JOLIDA SUPPORT

THE BIAS SETTING

Among the most regular question we get for support, there is : how to set the bias.

First, what is the BIAS ?

the bias is the cruise current which pass through each tube. Because all vacuum tubes for Hifi are still handcraft made, every tube has a different internal resistor.

So if you remember your physical courses in college, you may have heard about U=RI

In case you don't remember, don't worry, to be basic, what you must just remember is that because a Hifi tube is handcraft, its internal resistor may vary from one tube to another tube. Because the current with change depending the resistor, each tube will have a different "cruise" current.

Because you have 2 tubes pair channel, it is very important that these tubes have the same current, in order not to make cross-distortion.

But that is not the only point. If you have too much current going through one or several tubes, there is a risk to solicit too much the power transformer and make it heat too much and also shorten a lot the tube life length. For example if you use a KT88 in push-pull and if the BIAS settings is 40mV; let's imagine you don't set the bias and the current goes to 70mV; the tube life length can be divided by 10 times.

For those of you who remember a little bit their physics, you may wonder why we mention Volts for a current. Well, here it becomes a bit more complicate, we still use U=RI and make the bias measurement thought a resistor and that give a voltage on the meter. If we use a 10 ohm resistor, that means this a 40mV voltage means there is a BIAS current of 40mA.

THE KEY POINTS TO SET THE BIAS

1) most of our amplifiers have the EASY-BIAS system so you don't need to use a meter

2) the BIAS must be set with no signal, CD on pause

3) the BIAS setting will be accurate only after 10 minutes period of heating. Before this heating period, you can have a variation of +/-10% of the bias. During at the starting of the amp, you can just make a pre-setting, but the final setting can be done only 10 minutes after you turn-on the amp.

FOR EASY-BIAS USERS

you have a green or red (depending models) LED besides each tube

The setting is good when the light is switch on. HOWEVER, when you play the music, these LED can turn on or turn-off depending the music. Please don't pay attention of the LED when the music is playing. You should consider these LED ONLY when there is no signal.

Bias Values :

JD102B : 200mV

JD302B, JD302BRC, JD303 : 40mV

JD502B, JD502BRC, JD505 : 50mV

JD801A, JD801RC, JD801S : 50mV

JD1000RC : 40mV

Music Envoy : 250mV

Please don't go above these values. You can safely go below, up to 20%. The more you will lower the value, the more it will increase the tube life lenght, but the more the sound quality will be affected.

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