Quote: Originally Posted by the_equalizer /img/forum/go_quote.gif Another thing I don't yet understand: does the source-gate voltage have to be around 5 volts ? Can it be higher? How much higher? Could the 12AU7 version work with the same voltage divider as the 19J6 version (e.g. with the gate sitting at 24 volts with respect to ground) ? I can see that this voltage (along with the source-drain voltage) determines how much voltage the MOSFET can swing, but I haven't yet sat down to carefully read the datasheet and run some experiments with the amp Vgs is a property of the Mosfet and changes depending upon how much current is going through it. That is, there is a correlational relationship between Ids and Vgs. This can work in two ways -- first, as here, as the current increases, Vgs also increases, and as the current decreases, Vgs decreases. Vgs is pretty much always close to 4.5V for the IRF510 and IRF610, and just moves up a small amount with different currents -- too small an amount for concern on this amp. However, you can use it the other way, too. That is, by carefully controlling Vgs, you can control the amount of current that goes through the mosfet thus making it a constant current source. So, for instance, you can use a resistor divider from + to bias the gate. Then, by varying a resistor between the source and ground, a resistor that drops voltage across it, you can control the voltage between the gate and the source, which controls the amount of current that can flow. So, in the Starving Student, why is it not acting like a CCS? The reason is because the mosfet is biased into saturation. The amount of voltage we have between the gate and the source is too high to be able to control the current, so the mosfet will let basically any amount of current flow. Instead, the current is limited by the heater which, at the bias voltage of the mosfet minus Vgs (~19V), can only draw 150mA thus limiting the current across the mosfet. You can think of the heater as the resistor used with an LED -- the LED will pass full current and burn itself up without a current limiting resistor inline with it. Here, the heater acts as as the limit. The mosfet differs from the LED, though, in that it has an extra leg whose voltage with regard to the cathode is set by the current between the anode and the cathode.