How to Bias Tube Amp with Multimeter? (7-Step Guide)

Before we carry on and describe how to bias a tube amp with a multimeter. Check out the table below.

What you need to know about how to bias a tube amp with a multimeter

Within your typical tube amp, we have various key equipment that help amplify your guitar signal. These equipment include power transformers, pre-amp tubes, power tubes and filtering capacitors.

Every now and then your amplifier might create distorted signals, or produce a sound that is not as clear as you would like.

The reason this happens is because your tube amp is most likely not biased to the correct set point.

So in theory – your output signal will either have high levels of distortion or clipping as the signal moves from the anode to the cathode during amplification.

The result:  A wide variation of unwanted noise.

Fortunately, there are ways to fix this.

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New Replacement Tube Amps

Another reason to bias your pre-amp/power tubes might be because you might want to change them, and get new ones.

It is important before we get into the details on how to perform such a task, that you are fully aware that underneath the container of your amplifier are electrical components that can carry very high voltages, even when they are powered off.

So I would strongly recommend that you switch off your tube amplifier and let it stand for at least 30 minutes before doing any work.

If you are not comfortable carry this out on your own. Contact a technician or a qualified handyman who can help you.

If you have decided you are fairly comfortable with the inner workings of your tube amplifier, then great… let learn how to bias a tube amp with a multimeter.

Equipment You’ll need to bias a tube amp with a multimeter

Before we get into the details, you will need to setup your workstation with the following equipment: 

1. Bias Meter
2. Matching set of power tubes
3. Multimeter with probes
4. A load (i.e. a resistor/even a speaker)
5. Basic Tool Box
6. Container to keep bolts in one place

The video below is one of the best I have found to show you exactly how the process works.

How to bias a tube amp with a multimeter (7-Step Guide)

Step 1 – Remove cover and expose the amplifier circuitry

Ensure that the amplifier has been turned off for at least 30 minutes.

Use your screw driver from your tool box to remove the bolts from the housing of the tube amplifiers. When you are done you should see something like this.

Step 2 – Remove and replace tube amps from the amplifier

Pull out the tube amps from their socket positions. A key note to consider here is that you should be conducting this replacement in a low dust environment.

The worst thing that can happen is dust filtering into the tube sockets.

Therefore quick replacement is the key in this step.

You want to plug in your bias probes together with the new set of matching power tubes.

Once this is done you are setup to connect your bias meter.

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Step 3 – Connect to your bias meter

Once you have plugged in the bias probes and the power tubes. You should connect the bias meter using connectors to the bias probes.

Make sure that during this step you bias meter is flat on a table and is relatively stable. Try by all means to avoid workspaces that have underlying vibration due to nearby construction work. 

Reason being we don’t want any physical disturbances when we are measuring the milli-amps in later steps.

Step 4 – Connecting Multimeter to the Power Tube Plate

Next take the crocodile clip and connect it onto the anode of any one of the power tubes.

Here it is important that you check the datasheet of the specific power tube you are using.  

For example, using the EL 84-TK datasheet, the anode (a) is on pin 7, as below.

Take the negative crocodile clip and connect it to ground. This is typically the metallic casing of your entire amplifier.

Once that is done, turn the multimeter on to DC Voltage.

Then, connect your amplifier to your speaker, and plug in the power from the wall and turn on your amp.

Leave the amp on standby mode for for about 2 minutes. Then start the next Step.

Step 4 – Turn of STANDBY mode

Once everything has been setup, you should not see any voltage on your multimeter. This is because the amplifier is on standby mode.

The reason we do this is to give the brand new tubes a bit of power to warm up before we take them off standby.

It is generally good practice to do this when using an amplifier. It will extend the life of the amp and make sure it operates effectively for much longer.

After a short period turn off the STANDBY mode and you should see a voltage on your multimeter. This voltage will be the Plate voltage that we will use in the next step.

Step 5 – Use the Biasing Tube Amplifier calculator

Now Google  - “Biasing Tube Amplifier calculator”. There are so many of these tools on the web today.

Do not get bogged down by the multitude of options, they practically do the same thing. Pick one and go with it.

All you need to do is provide 2 inputs.

• First: Name and details of your power tube (usually drop down menu);
• Secondly: The Plate Voltage you got in Step 4.

Then calculate your bias point.

What will then happen is that you will get an array of bias set-points.

60% Bias Point – 110 % Bias Point, with increments of 10%.

The standard Bias Point is typically 70% - therefore choose this one and you should then get a current value in mA associated with your selected bias point.

Great we are almost done, onto Step 6.

Step 6 – Biasing the Tube Amp

Now you want to locate the potentiometer within your circuit board. Use your thin screw driver to slowly tune this variable resistor, until you get a milli-amp reading on your bias meter that is equivalent to your set point value in Step 5.

Once that is done, your tube amps are effectively biased correctly.

Step 7 – Disconnect and put back the amp cover

At this stage switch off the power on the amplifier and the wall socket.

Disconnect the multimeter and the bias meter.

Remove the bias probes and install the tube amps directly into the sockets.

Put the enclosure back together, screw back the nuts and bolts.

That’s it. Well done.

Read More:>>>Best Multimeters under $50 to help you bias tube amp

Final Thoughts of how to bias a tube amp with a multimeter

As you can see the process is quite simple. However, as always. I do caution you to operate with the highest levels of safety in mind. Inside an amplifier there are power equipment and capacitors that can hold a charge for longer than the power is off.

So please handle with caution.