JCM900 mods with pics and clips

[Mattbedrock]

Special thanks to balok and his JCM900 mod thread.

http://www.thegearpage.net/board/showthread.php?t=975471&highlight=JCM900

I have a JCM900 4500 (50 watt reverb head) that has set idle for years because, quite frankly, I have since bought better sounding Marshalls. This JCM900 was my second Marshall that I bought years ago. I liked channel A but never could warm up to the fizzy, shrill channel B (lead channel) on the amp.



So when I saw these mods posted, I thought, maybe I can resurrect the old beast and turn it into a usable two channel amp.

I started by recording a demo with the amp cranked up (full up on the MV's, just over half on the pre's). I tucked in some earplugs and recorded the rhythm part on channel A and the lead part on channel B. I was actually surprised at how good the amp sounded but it was at a volume that would get most any gig cut short at my local venues.

http://soundclick.com/share.cfm?id=11639997

I then turned the MV on channel B down to 4 (a reasonable volume level) and re-recorded the lead track. The nasty, shrill thin tone was back in spades at this lower volume. This is what I didn't like about this amp.

http://soundclick.com/share.cfm?id=11639994

The mod's from baloks post were actually quite easy to implement. I must admit, I was a little taken aback by the guts of the JCM900. I am used to working on PTP turret and eyelet board construction, not this PCB based monstrosity. Why there are 8 microcircuits in this thing ...



For the lifted resistors, I just unsoldered one end and wrapped the looose end in electrical tape. I jumpered the diode and cap with a little piece of teflon coated wire. Pretty easy - I was done in 10 minutes or less.

The results? I'm pleased. The raspy shrill tone of channel B is gone and it is much fuller and richer sounding to my ears. It still sounds like a distortion pedal in front of a tube amp, but that's a big improvement from where it started.

Here's a clip with the master still on 4:
http://soundclick.com/share.cfm?id=11639995

I think it sounds much better - fuller without the thin, shrill tone that was there before. Thanks again to balok and the original mod post.

[8len8]

All 3 links point to the same clip?

[Mattbedrock]

3 different clips:

997 - unmodded but cranked
994 - unmodded, mv on 4
995 - mod'd, mv on 4

[omfg51]

Cewl!!

[mixn4him]

I had the 5881 version of this amp and found that changing the speaker to match the amp worked very well without doing any mods. I just did not care for Ch A , a little too sterile sounding..Yours sounds great and congrats!

[demea26]

Had the 4502 with JJ EL34's, Drake transformer and V30's. One of the best sounding amps I've owned.
Also had a 4100 with 6L6's and 4500 with 5881's and yes, I would agree that speakers help these amps alot.

[Sparky6string]

The mod did seem to mellow out the tone some. I personally liked all three recordings, and even the one you don't like. That biting tone can be helpful when trying to get through the mix.

[GearHeadFred]

Good post - thanks for taking the time to record the samples... Nice playing.

I had a 4501 with EL34s. It was my main amp for 10+ years.. I sold it about 5 years ago. The amp does "that tone" pretty well.. I never felt the cleans were very useful, always had that dark Marshall coloring. I eventually went in the JTM45 direction.

I was watching that balok thread too and wondering if it might make me regret selling my 900. ALMOST!

[Blues Power]

I dont know whats all this shit about the 900's.
bought this amp head new in 94 and still love it. no mods at all except maybe if you want to call it one I put in straight 6L6's

I dont know maybe I got a good one.

I voice the EQ petty mcuh at 12 OC and the gain for cH. 1 at about 1 OC.

The cleans are great and I back off the Geetar vol to about 6 and its warms up nice. after that i use a sprakle drive or Aria Disnortion to push it.

I have no need for CH. 2 but if I do use it the gains on about 11 OC

same geetar vol.

I let the guitar do the work and roll the geetar voll and tone to get the sweet spot.

Im playing thru various LP,s teles and strats and all work well with it but I have to say the G&L blues boy with both pups on is tonally orgasmic heaven.

[Custom50]

i thought that sounded great!

[Mattbedrock]

Originally Posted by GearHeadFred

Good post - thanks for taking the time to record the samples... Nice playing.

I had a 4501 with EL34s. It was my main amp for 10+ years.. I sold it about 5 years ago. The amp does "that tone" pretty well.. I never felt the cleans were very useful, always had that dark Marshall coloring. I eventually went in the JTM45 direction.

I was watching that balok thread too and wondering if it might make me regret selling my 900. ALMOST!

Thanks Fred, very kind of you. It is a bit dark and I, too, have a JTM45 as well that is, quite frankly, a better amp. But the 900 does it's thing well and these mods made the lead channel at least usable.

I also have a 1986x 50 watt bass that is real "Duane-y". I am considering picking up a JCM800 2204 just to round out the bunch.