Fixed bias guitar amp2/14/2023 In this case though the voltage doesn't depend on the quiescent current which means you don't have to calculate or probe it.įor power amp tubes, one has exactly the same type of concept since they are just "bigger" pre-amp tubes. This is just another way to do the same above but it doesn't have any feedback capabilities and is easier to adjust(since you don't have to change out a resistor). The other method is to use fixed bias which applies the voltage to the grid directly. To my ears the best value sounds around 1.5k. You can change the grid resistor from about 0 ohms to 3k ohms and still get a good usable signal. For your first tube you do not want this unless you want to get distortion from your clean sound. This is how we get pre-amp distortion but we do it by sending in a larger signal than we normally should. If your guitar signal's transients are too large then they will clip(the tube can't turn off any further no matter how large the signal is). You find the right amount to give you the most gain and the most linearity and the lowest noise. So if you have 1.5V on the cathode due to cathode bias putting 0 volts on the grid will set the tube to be somewhere between completely on and completely off. 0 volts on the grid on a cathode biased tube is really the negative of the cathode bias. 0 volts on the grid allows current to flow and anything above this allows even more. Below about -4 volts the tube will cut off as it cannot turn off any further. A normal 12AX7 can take a grid input from about 0 volts to -4 volts. The standard resistor value is about 1.5k which puts the grid about 1.5V below the cathode(or the cathode 1.5V above the grid). The capacitor fixes the voltage which ultimately depends on the quiescent current. This reduces the gain but increases the linearity of the signal. This has the same effect as lowering the grid voltage below ground by the same amount.īy reducing the capacitor size or even eliminating it you introduce negative feedback. What this does is effectively create a voltage source in series with the cathode with raises it above ground. Those applied to the preamp tubes generally use cathode bias which is a small resistor with optional capacitor in parallel.
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