18W Jensen Amp

18W Jensen Amp

I've pointed out before that as a woodworker, I make a good butcher, so I tend to get cabinets custom built for my amp projects, or adapt an existing piece of furniture.

In this case, I was unable to resist, about 5 years ago on eBay USA, a timber extension speaker with a mint 1958 Jensen AlNiCo 12" speaker. I knew I would eventually find a use for it. Here it is before I began...


I would have liked to build a Valco style amp with octal tubes, but there wasn't enough room. Sometime later I acquired an aluminium chassis of suitable size, designed for  2x EL84 output tubes, and decided to build a simplified Marshall 18W amp with just one input and three controls - Gain, Tone and Master.

This is where I ran into trouble. The ideal design for a valve amp involves having the input and pre-amp as far away as possible from the power supply and output tubes. Also the audio signal should run in as straight a line as possible from input to output, with all wiring as short as possible.

Problem! I was faced with having the input located right next to the power supply, and the audio signal zig-zagging across the chassis. On initial power-up, the amp was VERY noisy, with lots of hum, buzz and whistles. I changed my initial layout drastically, taking most capacitors off the board, and placing them as close as possible to the pots and tube sockets. Ditto with some of the resistors. I also used shielded cable for all but the shortest wire connections. Now with the volume full up there is still some noise, but the amp is really quite usable. Phew! Here are photos under the chassis early and then late in the process.

The '18W Jensen' has turned out to be a cracking harp amp. Lightweight, with a unique look, it really cuts through the mix, and the separate gain & master volume allows you to control the amount of overdrive. Another winner!