WHAT IS BIAS GUITAR AMP FULL

The bias current is small compared to the full power current that the power supply must deliver. Even though the transformer runs hot the survival rate is high even to this day. Do you mean that if the idle current gets too high the transformer will get too hot? If it does, just turn it down man. I do not agree that ' even a small bump up in bias can make surprisingly large demands that the stock transformer just can't handle'. However this introduction has nothing to do with bias. Thats about $650 in todays money and one would be hard pressed to do it for that today. When you consider he offered the kit for $99 its amazing even in that day. He would argue over a few pennies in a transformer for the preamps and got a bit in trouble there. My dear friend Bruce DePlama worked for David Hafler and admitted David was tough on the suppliers. Granted the stock ST-70 power transformer runs hot.

In both cases, the thing being manipulated is the voltage differential between cathode and control grid. 'Fixed bias' means the voltage is applied to the grid that is more negative than the cathode voltage, and sometimes this voltage is made adjustable, so you might hear the term 'adjustable fixed bias.' Whereas 'cathode bias' means the cathode voltage is made more positive than the control grid voltage by placing a resistor in the cathode circuit that when current flows through it, it creates a voltage potential at the cathode that is more positive than the voltage at the control grid.

Regardless of which method is used, the above three (triode) or four (pentode) parameters must always be set for the tube to be biased. The terms 'cathode bias' and 'fixed bias' are also used routinely to describe the method by which the voltage differential between cathode is control grid is provided. The technically correct definition of a bias point for a triode a set of three values that define the tube's quiescent (zero signal) operating point, and for a pentode, it's a set of four values that define the tube's quiescent operating point.įor a triode, the bias point is defined as:ī) The voltage drop across the tube from plate to cathodeĬ) The voltage differential between cathode and control gridįor a pentode, the bias point is defined as the above three, plusĭ) the voltage differential between the screen grid and the cathodeĪll three (triode) or four (pentode) must be simultaneously stated to completely specify the 'bias point' or 'quiescent point' for the tube in question.įor the output stage of a power amp, amp manufactures will sometimes provide a user adjustable mechanism to set the output stage bias by allowing the voltage difference between cathode and control grid to be adjusted within a certain predetermined range, which once set, will automatically set the other values. So it's really important to get the biasing set right so the best performance of the amp can be obtained.

Get the biasing wrong (too hot or too cold) and the output signal may not be able to swing to its full potential as designed by the manufacturer, or it may cause significantly more distortion than expected by the manufacturer. Bias too hot, and your tubes won't last (and your amp could sound like s**t.)īiasing matters greatly because it's the center point around which the output signal swings. Bias too cold, and your amp will sound like s**t. We care about bias because it relates directly to the performance of the amplifier as well as tube life. if they say 'biased at -35V', maybe they mean they set the negative voltage on the control grids to -35V context is everything. If they say 'biased at 2.6V', well, maybe that means they adjusted a bias control until they read 2.6V at the test point. if someone is referring to 'biased the tubes at 50mA', that's a measurement of the quiescent current. When biasing 'cold', it means 'less current' (and a more-negative Vgk). When someone refers to biasing 'hot', it means 'more current' (and a less-negative Vgk). What this ultimately controls is the amount of quiescent current that flows through the tube, and where along the load line you center the operating point. Bias refers to the difference in voltage between the control grid and the cathode, or Vgk, at quiescent conditions.

What is bias guitar amp