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Marshall amp mods
The negative feedback modification:With less feedback the amp gets louder at lower volumes and the power tubes break up more dynamically and gradually, rather than staying more quiet at lower volumes and breaking up more suddenly, as with greater amounts of feedback. It's a very significant change in amp dynamics and to some extent tone (because of the more gradual and controllable onset of power tube distortion and the lessening of the influence of the presence and resonance controls).But at really loud volumes, when the power tubes are in "full" breakup anyway, the power levels and volume out of the amp will be the same with a little or a lot of negative feedback because at that point the feedback loop collapses in both schemes. The difference between a little and a lot of feedback is essentially how gradually or suddenly the power amp breaks up (dynamics), and how loud the amp is on lower volume settings, but power levels won't be any different when fully cranked.
On this page you can see the amp. modifications that I performed on my Marshall JVM's.
Warning!!!!Inside tube amps there are or can be potentially lethal voltages at several point within the amp.,even when the amp. hasn't been played for a couple of days,the voltage can still be present in the electrolitic capacitors,and they can be discharged by you if you dont't know what you are doing,and can cause serious injury or death.If you don't have the knowledge and don't know what you are doing,stay out of the amp. and find somebody with the knowledge, who can help you out.I don't take any responsibilty for any damage whatsoever to persons or property that may result from readers of my site,who didn't follow up the warning,and undertaking work on their own amps. I did perform the following modifications:- negative feedback mod.- Plexi capacitor mod.- Choke mod.- 3 rd stage bias mod.For more detailed info about the mods,visit the JVM forum.Link: http://jvmforum.com/phpBB3/
Left you see a part of the schematics of the Jvm,the negative feedback is tapped from the output pcb in the amp. there is a connector on the pcb from where a wire goes to the main pcb.Pin # 2 is the wire you need to interupt and put a pot.meter in between.Right you see a close up of the output pcb.
Left you see the wiring of the pot.meter,the resistor on the drawing is the resistor that is mounted on the main pcb,the value of the potmeter is min.100 Kohm, up to max. 250 Kohm.Right you see the wire coming from the output pcb interrupted and attached to the pot.meter.
Above you see the potmeter mounted on the front panel of the Jvm. 82k -- stock JVM137k -- EVH's Plexi176k -- JCM800 2203177k+ -- a Marshall with almost a Vox's lack of headroom in the power ampThe sound? That has become awsome,now you can realy hear those tubes working,like others have already described it;it makes the Jvm roar like a Lion,I love it! After that I've played it for about a week,I just love the mod,power tubes sound much more alive and break up much sooner as without the -ve fb.I have it at the JCM800 value set,and didn't change it any more,but the advantage of the pot.meter is that you can get the original feedback dialed in also if you need to.The sound is much more like the classic rock bands we all know,the dynamic is more controllable with you guitar,and how you hit the strings.Together with the plexi mod it makes your sound palet more complete on the amp.
Plexi capacitor mod.The R97 "Plexi mod" has a less dramatic effect on gain than the C83 cap delete. However, the 1 uF cap in parallel with R97 does an absolutely beautiful job of taking any brittleness out of the high notes. With a pickup with a thick top-end it doesn't matter so much, but with a pickup that has a brittle high-end this can help turn the upper frets from harsh-ish to smooth and full. Again, it isn't dramatic, but you will notice that the higher notes are fuller and the brittleness improved/gone.
Left you see the double pole,3 position switch with the capacitors on it soldered.The center tags has the wires that goes to R97 on the main pcb.The other tags have on one side the 0.68 uF capacitor,on the other site I have two 0.50 uF capacitors paralell,what makes 1 uF total.Right you see the wires coming from the switch,soldered to the resistor R97 on the main pcb.
Above you see the switch mounted on the front panel.Center position of the switch is stock Jvm,switched left is 0.68 uF,switched right is 1 uF.I like the mod,even with Les Pauls with humbuckers you can hear the difference,it gets more "bite" to the sound, the 0.68 uF position I like the most.
Choke modification.Some people have complained that the JVM feels "stiff" and not as dynamic as some older Marshalls. This mod will remedy that... .I did use a MM MC10H choke along with 47 uF/400 Volts Capacitors.Check the Jvm forum thread for other values.Link:http://jvmforum.com/phpBB3/viewtopic.php?f=7&t=5270
Left you see the chokes and the new capacitors for bove my amps.Right a close-up of the choke..
Left you see the holes drilled into the chassis,and a rubber wire protection fitted,where the wires of the choke are going into the chassis to the pcb.Right you see the old filter capacitors and the resistor removed from the pcb.
