Modding A Lead 100 Mosfet? Help Wanted

Discussion in 'The Workbench' started by Speeddemon, Dec 27, 2012.

  1. Speeddemon

    Speeddemon Active Member

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    Hi guys, I was lurking here for a while, until I recently (just the day before Christmas!) acquired a Marshall Lead 100 MOSFET head with the matching 1965A 4x10 cabinet (with beam blockers pre-installed :thumb: )

    Anyway, I've become quite a believer of this amp as I was able to crank it really friggin' loud and it RAWKED! Almost tube-like. I of course used a footswitch for the channel switching and to avoid the channel bleed/cascading.

    It's close to mint condition and it turned out it was a 1984 (S-serial) made one! (black cloth, not the discolored brown/tan one though). :cool:

    I saw some threads from Wakjob about his mods of the Artist 3203 and another Mosfet head (the Twin Reverb I believe and the 3310) and I was wondering if some of these mods apply to the Lead 100 Mosfet head as well?

    My wish-list:
    -clean channel more clean (now I have to keep the Volume below 3 to keep it really clean).
    -drive channel more max. drive possible
    -a bit more low-end, and maybe a hint tighter too.

    I noticed lowering the R5 resistor wouldn't help, as for the 3210 it's the footswitch LED resistor and not in the Boost channel's gain circuit. I won't tamper with the drive LEDs, as I understood they're integral to the tone (and having built quite a few guitar pedals years ago, I know how cool LEDs can sound in a DOD 250/Dist+ like drive circuit).
    Would lowering R6 (47k) help here or should something be done with R4?

    Any help would be greatly appreciated.
     
  2. wakjob

    wakjob Well-Known Member

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    First, welcome to the forum. And welcome to the Mosfet Posse.

    Second, You'll need the right schematic. It's NOT the 3203/3310 schem. There's a very clear 3210 schem. and an old fuzzy hand drawn one like the one found in the Marshall book. Just google image it, it pops up.

    You are correct in your assumption, all OD/distortion pedal type mods apply here.

    +1 million on the red LED's! They are coin!

    I'll share some of my findings.

    1) Don't go hog wild. Marshall knew what they were doing with this amp. Keep ALL the tantalums and small electrolytics, no need to swap them.

    2) Get rid of the input cap C1. It looks like a resistor. In fact, locate all the capacitors that look like resistors and chuck em'. Replace it with a good film cap. You can lower the value for an overall 'tighter' feel to the amp also. Polypropylene is 'metal'. Polyester is classic Marshall.

    3) You could try removing LED's 4 & 5. It will clean up the normal channel, but at the expense of the Drive channel I'm afraid. And that's the beauty of this circuit. It's cascaded, so you get a full working tone stack for the boost channel too! But try it anyways, I didn't, and I'm curious.

    4) CAPS CAPS CAPS!!!
    Find all the ceramic caps that are in the signal path and swap them for a good 50vdc film cap. I've found

    Panasonic (metallized Polyester film) works great for high gain and the Nichicon (Polyester film/foil) give the distortion a more vintage/grainy texture.

    Xicon (Polystyrene film/foil) are touch and go. They mostly come in pF values and can be too sweet/smooth sounding on the top-end. I like to use them for the oscillation control caps that are in the feedback loops of the op amps. And sometimes for the Treble cap in the tone stack.

    Kemet "Golden Max" c0g/np0 temp. coefficient axial ceramics kick all kinds of ass too. They still sound like a ceramic cap which is a good thing, but are nice and clean when under high-gain, a lot less high-end 'hash'.

    5) C36 is a good cap to make much bigger. Stock is .47uF... I went to 2uF (Panasonic). Here's where you'll get the low-end you're looking for.

    6) While your in there, replace the 2200uF filter caps, C48 & C49. They are due. It'll lower any hiss and give the amp some punch back.

    Enough for now. Should get you started. Digikey and Mouser are your friends here.
     
  3. Speeddemon

    Speeddemon Active Member

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    Great reply, thanks!

    Quick question about the filter caps (given the value, I assume they're electrolytics); what voltage rating should those have? 50V or more?
    And the same for C36?
    The reason I'm asking, is that I also still have some WIMA 2uF film caps (non-electrolytics!) lying around, which are I believe rated 63V IIRC.

    Little side-story: I moved about 9 months ago from a rental place to my first 'bought' place; the attic is where I'm going to re-build my home-studio/music room, but right now it's filled with boxes and nothing is setup. I got so tired of not being able to quickly solder something, I cleaned, re-arranged and emptied the mofo, so I can have my workbench clear again.
     
