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Marshall JMP 50 - 1987

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7/19/2000 4:51 PM

Marshall JMP 50 - 1987

Hi All,

I've finally finished my conversion of my 1986 circa '78, to the 1987 circa 1970 circuit. I'm well pleased with it, but have a couple of comments. The amp is totally to the schematic with the exception of 47k FB resistor, and a 0.47 uF presence cap with a 25k series pot. Oh, and I left off the bright cap from the volume - it was just too much!

Channel 1 is good and bright, has plenty of gain, but is a bit short of real bottom end.

Channel 2 has shed loads of big bass, but lacks brightness a little. Seems to have more gain than channel 1, which I hadn't expected.

The tone controls do less than any amp I've owned before.

Is this about what I should have expected? I'm quite pleased with it, but haven't cranked it right up yet. I've never played a 1987 from the early '70s, so I don't really know if I'm in the ballpark. It actually sounds quite a lot different to any 1959 I've ever used. (More middle, less bottom and top)

Cheers

Liam

7/19/2000 5:41 PM
SpeedRacer	1. Part of the mid-heaviness is the OT; it's normal for 70's amps. Adding more -fb will help ease that. 2. For more highs and lows, go to the 1986 tone stack (250pF, .02, .02 and 56K slope) The tone controls will do "more". 3. The overly bright Ch-I and bassy Ch-II are what leads most folks to jumper the channels - so you can blend them to suit your instrument and playing style.

7/19/2000 6:14 PM

quote:
"The tone controls do less than any amp I've owned before."

It's been a very long time since I've played any Marshall, but my memory of when these were new agrees with that --The volume control didn't have much range either (too quiet, or holy-hell!)
The on-off switch worked pretty much as expected, though

Hmmm...wish I had one now

7/19/2000 8:15 PM

Liam,
Sounds about right! I always leave the cap off the summing resistors..... I can't stand the brightness on the bright channel and what this cap adds in brightness to the bright channels is also what it shaves off of the already bass heavy muddy channel. If you leave this cap out there will be better balance between the two channels. Then I add a 120pf bright cap to the volume control of the bright channel so darker guitars have a fair chance of sounding half way decent at very low settings of the volume control. Teles and Strats get to use the higher gain bottom heavy mud channel. With this setup, the amp is more usable to my ears. You can also try a 390pf treble cap with a 47k slope resistor. The amps are so easy to adjust to suit your needs. Isn't it great!?

Peter S

7/19/2000 10:02 PM

Peter is right,some of the reasons of the different channels sound, is the cathode caps and resistors values, so tweaking here may make a difference in what you want. Also the plate caps.. use .02.. and like peter said.. you could use a different value, 250pf cap for the treble. and bypass the bassier sounding channel with a 120pf cap..tone slope should be 56k or 47k.. and change the middle pot..double the value.. it takes trial and error to find the right thing your looking for. but you'll get it..Thats why tweaking them is so good. either you have a channel thats too bright or the other is to bassy.. then they jumper to try and blend them. but if you tweak them for sound you like you won't need to do this and have two channels you can use..I like the sound of them modded like the JTM45/5F6A with a few tweaks..standard. 56K slope.. 250pf treble,middle pot changed.. a feedback switch .,and .02 caps..plate,, and tweak the cathode caps and resistor values..and some voltage changes depending on the amp.With EL34 tubesThis setup gives the more Hendrix tone. chuncky and thick, but still has plenty of treble.. Thats the sound and tone i always wanted..and what i try to always get from my amps..{for me} Selecting the right preamp tubes also makes these amps have there own voice.So much depends on so many things.. guitar used,pickups and speaker cab..
Richie{~}==:::

7/19/2000 11:43 PM
DavidB	Richie, do you prefer the shared cathode or the split cathode? Also what happens by doubling the mid pot? Thanks David

7/20/2000 2:34 AM
Richie{~}==:::	I like to split them.. the mid pot change gives alot more mid control.. and a little boost.. Richie{~}==:::

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