Mini Amp

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The mini amp was originally intended to be a project that fit entirely in one Altoids box, but ended up using two. The 21LU8 TV compactron tube was the original choice for the design until I found a 6LU8 in my junk box. The 6LU8 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. 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. I highly recommend using this with an isolation transformer or figure out a way to get everything contained in a single box without exposed power supply lines!

The amplifier works very nicely and a quick test of the amplifier's sensitivity is by touching the audio input to produce a 60-cycle hum noise at the speaker. It could be used as a crude signal tracer for troubleshooting radios.

Front View Rear View

Mini Amp's Innards

Power Supply