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Biasing your Amp

What is Biasing?

The bias of a Valve amp is set by adjusting the grid voltages to set the idle current of the output tubes. This idle current can usually be set by adjusting a variable resistor (this can be visible in the form of a variable screw on the back of the amp or mounted inside the amp chassis). If the idle current is set too high (under bias) then the valves will run too hot and the life of the valves will be reduced significantly. If the current is set too low (over bias) the tone of the amp may be thin and lacking volume.

Why do I need to Bias?

Valves vary in specification. The chances of your new tubes being exactly the same specification as your old ones is quite small. There is a good chance that they will be within an acceptable similar specification window in which case there would be no need to bias but you can't be sure that this will definitely be the case. So the reason you need to have your bias checked whenever you replace your output tubes is to obtain the best possible sound your amp is capable of and to maximize your tube life.

Do I need to bias if I change pre-amp tubes?

No. The pre amp tubes work differently to output tubes and do not generally require any form of adjustment. The Bias only needs checking when replacing output tubes such as 6L6, EL34, EL84, etc.

Do matched tubes need biasing?

YES. It is very important to have matched output tubes fitted to your amp. But it is also important that you get the bias checked when fitted. If your amp is fitted with say Two EL34 in the output stage. You will often find that amp designs are such that one tube will be pushing the signal whilst the other pulling through the output transformer and ultimately to your speaker. Now at idle current when there is little or NO signal the tubes need to be perfectly balanced. If they are not one will push or pull harder than the other. So when the signal hits them one side of the output section will be working much harder than the other and the amp has a potential of sounding bad. Particularly at high volumes where the one valve runs out of steam whilst the other is still ready to push more. This is a simplified explanation but hopefully gets the point across about the importance of using matched output tubes. The Bias would need to be set to ensure the tubes are both idling at the correct current whether the tubes are matched or not.

Mesa Boogie claim their amps don't need biasing.

Only true if you use Mesa screened tubes or tubes that have been selected to match the Boogie biasing specification or if your tube supplier has tested and selected the tubes for you. The way Boogie get away with not fitting bias adjustments to their amps is they recommend you fit only Mesa tubes. These tubes are screened and tested to within a narrow specification so that the biasing adjustment is not necessary. Most good tube suppliers can offer you any tube screened to the Boogie spec if you mention you have a Boogie amp. The tubes used and badged by Mesa are manufactured from one of the 3 main plants remaining today. The only difference is that they are tested and screened to match their amplifiers and they also have a Boogie logo printed on them. Otherwise the tubes are the same as any other produced by Russia, Europe or China.

I am not technical and want a quick way of telling if my amp is going to melt due to the bias being set too low (tubes too hot).

OK a very simple test to ensure your bias is not set too hot. If you have replaced your output tubes switch the amp on and standby switch on if you have one as if you were about to play the amp. Wait for a few minutes for the tubes to warm. Darken the room and take a look at the output tubes. You should see small red hot pin prick glows from the heater possibly at the bottom and top of the tube. This is normal. Also if you look through the holes of the Grey box plates you may see a faint blue glow. Again this is normal and indicates your standby switch has applied the High voltage to the tubes.

You SHOULD NOT see the long grey plate area glowing red or red blotches appearing on them. If the large grey plate box area glows red the tubes are under biased and running far too hot. In this situation damage can occur to the tubes and the amplifier within a short time. If you see no red hot plates the chances are you will be safe. However if the tubes are overbiased (running cold) you may not be getting the best tone from your amp.

How do I bias my amp?

If you have adjustments available on the back of the amp like on Fender Twin and the like then biasing is straight forward and requires very little technical knowledge. You will need a digital voltmeter capable of reading 30mV (usually a 200mV scale on the meter will do) and a screwdriver.

Locate the bias adjustment screw on the back of the amp. Usually there will be instructions indicating the voltage that needs to be set at the test points (again can be sockets on the back of the amp). Put your meter on the test points. Switch the amp both standby and mains switch on. Wait for a few minutes and then measure the voltage.

IMPORTANT: LOUD SPEAKER MUST BE CONNECTED AT ALL TIMES DURING BIASING AND WHEN THE AMP IS TURNED ON. Adjust the screw provided until the meter reads the voltage specified on the instructions (this is usually around 30mV since most amp manufacturers take 2 wires off an inline 1 ohm cathode resistor. The 30mV you are reading translates directly to 30mA through one tube). This voltage can be 30, 60 or 120mV depending on how the manufacturer has wired the test points. See their instructions for the amp in question.

I will not go into this in great detail in this guide if you do not have a bias adjustment easily accessible. I will however give an excellent link for you to follow if you are technically minded and want to have a go yourself. Duncanamps.com is a great place to find information about biasing and all kinds of Tube info.

Please remember there are lethal voltages present in valve amps. If you are not familiar with working with high voltages take your amp to a tech.

Hopefully that has given you some insight into what Biasing is on a Valve amplifier. Keep safe and making great music!

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Biasing your Amp What is Biasing? The bias of a Valve amp is set by adjusting the grid voltages to set the idle current of the output tubes. This idle current can usually be set by adjusting a variable resistor (this can be visible in the form of a variable screw on the back of the amp [...]

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