The triode mu of some popular valves are:įor example, if you’re using a 6L6GC with a screen voltage of 400V,īy contrast, an EL84 running at 300V would need only: This will be enough to bias the valve almost to cut-off, and we don’t really need more than that! the mu quoted on the datasheet when operating in triode mode. That’s easy: just take the screen-to-cathode voltage and divide this by the triode mu of the valve, So how much negative voltage do we need to cover the full range? This covers the full range of useful adjustment from cold class-B to total meltdown. While if it is made very negative then it will reach cut-off. If the bias is reduced to zero volts then the valve will run red hot (and perhaps be destroyed), In most valve amps the bias voltage can be adjusted over a small range to accommodate differences between valves andĬhanges due to aging (as they get older their gm falls and less negative voltage is needed for a given operating point).īut generally it is trimpot inside the amplifier which must be adjusted by someone with at least a rudimentary understanding of what they are doing. –they should be allowed to track one another naturally. In other words, if the HT is not regulated then the bias supply must not be regulated either The bias voltage must therefore go proportionately more negative as the screen voltage goes more positive. To maintain roughly the same operating point and dissipation in the power valves, Increases, then the quiescent current in the power valves will try to increase too. If the HT (or more particularly the screen voltage in the case of pentodes/tetrodes) it is free to rise and sag in accordance with load current and mains voltage fluctuations. In most amplifiers the HT voltage is not regulated The bias voltage keeps the quiescent current in the power output valves at the safe design value.
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