jcm900-2500 mods

kirk

kirk

Member
haven't posted in a long time so i did some minor tweaking of a jcm900 2500 model and it turned out so good i thought id share it...
1st i know we all know the c13 mod,cut that bastard out![I've seen guys put these on a switch but its too bright for me and wouldn't want to drill a hole for a switch i wont use]
2nd i replaced c3 with a .05 coupling cap of your choice,i've done both ceramic and an orange drop, believe it or not i thought the orange drop was better [400 to 600 vdc]
3rd remove br3 and rebuild with 4-1n4007 diodes,this is the filament dc rectifier for the 1st 3 preamps[they did this to reduce ac filament hum in the high gain preamp stages]br3 is under rated and will eventually burn up and burn the pc,save yourself some grief and time and do not skip this step
4th tube up the outputs with jj 6ca7's,the bias will dial in but be sure to bias it correctly the stock setting is too hot
these mods really worked well and the amp sounds killer ,like an old jcm800 on steroids
 
all i understood was "killer". i really need to learn about amps. all the old dudes in my area are getting OLD... cool that it made a 900 sound beefy tho, if it sounds like an 800 on steroids i'd like to hear it. mine roars like a lion, i rarely have the bass above 5 or 6. are c3, br3, etc, markings on the pc board? i need to pick up a junky old 900, and start learning this shit. do a clip or vid of this thing if you can.

thanks; steve
 
Yeah , the only 900's I ever liked were the mkIII and slx versions. I had the 2500 and I liked the tones I got with it but it didn't seem near as loud as my older jmp 50 watters. My tech told me that these were plagued with a defective transformer in early runs. I sold it but I did liked the amp, would like to get an slx some day.
 
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