My son had been asking me to put an effects loop into a JCM-800 clone I built for him about 15 years ago. I was really not interested in the surgery necessary to add a tube-buffered loop, so I scoured the web for a solid state loop. I was familiar with Metro loop, but ran across schematics for the Mojo SS loop. Both designs use LND150 MOSFETs in the send and return circuits. I decided to try and roll my own from the Mojo schematic.
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.
SS Effects Loop - JCM-800 and JMP50 Clones
Moderators: pompeiisneaks, Colossal
- statorvane
- Posts: 557
- Joined: Thu May 11, 2006 3:28 pm
- Location: Upstate New York
2 others liked this
SS Effects Loop - JCM-800 and JMP50 Clones
You do not have the required permissions to view the files attached to this post.
- FUCHSAUDIO
- Posts: 1241
- Joined: Fri Apr 08, 2005 2:48 pm
- Location: New Jersey (you got a problem with that ?)
- Contact:
- pompeiisneaks
- Site Admin
- Posts: 4222
- Joined: Sat Jan 14, 2017 4:36 pm
- Location: Washington State, USA
- Contact:
Re: SS Effects Loop - JCM-800 and JMP50 Clones
Very cool! BTW I just noticed somehow you duplicated this post, so I deleted the one without any comments
~Phil
~Phil
tUber Nerd!
Re: SS Effects Loop - JCM-800 and JMP50 Clones
Nice! Looks really good!
If anyone else wants a SS buffered effects loop I designed a PCB that is a similar layout a few years ago and the design/layout is up on Oshpark as a shared design (you can just order boards). The PCB also has a reverb driver (with a signal compressor/limiter) and recovery on the PCB.
Links:
https://oshpark.com/shared_projects/YfJ9Qc2t
https://tfrelectronics.com/Buffered-Eff ... op-Reverb/
If anyone else wants a SS buffered effects loop I designed a PCB that is a similar layout a few years ago and the design/layout is up on Oshpark as a shared design (you can just order boards). The PCB also has a reverb driver (with a signal compressor/limiter) and recovery on the PCB.
Links:
https://oshpark.com/shared_projects/YfJ9Qc2t
https://tfrelectronics.com/Buffered-Eff ... op-Reverb/
https://tfrelectronics.com/
https://oshpark.com/profiles/TFRelectronics
https://oshpark.com/profiles/TFRelectronics