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Marshall Jubilee/JCM900 MV/DR Mods

4/22/2000 9:31 PM
Ray Ivers
Marshall Jubilee/JCM900 MV/DR Mods
If you think it will get you a better tone, modify that amp! Don't drill, or cut, or maim - but I would not hesitate to substitute components in an educated and informed manner. Here you've got an incredible forum with helpful and knowledgeable people WILLING TO HELP YOU get the sound you want, and you can ask questions BEFORE you fire up the soldering iron. Bear in mind that the guy who designed any given amp has no idea what kind of music you play, or what kind of guitar you use, or what sound you like - the guy who designed the latest 'metal shredder' may be a jazz guy, or a Leo Fender type non-player! So it would almost a crime in my mind not to take full advantage of all this stuff to get you your own personal killer sound. Keep everything you take out, in a little plastic bag, for the next owner should you want to sell down the road - but let the prospective buyer hear what you've done first; if he agrees with you that 'this is the best Jubilee head I've ever heard', you'll be so glad you didn't rip all your hard-earned mods out first! There are exceptions to the above, of course - I don't think I could recommend putting MIDI switching in a Lucite AC-30, and valuable old Marshalls with the brown paint on the solder joints... you get the idea; but still, if your mod can be undone in the future, and it makes you happy, it's a good mod.  
Now that THAT'S over with, here's my reply to your post:  
The Jubilee preamp is different from the average Marshall preamp. It seems that the output from V1B is fed thru R29 to the diode bridge, and also thru R7 and R6 back to the grid of V1B. The channel-switching relay either shorts out the output of V1B to ground, killing the dirty channel and allowing the clean signal at the grid of V1B to continue thru R6, R8, and R10 to the grid of V2A; or, it grounds the R6/R7 junction, killing the clean channel output to V2A. R9 and the two IN4007 diodes and cap from a 'diode bounding' circuit (Kevin O'Connor's term) that would normally be quite mellow in effect - pulling the 'rhythm clip' switch would make it much more pronounced. It seems to me that the amount of diode clipping would remain the same regardless of the setting of either master, with the rhythm clip off - the lead master is just a voltage divider across the clipping diodes, varying the level of the clipped output into V2A. There are five negative feedback loops in this amp! a) C33 b)R6/R7 during clean channel operation c) R12 d) C15 e) the normal presence control network of R44, R20 et al.  
I would try removing the two LED's (unfortunately you can't jumper them, they have to come out). Your lead master will obviously now work much differently - turned down low, you will have very little overdrive, and if turned up you will now be including the overdrive of V2A in your distortion sound. The D4/D5 bounding circuit that before was mellow in it's action may also now be clipping at all times, regardless of the rhythm clip switch position. The output of V2A will go way up, so now your loop will be running mega-hot and may be useless for pedals, so you may have to change the R33/R34 ratio (isn't this design stuff fun?). It's also possible that now your clean channel will be so low in (relative) volume as to not be too useful to you; or, you may have to turn the gain control up so high to match channel levels that you will lose the 'bright switch' effect of C5. Actually, C5 is probably making your gain control into more of an on/off switch - if you have your board out anyway, you might want to pull it and see (or it may be soldered across the pot if you have flying leads). Without this cap, the preamp tone should be equally 'thick' at all gain control settings above about 1. Removing C4 will drop the gain , and shouldn't change the quality of the distortion too much with the diodes out. Removing R12 or bypassing R32 will increase gain.  
Was your 900 the MV, or the Dual Reverb? The MV preamp's diode section is identical with the 2205/2210 - not the whole preamp, just the diode bridge and resistor/cap part. I'm going to assume that you have the DR, because to the best of my knowledge no one bought the MV, with it's two switchable master volumes, when you could have the DR with true channel-switching and reverb! I've met people who have regretted this decision... but that's another story. Removing the diode bridge from the DR will allow the first MS201 switching op-amp section to hit the second MS201 a little harder (I believe this chip is kind of like an on/off VCA, or voltage-controlled amp, but I could be wrong) and at the solid-state voltage levels we're dealing with shouldn't affect overdrive too much, and might well make the clipping a little smoother. Removing the diodes in the MV, however, should quite noticeably affect both the tone AND the gain, bringing you back to what is basically a modified 2203/2204 preamp, with less gain and a bit less buzz. I've found that in the 2205/2210 preamp, increasing the dirty channel's volume control (not the GAIN dual-pot) past 8 made a marked change in the tone, from pretty tubey to pretty fuzzy - the tone difference is most noticeable at low volumes.  
Is this stuff on the mark for you? Referring back to what I said at the beginning - I read your post over again and it sounds like you may have bought this amp just as a collector's item, in which case you might want to think twice about modding it. Ask yourself this: if, after modding, it sounds much better, will you still wish you had left it stock? Maybe? I myself have never answered yes to this question (there's a different question, involving sparks and burning smells, that I have a different answer for) but that's me, not you.  
Keep me posted!  
4/22/2000 10:13 PM

