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.

Biasing guitar amplifiers (biasing tubes) is a highly confusing subject for the non-technical and for good reason. Biasing guitar amplifiers IS confusing!

Biasing Guitar Amplifiers 1
A Typical Bias Probe

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 your amplifier is not rocket science but it’s important to have the bias set correctly on your output valves.

Biasing amplifiers involves adjusting the bias on your output valves. We want 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 for biasing valve amplifiers is not exact.

Bias Current Too Low on Your Guitar Amplifier? Biasing Guitar Amplifiers Correctly.

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

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

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

Bias Current Too High on Your Guitar Amplifier? Biasing Guitar Amplifiers Correctly.

If you set the bias current too high when biasing your amplifier there are also two consequences.

1. Hot biasing guitar amplifiers. 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 a qualified tech to look at it.)

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

Biasing Guitar Amplifiers Correctly – Biasing a Tube Amp

6v6 valve
A Typical Power Tube

Now you know 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 when biasing guitar amplifiers is … correctly! That varies from tube type to tube type and also depends on the anode (plate) voltage. Putting it simply, we aim to adjust any given tube to idle (‘tickover’) at about 70% of its maximum allowed dissipation.

So how do we go about biasing guitar amplifiers?

Short answer, with difficulty for the lay person! Imagine that every time you fitted a new spark plug to your lawnmower, you had to take the mower to a specialist to be set up properly. Or every time you changed your car’s windscreen wiper blades, you needed to have a mechanic make 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 by adjusting the biasing circuit. You CAN just pop in the new set and hope for the best. You may get an acceptable sound, but it’s not the ideal way of biasing an amplifer, although a great many guitarists do exactly that.

Manufacturers Make it Hard!

Unfortunately, guitar amplifier manufacturers have made it hard, even for a tech, to bias their amplifiers, to adjust the bias on most amplifiers. They probably don’t want unknowledgeable guitarists fiddling with their amp biasing control (as if they would…) and risking tube burn out etc. Imagine you are a manufacturer shipping new amps with a variable bias. Suddenly a load of them come back because 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.)

Adjusting The Grid Bias Voltage

In simple terms, biasing guitar amplifiers involves adjusting the fixed, DC negative voltage on the grid of the output valves. This voltage is 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).

Biasing guitar amplifiers almost always involves taking out the chassis and getting to the innards. This is dangerous for the lay person when biasing guitar amplifiers because of the very high voltages present in valve amplifiers. And of course you need to adjust the bias 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:

Biasing Guitar Amplifiers Step One

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

Biasing Guitar Amplifiers Step Two

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 you must swap out.  If you are thinking of biasing guitar amplifiers yourself, consider investing in a bias meter as it makes the job of biasing guitar amplifiers simple. You remove an output tube, plug in the meter adapter to the empty socket and plug the valve back into the adapter. You can now read off plate voltage and current.

It is best to get a tech to bias your guitar amplifier. It is not an expensive process.

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