Biasing Guitar Amplifiers. What’s Involved?

Biasing Guitar Amplifiers. What’s Involved?

Since we supply valves for guitar amplifiers we are often contacted to ask us about biasing guitar amplifiers.

This is a highly confusing subject for the non-technical and for good reason. It IS confusing!

The first thing to say is that although all the valves in your guitar amplifier are biased, it is only the output valves (the big ones!) that concern us. Biasing guitar amplifiers is not rocket science but it’s important to have the bias set correctly on your output valves.

Biasing guitar amplifiers involves adjusting the bias on your output valves to set the ‘idle current’ flowing through the valves. This is the DC current which is there when no signal is going through the amplifier.

Maybe a good analogy might be the tickover rate of a car. You don’t want it ticking over so slowly that the car threatens to stall; neither do you want it revving away as it uses a lot of fuel and shortens the engine life.

Like all analogies this one is not exact.

Bias Current Too Low on Your Guitar Amplifier

When biasing guitar amplifiers , if you set the bias current too low on your valve amplifier there are two consequences.

1. The valve life will be extended because it is running cooler. That’s a good thing.

2. The sound will be thin and lacking in punch. That’s a bad thing.

Bias Current Too High on Your Guitar Amplifier

If you set the bias current too high on your valve guitar amplifier there are also two consequences.

1. The tubes will run hot and that will shorten their life. (Aside: if the bias is way too high, the tubes will ‘redplate’ which means the anode will glow cherry red. This is a very bad sign and will dramatically shorten your tube life. If you see this, turn the amplifier off immediately and get it seen to by a qualified tech.)

2. The sound will be fatter and will break up earlier, which is what guitarists like.

Biasing Guitar Amplifiers Correctly

Having pointed out the advantages of both too low a bias (long tube life) and too high a bias (fatter sound), undoubtedly the best place to set the bias is correctly. That varies from tube type to tube type and also depends on the anode (plate) voltage. Putting it simply, any given tube needs to be set to idle (‘tickover’) at about 70% of its maximum allowed dissipation.

So how do we go about biasing guitar amplfiers?

Short answer, with extreme difficulty for the lay person! It’s a bit of a crazy system actually. Imagine that every time you fitted a new spark plug to your lawnmower, you had to take the mower to a qualified specialist to be set up properly. Or every time you changed your car’s windscreen wiper blades, you needed to book it into the garage to have delicate adjustments made to the wiper motor. I’d call that bad design!

Well biasing guitar amplifiers is a bit like that. If you change the output valves for a new set, even from the same manufacturer, they will need biasing. You CAN just pop in the new set and hope for the best, and you may get an acceptable sound, but it’s not the ideal way although a great many guitarists do exactly that.

Unfortunately, guitar amplifier manufacturers have made it hard, even for a tech, to adjust the bias on most amplifiers. I’m guessing that the reason is they don’t want unknowledgeable guitarists fiddling with a bias control (as if they would…) and risking tube burn out etc. Imagine you are a manufacturer shipping new amps with a variable bias only to have loads of them back because the users have stuck a screwdriver in the back and maxed the tube current causing tube failure. Nightmare! (Equivalent: an adjustable knob on the back of your spin drier which allows you to adjust the revs. If turned too far clockwise the drier spins out of control and breaks.)

In simple terms, biasing guitar amplifiers involves adjusting the fixed, DC negative voltage on the grid of the output valves. It’s typically about -35V in case you’re interested. The more negative this voltage, the more the output tubes are turned down and the less current will flow through the output tubes (‘cooler’ bias). The more positive this voltage, the more the tubes are turned on and the more current will flow through the output tubes (‘warmer’ bias).

Adjusting this bias almost always involves taking out the chassis and getting to the innards. This is dangerous for the lay person because of the very high voltages present in valve amplifiers. And of course the bias has to be adjusted with the amplifier ‘on’.

I won’t go into the exact details here because each amp is different. In general terms though, biasing guitar amplifiers involves two things:

1. Measuring the steady state (DC)current flowing through the output valves (no signal present).

2. Adjusting the DC negative bias on the grids of the output valves until you read the correct current for that valve (e.g. 45mA).

The bias adjustment is usually a potentiometer on the board or a fixed resistor which needs swapping out.

This is all best done by a tech and is not an expensive process.

Posted by Stuart on August 21, 2014