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How To: Tube Amp Biasing


Whether you currently own or are considering exploring the wonderful world of tube amplifiers, there are a few things you should know to make sure you get as much enjoyment and life out of your valve gear as possible! I am of course referring to tube bias - an important part of amp care that will increase tube life and the overall sonic performance of your stereo.

Looks That Can Kill 

We can’t stress this enough - make sure proper care is taken whenever operating or making adjustments to your tube amplifier. Even when unplugged, the amplifiers capacitors are charged with enough high voltage to be fatal! Most is common sense but if you are ever uncertain, seek the help of a professional.

 

 

General tips,

-Make sure the amp is unplugged before changing tubes or parts.
-Tubes get hot, don’t touch them.
-Safety glasses are always a good idea!
-Wear short sleeves and remove any rings or jewelry.
-When prodding an amp keep one hand at your side - you don’t want to complete a circuit with your arms. Current going through your heart is bad!


What is Amp Bias?

No matter the manufacturer, how good the design is, or the quality of the parts used, tubes will eventually wear out. They also don’t wear at even rates meaning eventually some tubes will be working harder than others. Without getting into too much detail, changing the bias controls the amount of current that flows through the vacuum tubes of your amplifier, with each having an optimum setting. If the bias is set too low (often referred to as running cold) the amp can sound thinner and less dynamic, and generally won’t be performing at its best. If set too high (often referred to as running hot) it can sound harsher, less accurate, and can greatly reduce the lifespan of the tube.

Simply put, we want to make sure all the tubes are working (roughly) at the same strength and thus have the appropriate amount of current flowing through them. Left unchecked, we risk not only blowing a tube, but risk damaging other components of the amplifier.

There are 3 different types of tube amp bias:

Self Biasing/Cathode Bias: As the name suggests, these amplifiers are essentially ‘plug-and-play’ and will not need to be tinkered with if tubes are being replaced. Enjoy your music and if you blow a tube, just replace it with a matching one and you’re ready to go!

Adjustable Fixed Bias: Amps that fall into this category include an internal or external potentiometer/knob that with some basic knowledge, can be adjusted with ease to set the bias.

Non-Adjustable Fixed Bias: This category can be the most labour intensive when it comes to setting the correct bias. They use a fixed resistor soldered into the amp to set the bias and for the most part, can be used with a wide variety of tubes that have the same operating characteristics as the ones being replaced. This means it is important to buy matched sets of tubes because it is harder to optimize them.

For this quick guide we will only be looking at Adjustable Fixed Bias amplifiers, which will be the most common for hi-fi valve-amps. If you are uncertain which you currently own, check the owner's manual. Most manufactures will even include instructions on how to measure and adjust tube bias for their products. They will more often than not give you the actual range to set the bias to as well.


When Should I Check The Bias?

If your amplifier is getting regular use (2+ hours a day), you should at a minimum check the bias every 3-6 months. If you notice the sound has started to change, become noisy, or a tube isn’t lighting up as it used to, it could be a sign the bias needs to be set. If you are replacing tubes the bias should always be checked before listening.


Tools Needed 

-A multimeter with optional alligator clips
-A set of plastic flat-head screwdrivers

 


How To Proceed

To ensure you get a proper reading, the amplifier must be on and warmed up. We recommend waiting around 30+ minutes after the amp has been powered on before checking. NOTE: Always make sure your tube amplifier is plugged into your speakers! Without a load you won’t just get an improper reading, you risk destroying the transformers which can be a big expense.

If you are fortunate enough to own an amplifier whose manufacturer included external test points for your multimeter (eg. Lab 12Air Tight, Audio Research), adjusting bias is a breeze!

*Audio Research VS115 Pictured

Find the test points on your amplifier labeled V1, V2, etc. Set your multimeter to DCV > 200m. Insert the black and red ends into the proper test points and note the reading on your multimeter. Locate the bias trim/knob labeled V1, V2, etc, (usually located on the top near the front of the amplifier) and with your screwdriver make very slight turns and watch the readout change. Once you get the bias set to the manufacturer's recommendation, move on to the next. It is wise to go back and re-check the readings after doing all of the tubes, as sometimes ones previously biased can be effected by as much as 10-20%.

If your amplifier does not have the bias points and trim pots accessible there will be a bit more work needed.

First you must open the amplifier chassis to expose the electronics within. You will have to (carefully) look for two things. First find the bias test point which should labeled and located near the tube you are testing.


You will then need to find the corresponding bias trim pot with the matching V#. They come in many forms but should be labeled and resemble the photo below.


Attached the black lead of the multimeter with the alligator clip to the metal chassis of the amplifier - remember to keep one arm at your side. This is to ground the connection on your multimeter. Next put the positive lead of the multimeter to the bias test point and check the reading. Adjust the trim pot until the bias is set to the correct number. Repeat this on the rest of tubes and as mentioned before, re-check after adjusting all of them to make sure there aren’t any fluctuations.


Sit Back and Enjoy!

This is a very basic guide for checking the bias of a tube amp. There are always exceptions and with many different amp types, these instructions may vary. Whenever in doubt, ask a professional for help and avoid any risks - especially if your amp doesn’t give you such easy access to bias points and trims. With a little elbow grease you can enjoy the many sonic benefits of your tube amp for years to come!


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