Left you see the chokes and the new capacitors for bove my amps.Right a close-up of the choke..
Left you see the pcb in place,and the wires of the choke wrapped in shrink tube and secured to the metal panel,also glued the caps to the "old" caps,instead of putting the glue on the pcb.Right you see the amp ready with the choke.Thanks to Casey's advise somewhere in the thread on the Jvm forum,I ordered the capacitors with diam. 22mm and the distance between the legs of the cap. 10 mm,so they did fit just like they where ment to be there.How does it sound?After finishing the rebuild,I tested the amp,and compared it with my Jvm210H,all settings the same,the 210 +205 without -ve feedback,and man,that's already a hughe difference.After that I dialed in the -ve fb on the 205,and the difference became much more to the front.After that I dialed also on the 210 the -ve fb in,but still it couldn't compete with the 205.To my opinion:The sound is more open,clearer,more sparkeling,bigger,more defenition,even with the clean mode.With the od red you can get sounds like the scorpions,whitesnake etc. realy easy.Also the reaction of the amp on your playing has improved very much,more sustaining of the notes played.In three words: I Love It !
The Plexi "one wire" cascade mod -3rd stage bias adjust knob . On some modes of the channels of the JVM, an extra tube gain stage is added to give more distortion. This is a very cold biased stage configured deliberately as an asymmetrical clipper ala the 2203 (the waveform is clipped more on one side than the other, generating mostly even order harmonics). What I was interested in doing was also modding this stage so I could adjust its cathode resistance to go from warm biased to cold biased. Why? Because that changes the distortion character of the sound and gives the option of mimicking the famous "one-wire" cascade modded Plexis of the 1970's and early '80s (Randy Rhoads' amp was custom built like this by Marshall, and others, such as supposedly James Hetfield of Metallica, had it done - the "one wire" cascade mod became a common 1959 mod for more gain after Ritchie Blackmore had his Majors modded like this by Marshall in the early 1970s). The mod allows you to change the bias of that tube stage on the fly and get a less or more aggressive distortion sound with the turn of the knob - anything from more along the lines of the classic 1959 type amps (Blackmore's Mark II Deep Purple and Rainbow days, Randy Rhoads and countless early '80s metal albums) to stock JVM/2203 style anytime you want.As this mod affects the third tube stage in the JVM, it is only "on" in the modes that use that stage - Clean Green and Crunch Green don't use this stage and so are unaffected by the mod. In all the other modes you'll be able to adjust the third stage bias however you wish.The effect that this mod has can go from just noticeable to much more obvious. In higher gain channels - like OD1/2 Orange and Red - it will be subtle, but in Crunch Orange and Red and Clean Orange and Red, it will give you the ability to go from a very "raw" and aggressive distortion character (with the knob turned up) to a smoother, more "classic" distortion with the knob turned down.On the 2-channel JVMs it will affect all the modes except Clean Green.If you use a 10k pot with a built in clickable SPST switch (http://www.circuitspecialists.com/31vm401.html)You can also switch a 100 nF cathode bypass cap in and out of the circuit. This gives a gain boost starting at around 200Hz and was/is used on SIR #34, #36, #39 and the AFD100 head... it's part of the snarly mids and trebles they have. It also acts similar to the preamp in Rtichie Blackmore's modded Majors and gives a touch of that treble booster type sound (Ritchie stopped using a treble booster when he got his Majors cascade modded in the mid-70s). It's closer to #36/#34/AFD100 than how Blackmore's was done, but the peaking effect is a similar idea.You wire the switch so that when the pot is all the way to the 10k position (full "on"), you click it and it switches in a 100nF cap across the pot to ground. In other words, it switches in a 100nF cathode bypass cap on that tube stage, ala #34/#36/AFD100. Don't try switching the cap in when the pot is at less than 10k, it will be too much gain and will squeal (so use a click type pot that only lets you click at 10k rather than a push pull pot that lets you switch in the cap at any resistance on the pot).Check the Jvm forum for more info.To do the mod...Remove R96 and replace it with a 820 ohm resistor wired in series with a 10k linear pot.
Left you see the pot.meter,the capacitor and the resistor of 820 ohm 1 watt.Right :the switch switched the wrong way,turning the pot and making the switch clic,did make the connection between the contacts of the switch,at that point it should break the contact.So I was lucky,the switch could be taken apart and refigured,so now the switch breaks the contact when turning the pot. from left to the right.
Left you see the old resistor R96 that has to be removed.Right you see the little schematic that I made before soldering al the parts together.