Spunky Tube Amplifier

Posted on Feb 7, 2014

The diagram below shows that power supplies don`t have to be complicated, especially in low-power designs. The filaments are powered directly from the 120 volt AC line, minimising the size of the transformer(s) required to drive the amplifier. Note the slightly unusual series-parallel arrangment; each pair of 35C5 output tube filaments is in series with one half of the 12AU7 dual triode. The center-tap of the 12AU7

Spunky Tube Amplifier
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sums the two branch currents (150 mA each) into the preamp tube, giving 300 mA as required for this valve`s heater supply. While unusual, this geometry is eminently stable and safe. If any tube is removed (or its filament burns out), none of the others will suffer as a result, unlike some series-wired television sets where a single burnout could take out several other tubes at the same time. Each branch of two 35C5`s drops 70 volts. Add in the 6V drop across each half of the 12AU7, and the 9V drop across the 9JW8 input tube, and we have 85 volts. The two dropping resistors R1 and R2 each drop 36 volts (150 mA x 240 ohms = 36 V), for a nominal line input voltage of 121 volts. Because of this dependance on 120 volt line voltage, this design is unfortunately not practical for 240 volt AC supplies. Incidentally, the two dropping resistors get quite hot during normal operation. I suggest cementing them to the chassis, and placing a small heat-sink on the other side to help dissipate the generated heat. The heatsinks supplied with fans for 486 processors are perfect for this. (. just in case you wondered what the CPU heatsink was doing in the pictures!) The B+ supply is very conventional, consisting of a small plate transformer (150 volts either side of center-tap), a full-wave rectifier, and typical CRC filtering networks. The idea here is to get approximately 165-200 volts DC at the first filter output (B1) at quiescent...

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