (shunt method)

[Dr. Tweedbucket]

If you use the shunt method to bias an amp, do you have to tweek to twice the number that you want current wise? ie: 70 mA on the meter gives you 35mA through the tubes?

The amp is a Marshall superlead. I've been running the current meter between pin 3 of the power tubes (one side at a time) and the center tap of the output transformer... and setting my amp to 35mA. Recently a bias rite showed it was only drawing 18 mA ..... so maybe I've been shunt biasing my amp wrong ??

[davemccarthy707]

Yes you have to see double on this amp as it is 100watts. Therefore you are measuring both tubes together which pushes one side of the tranny (70ma). If the amp is 50 watts then you are using a single tube to drive one side therefore what you read is your bias (35ma). Clear as mud?

[Dr. Tweedbucket]

Yes you have to see double on this amp as it is 100watts. Therefore you are measuring both tubes together which pushes one side of the tranny (70ma). If the amp is 50 watts then you are using a single tube to drive one side therefore what you read is your bias (35ma). Clear as mud?


Yeah, silly me! I got so used to only running two tubes in my 100 watt amps and biasing them that way that I forgot that the current was divided by two if running all four tubes.


Thanks!

[phsyconoodler]

The transformer shunt method is a rather dangerous way to check the bias current.It is better to use 1 ohm resistors on the cathodes of the power tubes.Saves you slipping and frying something.

[Dr. Tweedbucket]

The transformer shunt method is a rather dangerous way to check the bias current.It is better to use 1 ohm resistors on the cathodes of the power tubes.Saves you slipping and frying something.


So, what is involved in that? soldering 1 ohm resistors to the cathode pins and reading current between that point and the centertap or ?

[crobar]

You have to set the bias from the plate voltage readings..Then you get the -ma bias reading..

[phsyconoodler]

See the wire that connects the cathodes of the power tubes to ground?(pin 8).Remove that wire and put a 1 ohm,1%,2 watt resistor in it's place.Now fire up the amp and measure across that resistor on the ma setting of your meter.It converts directly to mv with the 1 ohm resistor in place.You measure from pin 8 to ground.Nowhere near the centre tap.

Because you are measuring only mv,you are in the safest spot you can be in the amp.It's quite accurate and safe.I put ports on the back of my amp so I can do it externally.


So say you get a reading of 35 mv,or .035mv.That converts directly to milliamps.

[GearHeadFred]

Hey - what's up Doc Tweed?! It's Fred, I purchased your Ceriatone JTM45 a few years back!

When I received that head from you, I checked the bias with a bias-rite and it was in the right range.. I think I may have tweaked it up slightly hotter, but nothing radical. I think you told me you had set it using the Trans shunt method.

I guess the argument for Trans Shunt method is that it's the most accurate?? But phsyconoodler is right - it's very dangerous!! Bias-rite or 1 ohm cathode resistors are a much better way to go. Be careful !!

[Deric]

...measure across that resistor on the ma setting of your meter.It converts directly to mv with the 1 ohm resistor in place.

Measure across the resistor using the mv setting of your meter. It converst directly to ma with the 1 ohm resistor in place...

[Jerry]

Randall Aiken also has another way of measuring bias. It's under the advanced section, "The last word on biasing". It's the third section, where you'll measure the resistance of each side of the output transformer.

http://www.aikenamps.com/

[hasserl]

Randall Aiken also has another way of measuring bias. It's under the advanced section, "The last word on biasing". It's the third section, where you'll measure the resistance of each side of the output transformer.

http://www.aikenamps.com/

Measure/record the resistance of each half of the output transformer primary. Then measure the voltage drop between center tap and each leg and divide that amount by the resistance you recorded earlier. That gives you the current draw, is the most accurate measurement, and requires no resistors to be added to the amp.

[AR-305]

If you use the shunt method to bias an amp, do you have to tweek to twice the number that you want current wise? ie: 70 mA on the meter gives you 35mA through the tubes?

The amp is a Marshall superlead. I've been running the current meter between pin 3 of the power tubes (one side at a time) and the center tap of the output transformer... and setting my amp to 35mA. Recently a bias rite showed it was only drawing 18 mA ..... so maybe I've been shunt biasing my amp wrong ??



http://www.ionpool.net/arcade/tech/Weber_Bias_Calculator.html

[Prairie Dawg]

A number of the methods described here are not for the faint of heart as they involve working in close quarters on a live chassis with working voltage applied. I do not like the feeling I get from incidental contact with it. That is why I will never use anything but a bias probe that I made myself from a tube base, a tube socket, some coaxial cable, a banana plug and a precision 1 ohm resistor that allows me to sample cathode current. It's plenty close enough for my needs.