How Do I set a DMM for Bias Probe Test

[bobtec]

I have been watching on You Tube how to Bias a 100watt Marshall..I want to get a Bias Tool without the Digital readout and use my own DMM...But every video they do Not tell you where to set it..I have seen some that look like there plugged into the 10amp positive jack,How do I set my DMM into the Bias Tool that doesn't have a read out so I won't blow anything up.....My DMM has a mA-Ohm positive jack and a 10amp pos,jack and a Common ground....Or I will have to buy the one with the Digital readout....

[danman]

It would be helpful to know what type of biasing tool that you bought. Some use a current sensing resistor and you need to set your meter to DC millivolts scale. Some meters are auto-ranging so you only need to set it to DC voltage and it will choose the lowest scale on it's own. If it is not auto-ranging then you need to set it yourself. In this mode,if you get a reading of say 30mv then that will be equal too 30ma of current draw. Other types of biasing tools will break the cathode circuit and give you a reading directly in ma. In this case you would need to set your meter to the lowest DC ma scale which is usually somewhere around 200ma. When using either type of biasing tool, you want to plug the red lead into the normal volts/ohms input jack on your meter. The other jack is only used when taking higher amperage readings and will not be needed for biasing. Be sure to set all volume and tone controls to zero except for a ppimv which will need to be set at 10 (if your amp has a ppimv installed). Always have a speaker load connected while biasing and let the amp warm up about 10-15 minutes before starting the process.

[bobtec]

It would be helpful to know what type of biasing tool that you bought. Some use a current sensing resistor and you need to set your meter to DC millivolts scale. Some meters are auto-ranging so you only need to set it to DC voltage and it will choose the lowest scale on it's own. If it is not auto-ranging then you need to set it yourself. In this mode,if you get a reading of say 30mv then that will be equal too 30ma of current draw. Other types of biasing tools will break the cathode circuit and give you a reading directly in ma. In this case you would need to set your meter to the lowest DC ma scale which is usually somewhere around 200ma. When using either type of biasing tool, you want to plug the red lead into the normal volts/ohms input jack on your meter. The other jack is only used when taking higher amperage readings and will not be needed for biasing. Be sure to set all volume and tone controls to zero except for a ppimv which will need to be set at 10 (if your amp has a ppimv installed). Always have a speaker load connected while biasing and let the amp warm up about 10-15 minutes before starting the process.

ThanksThe bias tool I wanted to get is made by a Hong Kong company called Ablerocku on ebay...He has different ones I saw one with a digital readout called a Tube Bias meter -199 for $64.99 usa.The one that needs a DMM is called a QuadTube Amp Bias Tester it is $49.99usa I have a $40.00 "Ideal" DMM It just has a Sel button and a Hold button when you turn the dial to AC or DC Volts above it says Auto I think it is only good up to 400Volts but has a 600 volt overload protection..The Dial has AC V. DC V. (R) OHMS,Capacitance, AC/DC Current HZ,UA,MA 10A C,F(temp) Thanks

[danman]

Check your meter's instructions to see if it will read DC ma & mv. Ideal makes alot of products for electricians but they do not always have the features that you need for electronic work. If your meter will read in both ma & mv then you should be ok. When you get your bias tool it should tell you in the instructions if you need to use the mv scale or the ma scale.

[bobtec]

It reads both AC/ DC MV and MA...

[danman]

That should do what you need. Just be sure to check the bias tool's instructions for which method it uses to take the reading.

[bobtec]

That should do what you need. Just be sure to check the bias tool's instructions for which method it uses to take the reading.

My DMM says it has a overload protection of 600Volts... I thinks it only goes to 450Volts.. Is this safe to check my Power Tubes Voltage?...I have a 100watt 1959SLP ri Thanks

[danman]

I don't know the exact plate voltage of that particular amp but I do believe that it will have 450vdc or a bit higher at the plates. You may want to invest in another meter with a slightly higher rating. Harbor Freight tools sells a decent little meter for under $30.00 that will take the voltage of your amp.

[syscokid]

I was taught that because of all the stuff flying around in high voltage amps that we need to use meters that have at least a CAT.II or 1000volt rating....

Correct me if my wrong...

[bobtec]

can I check the Plate voltage without the One Ohm resistor on pin 8...I am going to get a Bias Probe first just to start out...I am still afraid of my 100 w.Plexi....I get nervous and have slipped the red probe off... just in a practice run...I still do not know what I am doing yet....

[ivan H]

Hi, yes u can check plate voltage without the 1 ohm resistor, it wont affect the reading. Here's a tip, cover the red probe with heatshrink tubing (preferably double walled), leaving just the tip of the probe exposed, that way if the probe accidentally slips it wont short to other pins etc. Cheers