Marshall Amp Forum - View Single Post - The JCM 900 and modification... Thoughts?

danmahoney · 11-20-2009, 02:09 AM

Hi Guys,
I`m actually waiting for the parts right now to mod my 1990 JCM 900
(4101 combo) to run on 6L6s (Tung Sol 6L6 STR`s).
I know most people go in the other direction but I really love the clean sound I get out of the 900 series with the right 6L6s in there.
I`ve got the spec here that covers what Marshall recommends in order to do the mod.
I`m planning on only changing R31-34 from 2K2 to 470 ohms, and R28 from 220k to 47k. Marshall recommends a lot more.
My question is will I potentially be damaging my amp by doing the minimum I`m planning?
I could change the other resistor values while I`ve got the board out but I don`t have the right cap values to start changing those, and I`d prefer not to start messing around with the leads on the power tranny (unless it really should be done in order to keep the amp running safely).
Here`s a link with detail on the mod from Marshall:

http://www.cowboymafia.net/frankie/5...conversion.pdf

Any suggestions and advice would be much appreciated. Thanks.
Dan

11-20-2009, 02:09 AM	#49 (permalink)
danmahoney Junior Member Join Date: Nov 2009 Location: Montreal, Canada Posts: 21	Re: The JCM 900 and modification... Thoughts? Hi Guys, I`m actually waiting for the parts right now to mod my 1990 JCM 900 (4101 combo) to run on 6L6s (Tung Sol 6L6 STR`s). I know most people go in the other direction but I really love the clean sound I get out of the 900 series with the right 6L6s in there. I`ve got the spec here that covers what Marshall recommends in order to do the mod. I`m planning on only changing R31-34 from 2K2 to 470 ohms, and R28 from 220k to 47k. Marshall recommends a lot more. My question is will I potentially be damaging my amp by doing the minimum I`m planning? I could change the other resistor values while I`ve got the board out but I don`t have the right cap values to start changing those, and I`d prefer not to start messing around with the leads on the power tranny (unless it really should be done in order to keep the amp running safely). Here`s a link with detail on the mod from Marshall: http://www.cowboymafia.net/frankie/5...conversion.pdf Any suggestions and advice would be much appreciated. Thanks. Dan __________________ JCM 900 (4502) (Tung Sol 6L6 STRs) (1999) JCM 900 (4101) (Tung Sol 6L6 STRs) (1990) JTM 30 (Vintage Tung Sol 5881s) (1995) JTM series (1x12") ext cab (1995) 1912 ext cab (1997) Last edited by danmahoney; 11-22-2009 at 01:44 AM.