  4. wakjob

    wakjob Well-Known Member

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    The 2200uF electro's are 50v if I remember right. Pretty sure even Radio Shack has em'.

    The 2uF I suggested for C36 needs to be a film cap, so the Wima in question would only work if it's not a box cap with short leads. If it's a molded cap with long leads you'll be ok, because its a stretch from one solder pad to the next.
     
  5. Speeddemon

    Speeddemon Active Member

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    I just opened the amp to clean all the pots; crackle is gone on them now, aside from the push/pull pot. Occasionally when touching it, the pushed mode will be audible, when the knob is pulled.
    The 2200uF caps are indeed 50V. I did noticed, aside from a bunch of crappy ceramics and tantalum caps, that pretty much all the other caps in the nF range are all red boxes; they look like old WIMA's. The logo is a circle with a star in it, of which the lower two 'legs' are longer and there's a horizontal line between them. Most likely ISKRA (from Slovenia) although this is missing the horizontal line:
    [​IMG]

    Never seen those before. Were they standard in mid 80's Marshalls?

    Would replacing IC1 (right now a 1458PA) with a NE5532 or OPA2132 be a big improvement as well?
     
  6. wakjob

    wakjob Well-Known Member

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    Careful with that push/pull gain pot. They are hard to find if it breaks.

    Leave the tantalums alone. Trust me.

    Go ahead and get an 8-pin dip socket and experiment with IC1. I ended up with a TL072IN. Fancy/expensive op amps do nothing for distorted tones. The cheaper op amps with bad specs sound better for the dirty stuff. 1458 is as good as any.
     
  7. Speeddemon

    Speeddemon Active Member

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    Wouldn't a 1 Meg DPDT push-pull pot work there then? Also, I wasn't planning on replacing the tantalums, just the ceramics.

    I see, but what makes me wonder how this circuit is different from the 3310/3203, as you did recommend changing IC1 to an NE5532 there?
     
  8. wakjob

    wakjob Well-Known Member

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    I've only seen those pots with regular solder tabs not PCB mount. So flying lead would be in order. I've done this no problem.

    The NE5532 didn't respond the same to this circuit. Still sounded good, it just didn't make the best out of this amp. But sounded glorious in the 3203. Grab a socket and try a few, you might come across a bomb op amp for this circuit. I just tried the half dozen or so I had on hand.
     
  9. wakjob

    wakjob Well-Known Member

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    Here's what mine currently looks like:

    [​IMG]

    [​IMG]

    I had gotten a little crazy early on and when too far: :D

    [​IMG]
     
  10. Speeddemon

    Speeddemon Active Member

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    Hey Wakjob, I ordered the parts yesterday for a re-cap and tweaks. C36 will become a 2.2uF Roederschein film cap. :cool:
    Banzai effects doesn't carry the 2200uF 50V's (only 40V, and I wasn't sure if that voltage tolerance would be enough), so I still need to get those.
    Also, they only had radial Panasonic electrolytics, so I went with the regular axials. And I do have most of the needed caps at home to replace the ceramics. Very curious to how it will end up. :eek2:
     
  11. wakjob

    wakjob Well-Known Member

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    Cool! Have fun with it. Thats the most important part.

    I highly recommend Mouser and Digikey for parts. Once you learn how to use their search system, its a breeze.

    I also noticed you added some gear to your sig... Invader 100!!!!????? What the hell you messing around with the Mosfet for? I would kill to have channel three of that Engl in a single channel amp form like a 2203.

    It'll also be interesting to see how the Mosfet responds to those 4x10's.
     
  12. Speeddemon

    Speeddemon Active Member

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    As I'm in Europe, those two are usually a bit more cumbersome to order from than using the smaller shops here.

    :cheers: Thanks! Yeah, the Engl is my gigging amp for over 5 years now, and it's not going anywhere. Actually, Channel 2 is pretty friggin' awesome too, as it contains a lot of Marshall-esque tones, but more refined and smooth. Channel 4 is too loose, not really muddy, but it's not that tight, and more suited for smooth,singing leads. It contains a shitload of mids, so my mid-knob of that channel is at 8 'o clock!
    Before the Invader I had a Savage 60, which would fit perfectly size-wise on the 1965/1966 cabs (24" wide) and that is an awesome amp as well. It actually could do a gain-cascading thing like the MOSFET; and its lead channel was sort of voiced like the Invader's CH3. So right now I'm keeping my eyes open again for one...
    Why dabble with the MOSFET then?
    Well, it was a sort of arranged-marriage; I was interested in getting the 1965A cab, but the guy selling it, wanted to sell it together with the MOSFET head. So I tested it and was quite amazed how well and almost tube-like it sounded. Really, for a GNR or AC/DC tribute, you could cover a lot with just that amp. I still wonder if I'm going to bring it on the road, as the Engl *is* miles and miles more awesome, versatile, better, yadda-yadda, and the Carvin is more portable and versatile, but I'm not quite there yet with the tone of the Carvin either. I did order a new set of JJ El84's along with the MOSFET caps, so once that stuff is in, I can tune both amps.
    I'll keep ya posted. :wave:
     
  13. BELT SANDER

    BELT SANDER New Member

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    What happened with the MOSFET!?!?! How did it turn out?
     
