Tube Amp Biasing at Home

Today’s blog post is courtesy of Nathan Sanford from Sanford Magnetics.  Nathan is a top notch designer of Humbucking and P-90 style pickups and he is located all the way up in Moncton, New Brunswick, Canada.  His company is Sanford Magnetics and you can see his website at www.sanfordmagnetics.com.  He will also be introducing a new amplifier to his brand in the near future.  I’ve heard the prototype of the amp and it is nothing short of stellar!  Nathan brings a lot to the table when it comes to pickup design and manufacturing.  He is our first guest blogger for J.Rockett Audio Designs.  Nathan’s topic is on how to do your own tube biasing at home.  Enjoy the article!

Tube Amp Biasing At Home

This is pretty basic and by no means a comprehensive lesson for doing amp work, but it’s served me well over the years. Some may find it basic, to those who always wondered, it may be insightful. I’m not an expert or a tech, just someone who needed to do it myself.

Learning to bias an amp will save you a lot of trouble over the course of your playing life. Much like learning to do your own guitar setups, you’ll be able to adjust your amp for your needs and tube roll at home without having to see a tech. If you do your own guitar work, you know how valuable a skill that is. With a little know-how and working safely, you can have a lot of fun finding what works best for your rig.

The key to making this easy is a device known as a bias probe.  A tool you can plug in between the tubes and the sockets of your amp to get the readings you’ll need. It’ll pay for itself in no time.  The Weber Bias-rite is the one I’ve been using for years, but there’s a few different ones on the market now.

Tube amps are a little bit dangerous to go prodding around in, so be aware that 5 milliamps across the heart can kill you. Never make an adjustment with a guitar around your neck. Never do this bare foot or in the bathtub, use common sense and nothing will bite you.

Make sure your amp is well supported, I use a couple stacks of 2X4’s drilled together for a cheap DIY cradle.  When you do anything inside your amp, stick your left hand in your pocket so you don’t accidentally touch something you shouldn’t. If you are a careless person, don’t try this.

Biasing an amp is really just simple math.  Watts/voltage=current. You’ll need to know the maximum wattage rating for the tube you’re using.  The KT66 I’m using in my RT66 is a 25 watt tube.   Typically, 70% of maximum tube dissipation at idle is what I work around..some say 70% is the maximum, but that’s somewhat subjective.

1.)  With the Bias-rite in place, I get a voltage reading, then hit the switch to get the current draw of the tube.

2.)  With my Rt66, 70% of 25 watts gets me 17.5 watts a tube. 17.5 watts/459v = .038 amps.

3.)  With the amp on in play mode, You’ll want to make small adjustments to the bias pot  and observe the bias probe as you make the changes.

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 I set the amp to 35.6ma per tube…for 16.34 watts. Under the 70%, but after playing the amp, I’m happy with the results.

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Now, that’s the basics. Where this gets fun is experimenting with different amps and tubes and what you want to get out of them.

Looking for more headroom, playing cleaner stuff, set the bias on the cold side, say 65%. If you have an old Marshall, set it higher, then play test the amp. You’ll let your ears be the judge of what bias setting works for you and your amp. There’s no real rule, just what your ears tell you is right.  Too cold and your amp will sound like garbage, too hot and you’ll wear your tubes out prematurely…but that could be worth it to some guys, so be mindful of shortened tube life.  I once had a tech tell me he biased everything cold. He was worried about a tube failing and the amp combing back for warranty repair…didn’t matter if the player preferred the amp set on the hot side.  Really, it’s more about experimentation than anything and finding what works for you. These simple tools and some math can open up another level of tweaking for us eternal tinkerers.

Thanks,
N Sanford.

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Chris Van Tassel