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previous: anonymous Re: Marshall (and other) Hum Sources -- 4/22/2000 5:49 PM

Marshall Jubilee/JCM900 MV/DR Mods

4/22/2000 9:31 PM
Ray Ivers	Marshall Jubilee/JCM900 MV/DR Mods Dale, 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! Ray

4/22/2000 9:31 PM

Ray Ivers

Marshall Jubilee/JCM900 MV/DR Mods

Dale,

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!

Ray

Replies:

dale Ray,thanks!! lots o... -- 4/22/2000 10:13 PM
dale hey Ray............. -- 4/25/2000 5:54 AM