thanks!! lots of usefull info. just to fill you in, the 900 is a single channel, not the dual reverb. it's a better sounding amp IMO, but too gainy, and decreasing the gain hasn't been good for tonal reasons. but thats water under the bridge cuz the jubilee is my main amp now and the 900 is for sale. [and back to stock]when i said it's a collectors item, i meant just that, not that that was the reason for buying it. to tell you the truth, i'm thrilled with it as it is stock, but you know how guitar players are--always trying to get that last drop of tone. *S* and since i can take the diodes, or leds out of the circuit very easily, i figure why not. but i thought i'd ask your opinion first to see if you thought it was worth doing. but you gave me lots of other info that sounds good, so i may try some of it. i usually don't even take the components out, but just lift one end or jump them, whatever the case may be. and yes, i WOULD NEVER drill holes in it or make any permanent mods.  
i'm gonna go over all your ideas with the schematic in front of me, and i'll probably have some questions for you. untill then, thank you VERY much. you are a big asset to this board.  
4/25/2000 5:54 AM
hey Ray.............
i did what you said with c5, the cap on vr1 of the jubilee. man, i've dumped caps like that before, but geez, you were right as rain !!! the amp is now literally 30-40% thicker !! i like the gain set at about 2:00, and at that point it lets too much high end thru. and since i don't use the rhythm clip pull switch, i removed the wires from that and used it to add the cap back in so i could a/b it. the difference is nite and day. and no muddyness, just thick fat tone. now i can't wait to gig with it, but no gigs coming up soon. i'm dyin !!! *S*  
thanks ray, you made my day, or should i say month!!  
ps--what in gods name do you think marshall does this for? i mean, you have to choose a gain level you may not like to get the tone where you want it. i understand caps that compensate for bedroom levels, but it should be designed to have no effect past television volumes.  
oh, and one other thing---approximatly what frequency was c5 [.001] passing?  
4/25/2000 4:45 PM
Ray Ivers
Bright Switches/Caps
I got your E-mail, too - I'm glad it worked! I think Marshall puts these in as a 'legacy factor', so that if someone wants to use the amp as a non-master-volume amp looking for the 'Marshall 100 watt tone', the gain control will work in a similar manner to an old Marshall Super Lead (although the Marshall 1959 print shows a .005 uF cap) or maybe they just wanted to add some more 'bite' without working up a sweat. The frequency range and amplitude of same that C5 passes will be very dependent on the setting of VR1; at low gain settings C5 would pass everything over 3 kHz or so at very high levels, and as the control was turned up the frequency corner will move up and the amplitude of the cap bypass signal will go down. I don't have a schematic in front in front of me - VR1's a 470K pot, right? The old Marshall .005 value would start at 600 Hz, and tends to make the volume control very on/off in nature.  
This is one of the reasons I don't like Fender's usual implementation of the 'bright' switch - if you like the tone with it on, now you can't move the volume control or you'll lose that tone! But it's cheap and quick and easy...  
Time to check around for some open-mike nights...  
4/25/2000 5:15 PM

the cool thing is that the amp has the pull switches that arent even necassary for my purposes, so i can use them for mods like this. if i get to a gig to find that the amp needs the brightness that cap added. all i have to do is pull a switch and i'm there. but i doubt i'll need it. the amp sounds best with the treble at 3-5, and presence about the same. but now i think i'll have to turn the treble and presence up, which is exactly what i want. see, my feeling is that amps sound best with thier tone controls way up. maybe not pegged, but close. but most amps don't allow that without getting too bright. so i ilke the idea of having a guitar and spekers that are on the dark side, and getting my hi end with the tone controls. it just seems that cutting signal at certain frequencies to tame your highs just isn't as good. it seems the amp becomes sort of "congested". the best sound i ever get are usually with a setup that allows me to crank the tones without getting too bright. so i think this cap mod will do that. of course i never say "for sure" till it's been gig tested. but i'll bet it's gonna be great. i recently traded a guy the v30 that came in it for a sidewinder to hopefully tame the hi mids that the ev i'm using has. but now i wonder if the cap mod will make the ev sound perfect. no big deal tho, i've always wanted to see how those sidewinders sound anyway, and i hate v30's.  
thanks ray,-dale
4/25/2000 10:13 PM
Re: Marshall (and other) Hum Sources
"now i have a silver jubilee 25th anniversary marshall, which also has diode clipping. however, the diode clipping circuit looks vastly different from the 900. could you look at the schematic and tell me what you think about removing them?"
I've tried it before. The amp won't distort without the diodes. Also, the lead channel becomes so loud that it is hard to get it to work well with the clean channel. The whole preamp would need to be re-designed to effectively do away with the clipping diodes in this particular amp.
4/22/2000 6:02 PM
opps, forgot to mention---there are 2 diode clipping circuits. one of them is selected only on the clean channel by pulling vr1 out. that one o don't care about cuz i don't use it and it's selectable anyway. it's the one on the lead channel that i was speaking of, and thats the one at the top of the schematic with vr2 at it's output.  
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