Blues Jr Conversion

Blues Jr Conversion

This is one of my earliest amp builds, completed in 2014. I had been using an early 90s Tweed Fender Blues Jr for blues harp, but then upgraded to a modified 5E3 Deluxe clone (which is still my favourite harp amp). The 'green board' Blues Jr then became a backup guitar amp at my weekly blues jam, but I was never happy with the sound. I did a lot of the Bill M mods and changed the speaker, but nothing made me happy.

Eventually, I decided to rip out the PCB and replace the electronics with a hand-built circuit. My choice was to use the popular Marshall 18W TMB circuit in a simplified form, with a single input, single channel, no tremolo. SACRILEGE! Crossing Fender with Marshall sounds like a genetic experiment of the worst kind, but I must say I'm happy with the result, and so are the guitarists who have used it.

I bought a kit of parts from Triode, and recycled the Blues Jr chassis, power transformer, and output transformer (an uprated Heyboer). Here are the before and after shots:

Now you may have noticed that the speaker in these shots is not the original Eminence ceramic, but a vintage Rola AlNiCo. This turned out to be too inefficient, and has been replaced by a lovely Australian made Lorantz ceramic, which is very similar to a Celestion Greenback - a nice match for the Marshall circuit.

The conversion was not without its challenges, but the effort was worth it. I ended up with an unused socket, and no reverb, so my final modification was to add a 12AX7 driven reverb circuit with a new spring tank (not shown here) and both intensity and dwell controls.

In the hands of a killer guitarist, this amp now really sings. Here is a clip from New Years Eve 2017 featuring Mojo Webb using the amp...