What are your most asked-for JCM800 mods?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by protoangel, Jan 6, 2006.


  1. protoangel

    protoangel Member

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    I've played through several JCM800s recently:

    One channel switching 50 watter with greenback-loaded cab, and 2 100 watters (non-channel switching) with G12T-75-loaded 4x12s.

    Prescence at 0 treble at 0... No power brake (oops)

    I generally found them to be overly-bright, grainy, but quite an addictive.. er... "Marshall-eque" grind Plus they have that honky mid-range which dates the sound instantly to 1980 LOL

    SO ideally I'm thinking "these would sound great with WAAY less treble, and mid emphasis somewhere further down the "fat" part of the spectrum, instead of that harsh fatiguing stuff and that annoying honk"

    What kind of stuff are you guys being asked to do tonally?
     
  2. scottosan

    scottosan Supporting Member

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    The channel switchers are less desireable and see far less modding that the single channel models
     
  3. protoangel

    protoangel Member

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    Is that because they need less modding cos they sound better, or peeps are just not buying them cos they sound worse.

    Following this logic... If I was to buy a cheaper channel switching JCM800, is it quite simple and cost-effective to get a modder to improve the tone and make it stellar?
     
  4. John Phillips

    John Phillips Member

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    The channel-switchers probably get modded less because they already have more gain (the usual biggest mod request in the past) and are a bit harder to work on - they're more complex and in some ways there's less you can achieve. And IMO they don't sound as good as the single-channel ones - they're generally used by people who need versatility over outright tone.

    My biggest mod request these days, with JCM800s as well as older Marshalls, is... "can you put it back to stock please?"

    I've even had to de-mod a few that I modded myself ten or fifteen years ago :). Luckily I was quite conservative in what I did, so it isn't too difficult. Nowadays I wouldn't do anything much beyond component value changes anyway - there's really no point in trying to make an amp into something it was never meant to be when there are so many other amps out there which are variations on the same theme.

    "these would sound great with WAAY less treble, and mid emphasis somewhere further down the "fat" part of the spectrum, instead of that harsh fatiguing stuff and that annoying honk"

    Sounds like you want a Soldano Avenger to me ;).
     
  5. scottosan

    scottosan Supporting Member

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    I still mod single channel Marshalls, but i will not drill holes or add gain stages. I actually pull the stock board out and desolder the leads from the tube sockets and remove the potentiometers with the board. Then put in a Turret board. The good thing about the value is that in my opinion is more desireable that a modded marshall with a hacked PCB board. That way, it can go back to complete stock in about an hour and can not even tell it was modded.
     
  6. scottosan

    scottosan Supporting Member

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    As far as bringing them back to stock, it wan't but 3 years ago, you couldn't get mor than $350 -500 for a PCB JMP or 800. The prices aren't going to stay where they are for the 800's and JMP's.
     
  7. protoangel

    protoangel Member

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    LOL

    Yeah I do find that ironic considering the negative rep 800s had years ago!

    *****
    So disregarding the "more gain mods" are people coming to you guys saying "Can you make it more this or more that" eg smoother, more fendery, cleaner, fatter, mre detailed, more 3-D etc, shifting the tonal centre etc (adjectives you find with Boutique amp descriptions)

    Or is there simply too much already in the PCB to change it from that somewhat harsh "Marshall" tone
     
  8. John Phillips

    John Phillips Member

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    There isn't too much in the PCB to change, it's just that most people who buy a Marshall want a Marshall. If they don't want a Marshall they tend to buy something else, not buy a Marshall and mod it. Or if they want a more traditional fatter/cleaner Marshall sound than a JCM800 they buy an older Marshall, or a reissue Plexi...

    There are so many choices out there it's really quite pointless to take a perfectly good old amp (which are going up in value, for a good reason which is that they are the definitive amp for that style) and mod it to make it something it's not... IMO.

    The other thing that most people don't realise until they try it is how much difference speakers make. Many of them don't even know what's in their Marshall cabs, and/or aren't aware that the stock G12T-75s are some of the most buzzy and thin-sounding speakers Celestion have ever made (the current Chinese ones especially)... IMO. Try a JCM800 with G12-65s (jangly/smooth/bright/warm) or G12-80s (deep/clear/solid/heavy) and hear what it was really designed to sound like and you may find modding it unnecessary. Or if those aren't dark and chunky enough, try V30s. Even the cab makes a difference - the modern ones with the mono/stereo switching plates just don't sound as good as the older (and noticeably heavier) JCM800 cabs, even with the same speakers.


    All just my opinion of course.
     
  9. Rich M

    Rich M Member

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    Are you implying they are going to go up or down?

