Mini Amp

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I originally intended to build everything using one case (Altoids box), unfortunately the audio output transformer is too big to be stuffed inside the case. So, the filament transformer had to be inserted on another case, which became the separate power supply. The 21LU8 was the tube I planned on using until I found a 6LU8 in my junk box, which is just like the 21LU8 except it has a six volt filament. The 6LU8 is a compactron that consists of a high gain triode section and a beam power pentode section. FYI: The tube actually was designed for the vertical circuits in a TV. Below is a schematic diagram of the Mini Amp and the separate power supply.

The triode section was used as a pre-amplifier, while the pentode section was used for the output. Since I only had a four-terminal strip, I decided to tie one side of the filament, one side of the audio output, and the B- all together on one terminal, which I labeled COM (common). This method eliminates the need of extra terminals and works nicely. Note the polarized plug, this is important to prevent the possibility of being shocked from the terminals.

The amplifier outputs at least 1W and works very nicely. The amplifier will produce 60-cycle noise when you touch the audio input, so it also can be used as a crude signal tracer.

Pictures:

Front View Rear View

Mini Amp's Innards

Power Supply

Power Supply's Innards