a much more powerful anode current, resulting in amplification. When used in its linear region, variation in the grid voltage will cause an approximately proportional variation in the anode current; this ratio is called the transconductance. If a suitable load resistance is inserted in the anode circuit, although the transconductance is somewhat lowered, the varying anode current will cause a varying voltage across that resistance which can be much larger than the input voltage variations, resulting in voltage gain. The triode is a normally "on" device; and current flows to the anode with zero voltage on the grid. The anode current is progressively reduced as the grid is made more negative relative to the cathode. Usually a constant DC voltage ("bias") is applied to the grid along with the varying signal voltage superimposed on it. That bias is required so that the positive peaks of the signal never drive the grid positive with respect to the cathode which would result in grid current and non-linear behaviour. A sufficiently negative voltage on the grid (usually around 3-5 volts in small tubes such as the 6AV6, but as much as –130 volts in early audio power devices such as the '45), will prevent any electrons from getting through to the anode, turning off the anode current. This is called the "cutoff voltage". Since beyond cutoff the anode current ceases to respond to the grid voltage, the voltage on the grid must remain above the cutoff voltage for faithful (linear) amplification as well as not exceeding the cathode voltage. The triode is somewhat similar in operation to the n-channel JFET; it is normally on, and exhibits progressively lower and lower plate/drain current as the grid/gate is pulled increasingly negative relative to the source/cathode. Cutoff voltage corresponds to the JFET's pinch-off voltage (Vp) or VGS(off); i.e., the voltage point at which output current essentially reaches zero. This similarity is limited, however. The triode's anode current is highly dependent on anode voltage as well as grid voltage, thus limiting the voltage gain. On the other hand the JFET's drain current is virtually unaffected by drain voltage, thus it appears as a constant-current device, similar in action to a tetrode or pentode tube (high dynamic output impedance). Both the JFET and tetrode/pentode valves are thereby capable of much higher voltage gains than the triode which seldom exceeds 100. However the power gain, or the output power