    I had a channel switcher I did a simple mod (cathode bypass on V4a) and I thought it sounded great for high gain lead. Had layout problems (reverb recovery wire laying on top of heater wire - caused buzz). Sold it to a fellow board member. I still miss it
     
  10. Wakarusa

    Wakarusa Member

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    #1 Request is less (or more controllable) treble
    #2 Request is more gain

    To the former, snipping out the volume pot bypass cap is usually just right for 'em. That and dumping the G12T-75's if that's what they're playing through. Occasionally there's a little more

    To the latter, it usually leads to an education -- most want a sick amount of gain that's just never going to result in a playable, stable amp. These are the folks that I usually point to a new/different amp purchase.

    However, much as I hate admit it, my average customer has pretty shallow pockets. That and we're pretty much in the middle of nowhere.
    For small tweaks to shape tone and gain (make it a bit chunkier at lower volumes is a pretty common request) it's usually a lot cheaper and easier to have me swap a few bits on the PCB (haven't had to drill one yet!) than it is to sell the Marshall and obtain the replacement that already does what they want. Most of 'em have never heard of a Soldano, Two Rock, Roccaforte, Aiken, etc., etc. let alone know what they sound like.
     
  11. protoangel

    protoangel Member

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    Hey John... Thanks for those Celestion descriptions. Yes... every single G12T-75 loaded cab (2 or 4-12) I've ever tried I've HATED for that very reason... Harsh scoop/sizzle. Looked on the Ceslsetion site and couldn't see either of those speakers you mentioned. Wonder if they've stopped making them? I believe the seventy-80 may be similar to the G12-80, unless the G12-80 is the Classic Lead?
     
  12. John Phillips

    John Phillips Member

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    Yes, both of those speaker types are long out of production, since the mid 80s.

    The closest modern speaker to the G12-80 is the Classic Lead 80, but it doesn't sound quite the same despite Celestion claiming it's the same speaker - it isn't, it's a derivative of it, and not built quite the same. It's a bit cleaner and has a bit sharper top-end presence - but it's still a very good speaker. The other one that is close is the Mesa/Celestion Black Shadow 90, which is also derived from the G12-80 but in a slightly different way from the Classic Lead - it's darker and chunkier-sounding. They're very expensive in the UK though, since they are only available via Mesa's rip-off distributor.

    There is no real modern equivalent to the G12-65, and although they did try reissuing them a couple of years ago, the common consent was that they sounded nothing like the original (I never heard one, so I can't comment). The original is a wonderful-sounding speaker, a nice mix of brightness AND warmth, the top end is 'jangly' and more in the very high midrange than the treble - it's actually very like a slightly tighter-sounding, higher-power G12M-25 Greenback. More complex overtones than the G12-80, but less bass so it does sound brighter overall.

    If you do want these exact sounds, your best bet by far is to find a used early-80s JCM800 cab with the original speakers (model 1960 with the 65s, model 1982 with the 80s) - they aren't expensive, but you do need to check what speakers are in them since they are often labeled with the wrong power rating as Marshall used up left-over label plates... many cabs with 65s are labeled 100W (they replaced the 25-watt speakers) and then many that are labeled 260W actually have G12M-70s, which replaced the 65s for about a year around 1983 and are really not a good-sounding speaker... and you might even find '260W' cabs with G12-T75s. The model 1982 went straight from 80s to G12-100s in about 1984 at the same time as the 1960 went to 75s. Even then, the earlier British-made 75s do sound better than the recent Chinese ones (that's not an anti-Chinese bias BTW, they're actually built differently).

    If you just want a good sound and aren't fussed with historical accuracy, try either reissue Greenbacks, reissue G12H-30s (deeper, clearer sound more like the Classic Lead, but with more warmth), or Vintage 30s... which are a very dark, aggressive speaker with chunky bass, gritty midrange with a very noticeable (some think harsh) upper-mid peak, and rolled-off top - they're the speaker Mesa use when they're not using the Black Shadow. Any of these sound significantly better than the 75s IMO.
     
  13. samwheat

    samwheat Member

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    I've built several ax84 high octanes which basically uses a variation of the jcm800 preamp

    Each gain stage is bypassed with a .68uF cap

    I also remove the 470K resistor that goes to ground at the voltage divider to the 2nd stage ..... all they do is suck tone and restrict the amp ..... without it, the amp is more lively and free

    Finally, I bias the 1st stage with 220K plate and 2K cathode for the maximum 2nd degree harmonic distortion

    None of my amps use shielded wire because I use a hoffman type board layout

    I have an amp with that preamp and a 12au7 power section

    And recently sold a EL34 combo with a 1969 G12H-30 .... the guy loves the amp
     
  14. protoangel

    protoangel Member

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    A 12au7 power section? Is this basically a very low wattage recording amp?
     
  15. cap'n'crunch

    cap'n'crunch Member

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    I heard that a good 5751 in V1 will thicken/warm a JCM800 up. You lose a bit of distortion but, whats more important. I have yet to try it though.

    I like a 100K resistor between the master volume pot and the PI.
     

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