Fitting an effects loop to an 80s Jcm 800 combo...

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mattdavismattdavis Frets: 243
my first and favourite amp is my jcm800 4104 combo. Having bust a lumbar disc I was afraid of carrying it around, but summoned up the courage to bring it to practice last night and it just sounds immense. 
Sacriligiously, I was using the Low input with an OD3 for gain so I had a clean option, but I’d love to use the amps High input gain. Problem is the delay and reverb in front really do mush it up. 
Wondering if anyone (ahem.. @ICBM ) knew if it’s fairly straight forward to add an effects loop to a 4104? Any idea of a ballpark figure for how much it would cost? Would it destroy the amps  value (although it will never be sold barring some sort of financial disaster) and would it cause some sort of collapse of the space/time continuum if I modify my stock g12-65 vertical input 800?
Cheers for any help
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  • sweepysweepy Frets: 1940
    Surprisingly these old JCM800 combos are getting valuable, tbh I wouldn’t mod it, maybe sell it and try a Friedman etc ?
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  • ICBMICBM Frets: 33753
    If you use either the official Marshall unit that they put in the 2203 reissue, or something like the Tube Town kit, fitting it is straightforward - the Marshall slightly less so because it has push-buttons for bypass and level which you will need to drill holes for very cleanly.

    I doubt it will affect the value in any way - it may even increase it, they are not 'valuable' vintage amps and originality is not the be all and end all - especially if you used the Marshall unit, I think. It's not the same as some sort of hack mod with holes drilled in the front etc.

    I would think about £100 for fitting including the unit will be in the right ballpark, although I haven't priced the Marshall one. The Tube Town kit is around £30 including shipping.

    If you *really* don't want to mod the amp permanently, it would be possible to rewire it so the High input is the only input, and the Low input becomes a send/return point, using a TRS (stereo) jack, but you will then need to have some sort of outboard level-matching box because the signal level will be too high for pedals.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • mattdavismattdavis Frets: 243
    sweepy said:
    Surprisingly these old JCM800 combos are getting valuable, tbh I wouldn’t mod it, maybe sell it and try a Friedman etc ?
     Can’t sell it..
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  • mattdavismattdavis Frets: 243
    Thanks @icbm. V helpful. Can’t see these Marshall kits online. Is it something that is specially ordered? Although it’s illogical, I think it would placate my neuroses to use an official Marshall product.

    If you weren’t in sunny Scotland I’d be very gladly putting the work your way - apologies for my southern location. 
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  • ICBMICBM Frets: 33753
    mattdavis said:
    Thanks @icbm. V helpful. Can’t see these Marshall kits online. Is it something that is specially ordered? Although it’s illogical, I think it would placate my neuroses to use an official Marshall product.
    Yes - you can (or could when I checked for someone a couple of years ago) buy the unit from Marshall's spares department.

    The only problem with fitting it is that there are four holes - two jacks and two push-buttons - and the two for the buttons have no covering around them so they're going to need to be very cleanly and accurately drilled. Although if you didn't want to do that I think you could preset the two switches to your preferred modes, remove the buttons and just leave them hidden inside.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • mattdavismattdavis Frets: 243
    Cheers mate. I’ll contact Marshall. Much appreciated. 
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  • John_AJohn_A Frets: 1618
    If you contact Marshall they'll probably do the work for you
    Do me a favour and like this:

    https://www.facebook.com/MarkedCoversBand
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  • mattdavismattdavis Frets: 243
    Cheers @John_A Have sent them an enquiry online. Have also used a good tech not far in Windsor (Paul Huish) who I’m sure would do a good job. 
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  • ICBMICBM Frets: 33753
    John_A said:
    If you contact Marshall they'll probably do the work for you
    If so, I expect they will have a jig to drill the holes perfectly as well - although the reissue amps are done by having them stamped into the chassis when it's fabricated. Not sure how much they will charge to do it afterwards...
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • mattdavismattdavis Frets: 243
    edited January 13
    I remember on my vintage modern when the effects loop was engaged (ie not bypassed) there was a subtle but definite slight muffling of the tone that I found hard to EQ out. Not a massive deal but is that likely to be an issue again?
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  • martinwmartinw Frets: 1941
    The only problem with the Marshall loop is that it uses op-amps so it will start to clip at 30V. This is an issue with Marshall preamps as when cranked the signal can easily exceed this. 
    I'd recommend the Tubetown loop of the ones mentioned, or the Metroamp loop, both of which use high voltage discreet transistors which won't clip.
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  • ICBMICBM Frets: 33753
    martinw said:
    The only problem with the Marshall loop is that it uses op-amps so it will start to clip at 30V. This is an issue with Marshall preamps as when cranked the signal can easily exceed this. 
    I'd recommend the Tubetown loop of the ones mentioned, or the Metroamp loop, both of which use high voltage discreet transistors which won't clip.
    The Marshall loop uses a level pad to bring it down to a third or a fifteenth of the input level (depending on how you have the switch set) before it reaches the IC, so that should give it enough headroom.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • mattdavismattdavis Frets: 243
    Cheers @martinw and @ICBM ;
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  • martinwmartinw Frets: 1941
    ICBM said:
    martinw said:
    The only problem with the Marshall loop is that it uses op-amps so it will start to clip at 30V. This is an issue with Marshall preamps as when cranked the signal can easily exceed this. 
    I'd recommend the Tubetown loop of the ones mentioned, or the Metroamp loop, both of which use high voltage discreet transistors which won't clip.
    The Marshall loop uses a level pad to bring it down to a third or a fifteenth of the input level (depending on how you have the switch set) before it reaches the IC, so that should give it enough headroom.

    The problem is balancing noise in the loop against headroom. The possible signal swing on a Marshall preamp is huge, depending on where you set the gain/volume, and at the top end it still clips, unless you pad it down so much the loop is then noisy.

    At least, in my experience. You can hear it, and some of my customers actually like it! After all, by that point it's just adding some diode-like clipping, as in certain other high gain preamps.

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  • DJH83004DJH83004 Frets: 176
    I know Tubetown do a valve version based on a 6112 pcb mount twin triode, that would certainly handle the pre-amp p-p swings. I have fitted their transistor kit and it worked very well.
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  • This is the TT effect loop
    https://www.tube-town.net/ttstore/Kits/Misc-Kits/Kit-Seriel-FX-Loop-LND150::5973.html

    Doesn't take long to solder together and only need 4 connections inside the amp, so a pretty easy install. if it is a combo amp you could probably fit the kit sitting vertical with the jacks coming out the bottom of the chassis, so you don't have to drill the rear panel out, usefull if the rear panel is covered in text and you want to keep the amp looking original.

    The 2204 and 2203 amps have gone up in value in the last few years, I almost bought one 4 years ago for less than £500 and you can't get them at that price now, the combo version has not done the same though from the ones I have seen.
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