deeaa
April 13th, 2012, 11:39 AM
Just finished modding my JVM and I gotta tell you...highly recommend them for any JVM player. Man they woke the thing up.
1st of all, all thanks to jvmforum.com guys who had awesome guides and whatnot to help.
I liked the JVM just fine; the D/I sound in particular is great, but the sound from speakers had some shortcomings compared to classic Marshalls. Still, as a compromise for live playing it sure is a great amp also stock.
Basically, the amp has preamp sections from many a classic Marshall, JTM thru JCM series, but I suppose both to keep the costs down and to keep the amp as versatile as possible and playable at very low levels too, they have made some compromises.
The most notable ones are the lack of choke, which is one of the reasons making the amp 'tight' and not very organic at all. Instead it has just a resistor for the job. Also, the amount of negative feedback is quite high stock, which kind of keeps the poweramp always in check and not roaring free, and that makes the amp sound a little cold and 'stiff' for lack of better word.
Third issue is the overall tone is more geared for modern metal sounds, and can be a bit too bright and even brittle for higher notes.
So here's how to rectify them:
https://lh6.googleusercontent.com/-0Kchlf3u6Z0/T4hSZ5ITDwI/AAAAAAAAE80/rDX4HLCGyJ0/s748/2012_kauhava_jne+086.JPG
First you open up the chassis - having made sure you've drained the caps. This can be done by switching the amp off and then switching Standby on despite it's off and wait for a while, minutes will do. But best measure with a multimeter that it's drained once you get the chassis out. When you look at the board, at one end there's four big caps; measure the leads of the resistors between/in the middle of those that they're empty.
Unscrew the jacks, the D/I jack screws, and remove the tubes and their holders. Label the tubes so you can put them back in the same order.
https://lh5.googleusercontent.com/-JA5SG-Opj-g/T4hSakKxiKI/AAAAAAAAE9A/irzKT_-rX3c/s661/2012_kauhava_jne+089.JPG
Then remove the board screws and that L-shaped guard by the said caps. Mark the leads so you can find the right spots once it's done, naturally. The tube-like connectors you just pull up, and the flat white connectors you kinda wiggle left-right-left till they come loose.
The hard part is to squeeze the board out of there. It is VERY tight. The trick is to press from below by the tube sockets so that you start from the left, push it in (I know; it feels absolutely impossible it'll ever push far enough) and WHILE you keep them pressed in, press the next one, the next...going right towards the caps, and keep the board supported so it doesn't fall back. It's tough. Do that a few times from left to right, keep pushing it in and don't let it fall back, and then you get first one socket thru and then the rest. Getting it back is hard too, but similarly inch it in slowly.
https://lh4.googleusercontent.com/-a_yi509H6I8/T4hSbzn5J6I/AAAAAAAAE9M/ej_-Hs30n1M/s748/2012_kauhava_jne+090.JPG
It's easier to check from the inside where to position the proper choke. I used a Hammond one, 4 Henrys. I understand anything from 2,5 to 10 or even more will do. 200-400mA. Chokes are cheap, this one was 19€ only.
Drill three holes, two for the choke and one for the leads going through.
https://lh5.googleusercontent.com/-QQeain_LdQg/T4hSc5p6mrI/AAAAAAAAE9Q/NH5_dpGw5qI/s748/2012_kauhava_jne+091.JPG
There's plenty of room there. Position it as close as possible to the main caps; the resistor to be replaced is the white rectangle just left of those caps. R106. The board is well labeled. Get some locking agent for the screws too :-)
https://lh6.googleusercontent.com/-bSj5-LBEzfU/T4hSgQHVK1I/AAAAAAAAE9Y/LG8fIZ4txKg/s748/2012_kauhava_jne+092.JPG
Choke installed...well enough room for sure!
Solder the choke leads last; now for the 'Plexi' mod that'll eliminate those nasty too-high squeals when rippin' it:
https://lh6.googleusercontent.com/-EWfjPkjteWE/T4hSiZrVUsI/AAAAAAAAE9g/caqp1lqQQJs/s748/2012_kauhava_jne+095.JPG
Solder a .68uF or 1uF polyester capacitor in parallel with R97, i.e. just solder it next to it/over it.
Like so. I used some hot glue to secure it with a few dabs to caps nearby, and shrink wrap on the wires.
