jcm800 mods?

[TomW]

Are there any simple mods that can be done to reduce treble on a stock 4104 jcm800 (combo version of 2204)?

I love the amp to bits but it can sound abit too trebly when turned up loud. It's not a huge problem as it still sounds ace, but it would be good if there was a simple reversible way of reducing the treble slightly to make it sound perfect, as I already have the treble turned almost all the way down.

Cheers

[gwEm]

did you already try removing the treble bleed cap on the preamp gain control? thats an easy one to do, but there are others.

[hamfist]

Two simple mods, neither of which require removel of the PCB !

First snip out the bright cap across the gain pot. Second, replace the 470K/470pF Resistor/Capacitor pair between the low gain input and the gain pot with a 68K resistor. THese are Mods 1 & 2 on this website - http://tone-lizard.com/Ultimate_JCM800.htm

Please also make sure you know how to drain the caps before sticking any fingers inside the amp.

THird (yes, I know I said there were only two !), play the amp with the master at least at 2 or higher. This reduces treble a LOT !!

FOurth, consider changing speakers, if your amp has G12T-75's. These are quite harsh, spikey speakers (tonally !). Something like G12-65's (for a more vintage feel), or G12K-100's  (for a more modern feel) would be a good replacements.

[TomW]

Cheers for the replies  :D I'll give it a go when I next have time.

FOurth, consider changing speakers, if your amp has G12T-75's. These are quite harsh, spikey speakers (tonally !). Something like G12-65's (for a more vintage feel), or G12K-100's  (for a more modern feel) would be a good replacements.

I play the amp through my 4x12 with g12h's and greenback's and I'm really happy with the sound of them so the speakers aren't a problem I don't think. As I said before I'm really happy with the sound of the amp as it is, this is just to make it that extra bit perfect hopefully.

Please also make sure you know how to drain the caps before sticking any fingers inside the amp.

I've never opened up an amp before so not sure how to do this. What is the best way?

[hamfist]

Please also make sure you know how to drain the caps before sticking any fingers inside the amp.

I've never opened up an amp before so not sure how to do this. What is the best way?

With your particular amp, you can get the caps mostly discharged before even opening the chassis. Just play the amp, then unplug the amp from the wall, leaving the power and standby switches both on "ON".  Straight away, play some loud chords on your guitar and you will hear the notes playing, then getting quieter and quieter. What you are hearing is the caps discharging their stored energy, until they are only holding about 20V, which will not give you much of a shock at all, if any.
  The amp should now basically be fairly safe to open up. Still be fairly careful not to touch anything inside the chassis until you have checked the caps with a voltmeter !
  Once you have the chassis open, use a voltmeter to check the voltages of each of the big electrolytic can caps. The voltages to be checked are between the + terminal of each cap, and the amp chassis. Each one should show about 20-30ish volts at this point.  Finish discharging the caps by shorting each cap to the chassis using a wire that is touched on the chassis and the other on the + terminal of the cap. The wire should obviously be insulated and you hold the insulated bit !  The voltages at this point should be very minimal, but it's best to be over-careful.
    If you had not already mostly discharged the caps by playing the amp straight after the amp was switched off, then you should not use a simple lead to discharge the caps safely, as you'd be looking at about 450 Volts, and sparks would fly. To discharge fully charged caps, you need to have a lead with an insulated 10K or 100K resistor in the middle of it, with insulated crocodile clips on each end. Using that, fully charged caps can be discharged to the amp's chassis safely, and in a controlled manner.  Some old fashioned techs still tell stories of merely using a big screwdriver to discharge caps, but I would not advise it !
  Then you check the voltages again with a voltmeter, and only then should you be ready to start soldering !! (oh, and watch the end of that soldering iron - they get hot, I believe !).

[TomW]

Thanks very much for the detailed description, doesn't sound too difficult :).

[HTH AMPS]

what are your amp settings first off?

I know that many people seem to be happy cutting the bright cap off, but if you play with the gain on full all the time, then cutting the bright cap will make no difference at all.  The bright cap is there to shunt high frequencies when the gain pot is turned down in order to stop the tone from getting muddy (just like putting a treble-bleed cap in your guitar). 

The 470k/470pF treble-peak is not something I'd remove - you lose that agressive Marshall crunch.  A better solution would be either to snip one end of the cap to take it out of circuit, that will remove the brightness.  Or, a halfway between all or nothing is to lift one end of the cap (i.e. cut it) and place a resistor in line with the cap to lessen the cap's effect - try 100k.

If you really want to snip a cap out to remove some brightness, the one to snip is the 470pF cap bypassing the series resistor in the voltage divider after the second gain stage (marked C8 on the schematic) - that will remove the ice-picky highs and keep some note definition and (most importantly) the trademark Marshall crunch.

also, merely soldering a 100pF cap between pins 6 and 8 on the first valve base will trim some high end off and also make the amp less prone to oscillations, though I think Marshall may have been doing this as standard by the time the JCM800s came out (the earlier JMP 2204s didn't have this and can squeel at higher gain settings - mine did).

I've modded quite a lot of these amps, and there is plenty more you can do if required including getting more gain and beefing out the bass end - these can be real screamers with some tweaks.

Hope that helps.

[hamfist]

The 470k/470pF treble-peak is not something I'd remove - you lose that agressive Marshall crunch.  A better solution would be either to snip one end of the cap to take it out of circuit, that will remove the brightness.  Or, a halfway between all or nothing is to lift one end of the cap (i.e. cut it) and place a resistor in line with the cap to lessen the cap's effect - try 100k.

If you really want to snip a cap out to remove some brightness, the one to snip is the 470pF cap bypassing the series resistor in the voltage divider after the second gain stage (marked C8 on the schematic) - that will remove the ice-picky highs and keep some note definition and (most importantly) the trademark Marshall crunch.

Hmmmm, I didn't find removing the treble peaking circuit essentially changed the character of the amp (or removed the "aggressive Marshall crunch" as you describe it) on both amps I have done it to.  I haven't tried removing the voltage divider bright cap though, so can't compare it to that particular tweak.

[TomW]

what are your amp settings first off?

When playing with my band I tend to have the preamp gain pretty low and crank the master volume then boost it with an sd-1. I also have the treble pretty low, but I think the fact that I have a riff raff bridge in my main guitar adds to the brightness. I might just keep all these ideas in mind and discuss it with my amp tech next time I need to take it in I think, then I can get him to do it for me. As tbh I dont feel 100% confident about taking the amp apart myself anyway.

Cheers for the help

[jpfamps]

The bright cap around the volume control has most effect al low volume settings (and NO effect at full volume). This is one of the reasons why amps can sound too bright at low volume settings.

[HTH AMPS]

what are your amp settings first off?

When playing with my band I tend to have the preamp gain pretty low and crank the master volume then boost it with an sd-1. I also have the treble pretty low, but I think the fact that I have a riff raff bridge in my main guitar adds to the brightness. I might just keep all these ideas in mind and discuss it with my amp tech next time I need to take it in I think, then I can get him to do it for me. As tbh I dont feel 100% confident about taking the amp apart myself anyway.

Cheers for the help

if you're keeping the gain low and the master high, you'll want to have a fairly small bright cap (100pF will be spot on) otherwise it'll be too bright as you've found.

[Will]

Not a mod, but what valves have you got in the preamp? The Mullard reissues seem quite fat sounding, and compare quite favourably to the old ones too

[sjwebb90]

Check out the following makes an interesting read.

http://www.tone-lizard.com/Marshall_Myths.htm

http://www.tone-lizard.com/Ultimate_JCM800.htm