DC filaments for 300B
My Vaic302B(300B equivalents) tubes draw 1.8A of filament current (the 1R 5W resistor was later changed to a series of two 0R33 5W resistors). It's quite hot if DC filaments were to be used. In fact, the entire amp is quite hot. Probably need to put in the real aluminum side panels to help dissipate heat rather than the current aluminum-sprayed wooden panels. It's actually quite a nice optical illusion. Most people did not know the panels were wood.
So, I did up a small prototype to see if I can fit the thing into the already cramp enclosure. A capacitor multiplier that provides a theoratical 4.7F of filtering with the help of a TIP142 darlington NPN transistor. The TIP142 has a hfe of 1000. This is followed by a LT1083 current regulator. Two os-con capacitors were used for convenience's sake(I have a bag of 100uF 20V). One after the TIP142, the other after the LT1083. Turns out that the heat sinks may be too small. Buzzing became apparent after around 1hr of audio. It's interesting to note that a hum pot on the filament with DC heating does not yield as much as impact on reducing hum as when it is on AC heating.
Guess it is back to AC heating for this amp, and living with the slight hum.