Next up, let's fix that insane negative feedback amount:
https://lh3.googleusercontent.com/-7Y3kBp8c4GQ/T4hSjTvSpEI/AAAAAAAAE9s/UCGz1huanpU/s748/2012_kauhava_jne+097.JPG
Replace R58 with a suitable resistor. JVMForum gives these guidelines on what values to use for what effect:
R58 = 47k -- late '60s Plexi
R58 = 74k -- early '70s MkII Plexi
R58 = 82k -- stock JVM
R58 = 137k -- EVH's Plexi
R58 = 176k -- JCM800 2203
R58 = 177k+ -- a Marshall with almost a Vox's lack of headroom in the power amp
I myself went for 180k because I love me Marshalls with no neg feedback, untamed :-) (My Ceria also had a switch to make it have pretty much no feedback and I liked it that way).
Now when those are done, remove the R106 resistor and solder the lines from the choke there instead. Best do it last, because likely the choke leads aren't hugely long. And I hope you put the choke somewhere close :-)
https://lh6.googleusercontent.com/-85eSM9j5k3w/T4hSk3fHBqI/AAAAAAAAE9w/I_qauACfibE/s748/2012_kauhava_jne+098.JPG
Like that. Here you also see the resistors between the caps that you can measure from if the caps are drained.
That's it!
I screwed up my before/after clips, but here's some going thru sounds after. Recorded with a (NB): single-EMG85 equipped superstrat/metal axe...so the cleans aren't exactly clean and the bridge pickup only of course is what it is. But who needs neck pups with an EMG85 :-)
I REALLY REALLY REALLY can't tell you how much I love the changes.
Immediately it was apparent the amp was a bit louder and touch more gainier, so I had to lower my channel volumes just a touch, and some gain too. But MAN it sounds so much better.
The clean channel is now just like a Bluesbreaker. You hit chords, they just BRAMMM out there with utmost Vox-like clarity and punch, and there's WAY more pant-flapping lower sound content. Kinda like someone put in bigger speakers. That's mostly the lack of negative feedback I guess.
The 2nd clean is instant pure old ACDC. Just like a hotrod Plexi. Warm and big, too.
Drive 1 is now exactly Iron Maiden on The Number of The Beast etc...and perfect for any rockin'. JCM800 at its best.
Drive 2 is now way more loose and usable, growls almost like a Mesa or something, and screams in solos. Nicely there's now even a touch of slight sloppiness and growl where there used to be tight upper-middy bite. And even the craziest bends don't grate on ears now, even without picking and just hammerons it's instant Gary Moore and sustain 4 ever.
Here's some sloppy playing after the mods, going from cleaner towards the hotter channels.
http://deeaa.pp.fi/clips/JVM_MODDED.mp3
1st of all, all thanks to jvmforum.com guys who had awesome guides and whatnot to help.
I liked the JVM just fine; the D/I sound in particular is great, but the sound from speakers had some shortcomings compared to classic Marshalls. Still, as a compromise for live playing it sure is a great amp also stock.
Basically, the amp has preamp sections from many a classic Marshall, JTM thru JCM series, but I suppose both to keep the costs down and to keep the amp as versatile as possible and playable at very low levels too, they have made some compromises.
The most notable ones are the lack of choke, which is one of the reasons making the amp 'tight' and not very organic at all. Instead it has just a resistor for the job. Also, the amount of negative feedback is quite high stock, which kind of keeps the poweramp always in check and not roaring free, and that makes the amp sound a little cold and 'stiff' for lack of better word.
Third issue is the overall tone is more geared for modern metal sounds, and can be a bit too bright and even brittle for higher notes.
So here's how to rectify them:
https://lh6.googleusercontent.com/-0Kchlf3u6Z0/T4hSZ5ITDwI/AAAAAAAAE80/rDX4HLCGyJ0/s748/2012_kauhava_jne+086.JPG
First you open up the chassis - having made sure you've drained the caps. This can be done by switching the amp off and then switching Standby on despite it's off and wait for a while, minutes will do. But best measure with a multimeter that it's drained once you get the chassis out. When you look at the board, at one end there's four big caps; measure the leads of the resistors between/in the middle of those that they're empty.
Unscrew the jacks, the D/I jack screws, and remove the tubes and their holders. Label the tubes so you can put them back in the same order.
https://lh5.googleusercontent.com/-JA5SG-Opj-g/T4hSakKxiKI/AAAAAAAAE9A/irzKT_-rX3c/s661/2012_kauhava_jne+089.JPG
Then remove the board screws and that L-shaped guard by the said caps. Mark the leads so you can find the right spots once it's done, naturally. The tube-like connectors you just pull up, and the flat white connectors you kinda wiggle left-right-left till they come loose.
