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Tech Talk with Steve Wilson:
   What is the Bias and How Does it Work?

A vacuum tube amplifies by taking a small voltage on the Grid (input) and giving you a larger voltage on the Plate (output). The Cathode emits the electrons (negative) which are attracted to the Plate (positive). The Grid serves dual purpose in this plan, it is the input for signal voltage (AC), and acts as a "Gate Keeper" for the tube. It does this Gate Keeping with a small, negative DC voltage. Like voltages repel and opposite voltages attract. Since the Grid is negative and the Cathode is negative, the Grid will try to repel or push away the signal coming from the Cathode that is heading for the Plate to be amplified. Without the negative Grid voltage, the tube would try to keep amplifying and amplifying until it burned up. With too much negative Grid voltage, the tube will not conduct enough and is said to be in Cut Off. If you have too much negative Grid voltage (Bias) the tube will not work at all, and if you have too little Bias the tube will burn up or "exceed its maximum plate dissipation ratings".

The correct setting for the Bias is somewhere between Cut Off and Burn Up. Most modern tubes made today can handle a range from about 15mA to 45mA of cathode current. The cathode current is directly related to how much Bias (negative grid voltage) there is. By adjusting the Bias Voltage on your amp you can adjust the quality of sound you get from your tubes. That's where the BIAS KING comes into the picture.

Just put the socket of the BIAS KING in your amp, put a tube in the socket and turn on your amp. The BIAS KING's digital readout is calibrated directly in milliamps so you know exactly how much current each tube is drawing. Adjust the bias on your amp until you achieve the sound you like. Take note of the reading on the BIAS KING so when its time to put a new set of matched Svetlana tubes in your amp, you will be able to recreate that sound you've grown accustomed to. With the BIAS KING you can accurately and consistently check your amp for the correct Bias for each set of tubes.
 

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