JMP50 Clone Build, Part 10
A shakedown run with the new JMP50 clone. It lives!
After I finished the building and running through the basic tests I dragged my 2x12 cab and guitar down to the basement for a shakedown test. I started with the presence and tone controls at 5 and the channel volumes and MV at 0. Gradually increasing the MV and channel volumes, I was thrilled—and a little surprised—that everything worked the first time! The tone controls did what they were supposed to do, and the volumes worked correctly. Ramping up the MV and channel volumes, the 50W was getting loud… fast. I didn’t keep it cranked for long for the sake of marital harmony.
I did notice that in addition to what I expect is normal gain-stage noise, there was a fair bit of hum. Too much. Poking and prodding the leads going to V1-V3 didn’t seem to have any effect on the hum, so I began to think there was a problem with a ground point. Up to this point each cap can and the transformer each had their own ground point. The turret board and input jacks shared a common ground lug, drilled into the chassis between the volume pots and the input jacks (I noticed this on Greg Germino’s amps). I re-ran the input jack grounds over to the buss bar on the turret board, next to the volume grounds, and then took another lead off the buss bar from the presence end back to the same point the transformer is grounded. This helped a lot. There’s still some hum when I start to turn the amp past home playing volumes that I need to work on. I’ll probably take things apart again and ground each of the cap cans back to the same mounting point, creating a true star ground and see if that solves the problem.
Othewise, I’m still getting used to the overall sonic signature. Ch. 1 is a little brighter, and ch. 2 is a little muddier, than I would want individually. Combine them, however, and things start to sound pretty good. I would describe it as woody or stringly, but that doesn’t mean much. Higher gain than I was anticipating, but still relatively articulate. And I’m enjoying the “kerrang.” Even my wife noticed the difference and said she prefers the JMP50 over my last build, a Matchless DC30 clone.
What’s next? I just ordered a head case from Metro and will probably make some minor tweaks before I wrap it all up. I may try a .022uf coupling cap and different bright cap in ch.1, and some different values in ch.2 to see if I can tame ch.1 and make ch.2 a little more aggressive. I also want to swap out the 3A diodes for 1A fast recovery diodes and see how that sounds. I might replace the 10k bulk resisters in the power supply with Vishay/Dales just to be consistent. And, if I get really adventurous, I may try mounting the 5.6k swamp resistors on the turret board with leads down to V4 & V5, but I’m not sure how I would stake the turrets without taking the whole board out of the amp. Otherwise, there’s not much else I feel compelled to do other than get some playing time in.