The last electronic retailer in my area closed down a few months prior - I now cannot source a single resistor locally
. I was forced to source all parts from the web. I wound up using a non-polarized 2.2 uF capacitor in the send circuit vs the electrolytic in the schematic - the only type available from my usual suppliers with a 400 VDC rating. This pretty much dominates the layout. Here's what I came up with:I drilled two holes in the back of the amplifier chassis for the loop. I used the preamp B+ node (B+5) which is around 320 VDC for the board power (Mojo indicates their design can operate between 150 and 350 VDC). The signal input is from the Treble pot output. The loop's signal output is to the Master Vol pot input. This is a series loop, not bypassed - always on.
After installing it we gave it a whirl with my son's Boss Chorus Ensemble pedal. Wow, very cool. We did not notice any change in power or tone.
I decided to build another one for my JMP50 clone. I had built this amp with a pre-PI MV control, so I duplicated the installation, using the B+5 node for power (~ 270 VDC), signal input from the Treble pot, board signal output to the Master Volume pot. I also took the opportunity to upgrade the OT from a Weber 5F6-A unit to a Hammond Marshall 50W replacement unit. The results were the same as my son's amp. There is no noticeable change in power (still shakes the walls), but it seems a little brighter now. That was because of the new OT. We noticed this before installing the loop. It really cranks.
I added the optional signal trimmer pots for the input and output signals to the board, but I do not think they are necessary. I have them set at a minimal signal attenuation level.
