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CENTER TAP NEGATIVE BIAS VOLTAGE

Cathode bias has its plus's and minus's. Fixed bias has its plus's and minus's also. But I won't go over them here. However, as opposed to having either a separate power transformer, or a separate winding, there is another alternative source for negative bias.
 
 




The resistor on the left from the tap to ground is the actual current limiting resistor from which is derived the bias voltage. It can be from 100 to 250 ohms, and can drop from -10 to -30 volts or more, depending on amplifier current draw and resistance value. The horizontal resistor and vertical one on the right can be either a potentiometer or two fixed resistors for voltage dividing. I would use values in the tens of K so as not to draw much current, since control grids draw virtually no current. The filter capacitor is of sufficient size to filter the 120 hertz hum, but not much more, so that if line voltage varies, the capacitor will keep up with the change.

For example, if 10K and 33K are used for the horizontal and vertical resistors respectively, then one would use 10% of the vertical, or 3.3K and plug it into the capacitance formula, then one gets a capacitor value of 0.4 µF. This is ideal because we can use the nice poly or paper caps instead of electrolytics. I haven't tried this one yet, but one could also bypass the main resistor on the left with a filter cap. But then one may need an electrolytic, since its value will have to be about 133 µF. But this is a low voltage, so it can be a cheap off the shelf Radio Shack capacitor with a 35 volt level (don't forget to put the positive lead to ground, though!). For that matter, the 0.4 volt one can be cheap too. The only thing about bypassing the resistor is though that it will raise the ripple at the B+, since this resistor also acts as a sort of a degenerative feedback.

If you must have regulation, a simple SS zener will do. Then simply put the grid resistors instead of from grid to ground, from grid to the output of this point. One can also use a few 10's of K linear taper pots to adjust bias, if one wishes.

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