  14. Speeddemon

    Speeddemon Active Member

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    Well, it's still on my work bench, lying guts open.
    I did receive the parts, but unfortunately, my big hope (C36) had way too wide spacing. I screwed up somewhere when measuring. I could ofcourse solder some leads to it... It was a $4 capacitor, not a 10ct one.

    There were some other projects that wanted priority, so maybe somewhere in the coming weeks...
     
  15. highgainplayer

    highgainplayer New Member

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    Hello from Sweden !!

    New to this forum,

    But Wakjob, greatly appreciated your guidelines and since I was looking for exactly same improvements as Speeddemon I ordered parts this week and had them installed this evening.

    Bottom is improved, hiss and noise a tad too, but what´s worst is the clean channel is still breaking up like it used before.

    Even if I keep both my master and clean volume set on 1 it still breaks up.

    Any hints... ?

    I even soldered in a pair of LED sockets so I can swiftly unplug LED 4 & 5 but to no avail.

    That slight annoying distorted sound is always present whatever I do.

    I used to play a head of this kind in my youth, back in the late eighties, and like to remember it sounded okay then. But then it was all about hard rock and with time one has come to appreciate clean tones as well....

    So now when I went and got myself some gear on a budget I wanted a traditional looking Marshall head without paying tube money... But perhaps that´s where I will end up anyway by the sound of this one :-(
     
  16. wakjob

    wakjob Well-Known Member

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    Sounds like either channel bleed or the opamp is clipping. Mine also did this to a degree.

    Maybe adjusting the value of R17 will help.

    Unfortunately, I no longer own this or any other Marshall at the moment.
     
  17. highgainplayer

    highgainplayer New Member

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    Cudos Wakjob !!

    Amazing to get support in almost real time despite me sitting in the office over here for time being :)

    With "channel bleed", you mean replacing IC2 which is the circuit the pedal actuates ?

    While opamp would be the IC next to R17 ?

    Btw, R17 is 1Meg, so any clues to whether raise or lower that (sorry, too long time ago since I studied semiconductors and IC´s.......)

    FYI I have an original pedal and cable plugged in at all times and gain and reverb switching works ok.

    Another thing I noticed this morning when I conducted some more checks is that if I turn down the tone stack i.e. bass/mid/tre to zero while in clean mode, the sound cut outs. Is this how it is supposed to be ?!

    I really fancy the "organic" expression this head has despite being a SS and would love to hold on to it rather than spending top dollars for tubes.

    Cheers
     
  18. wakjob

    wakjob Well-Known Member

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    Try halving (500k) R17 and see what happens. I have no guarantees it will work, but worth a try.

    Replacing IC2 won't fix any channel bleed issues. It's a capacitor to return (ground) issue. RickyLee here on the board knows more about this.

    I never played these amps when they were new back in the day so, I'm not even sure that the Normal Channel is supposed to be squeaky clean?

    I'm trying to remember, but I don't think I ever turned all the EQ controls to zero. So I can't comment on that.
     
  19. highgainplayer

    highgainplayer New Member

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    Ok. Got you. You´re saying this slight distortion could be a bleed through from the gain channel, caused by a bad cap along the 0volt ?

    Perhaps I also should buy a can of such cooling spray which I like to remember was quite handy when locating faulty IC´s. If it is an op amp.

    I also do recall that high ohm resistors has a tendancy to vary over time and since R17 appears as one of the highest valued resistors in this amp it may make sense to measure it.

    Will consult RickyLee and see what he has to say.

    Me neither remember because back in the days it was all about maximum distortion, but I tend to see many posts that praise this amp´s good clean channel. So would like to think it should not have that annoying slight dist to it. It becomes very obvious when I strike whole chords. Not so much during solo playing.
     
  20. wakjob

    wakjob Well-Known Member

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    Just spit balling here but, it could be the age of the amp. Know what I mean? LOTS of electrolytics in there. And those Mosfets (2SK134 & 2SJ49) might have taken a good pounding in their day. They are also noted for failing in these amps.
     

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