• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Fixed Bias: the ups and downs

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alexmoose said:
What are the purpose of the Schottky diodes?

The output of the transformer is about 140vac. Gotta make that DC. The schottky diodes produce less noise than regular diodes, and are not expensive. You could probably substitute regular diodes with good results, as the current is pretty low.

If you are using a center tapped mains transformer and can afford to lose a few B+ volts, you can also do something like shown in the link below to make a - supply. This was for a CCS, so you'd need more filtering to make it quiet enough for a bias supply. The amp in this example draws about 180mA, so 0.18A*150R/2=13.5V at the junction of the two 75R resistors. This one actually is somewhat fail-safe, in that as the output tubes conduct more, the neg. bias voltage increases, in turn reducing the bias current. http://www.diyaudio.com/forums/attachment.php?s=&postid=622719&stamp=1113901355

Sheldon
 
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