The hard part is to squeeze the board out of there. It is VERY tight. The trick is to press from below by the tube sockets so that you start from the left, push it in (I know; it feels absolutely impossible it'll ever push far enough) and WHILE you keep them pressed in, press the next one, the next...going right towards the caps, and keep the board supported so it doesn't fall back. It's tough. Do that a few times from left to right, keep pushing it in and don't let it fall back, and then you get first one socket thru and then the rest. Getting it back is hard too, but similarly inch it in slowly.
https://lh4.googleusercontent.com/-a_yi509H6I8/T4hSbzn5J6I/AAAAAAAAE9M/ej_-Hs30n1M/s748/2012_kauhava_jne+090.JPG
It's easier to check from the inside where to position the proper choke. I used a Hammond one, 4 Henrys. I understand anything from 2,5 to 10 or even more will do. 200-400mA. Chokes are cheap, this one was 19€ only.
Drill three holes, two for the choke and one for the leads going through.
https://lh5.googleusercontent.com/-QQeain_LdQg/T4hSc5p6mrI/AAAAAAAAE9Q/NH5_dpGw5qI/s748/2012_kauhava_jne+091.JPG
There's plenty of room there. Position it as close as possible to the main caps; the resistor to be replaced is the white rectangle just left of those caps. R106. The board is well labeled. Get some locking agent for the screws too :-)
https://lh6.googleusercontent.com/-bSj5-LBEzfU/T4hSgQHVK1I/AAAAAAAAE9Y/LG8fIZ4txKg/s748/2012_kauhava_jne+092.JPG
Choke installed...well enough room for sure!
Solder the choke leads last; now for the 'Plexi' mod that'll eliminate those nasty too-high squeals when rippin' it:
https://lh6.googleusercontent.com/-EWfjPkjteWE/T4hSiZrVUsI/AAAAAAAAE9g/caqp1lqQQJs/s748/2012_kauhava_jne+095.JPG
Solder a .68uF or 1uF polyester capacitor in parallel with R97, i.e. just solder it next to it/over it.
Like so. I used some hot glue to secure it with a few dabs to caps nearby, and shrink wrap on the wires.
Next up, let's fix that insane negative feedback amount:
https://lh3.googleusercontent.com/-7Y3kBp8c4GQ/T4hSjTvSpEI/AAAAAAAAE9s/UCGz1huanpU/s748/2012_kauhava_jne+097.JPG
Replace R58 with a suitable resistor. JVMForum gives these guidelines on what values to use for what effect:
R58 = 47k -- late '60s Plexi
R58 = 74k -- early '70s MkII Plexi
R58 = 82k -- stock JVM
R58 = 137k -- EVH's Plexi
R58 = 176k -- JCM800 2203
R58 = 177k+ -- a Marshall with almost a Vox's lack of headroom in the power amp
I myself went for 180k because I love me Marshalls with no neg feedback, untamed :-) (My Ceria also had a switch to make it have pretty much no feedback and I liked it that way).
Now when those are done, remove the R106 resistor and solder the lines from the choke there instead. Best do it last, because likely the choke leads aren't hugely long. And I hope you put the choke somewhere close :-)
https://lh6.googleusercontent.com/-85eSM9j5k3w/T4hSk3fHBqI/AAAAAAAAE9w/I_qauACfibE/s748/2012_kauhava_jne+098.JPG
Like that. Here you also see the resistors between the caps that you can measure from if the caps are drained.
That's it!
I screwed up my before/after clips, but here's some going thru sounds after. Recorded with a (NB): single-EMG85 equipped superstrat/metal axe...so the cleans aren't exactly clean and the bridge pickup only of course is what it is. But who needs neck pups with an EMG85 :-)
I REALLY REALLY REALLY can't tell you how much I love the changes.
Immediately it was apparent the amp was a bit louder and touch more gainier, so I had to lower my channel volumes just a touch, and some gain too. But MAN it sounds so much better.
The clean channel is now just like a Bluesbreaker. You hit chords, they just BRAMMM out there with utmost Vox-like clarity and punch, and there's WAY more pant-flapping lower sound content. Kinda like someone put in bigger speakers. That's mostly the lack of negative feedback I guess.
The 2nd clean is instant pure old ACDC. Just like a hotrod Plexi. Warm and big, too.
Drive 1 is now exactly Iron Maiden on The Number of The Beast etc...and perfect for any rockin'. JCM800 at its best.
Drive 2 is now way more loose and usable, growls almost like a Mesa or something, and screams in solos. Nicely there's now even a touch of slight sloppiness and growl where there used to be tight upper-middy bite. And even the craziest bends don't grate on ears now, even without picking and just hammerons it's instant Gary Moore and sustain 4 ever.
Here's some sloppy playing after the mods, going from cleaner towards the hotter channels.
http://deeaa.pp.fi/clips/JVM_MODDED.mp3