Some Mods for Marshall Class 5

Discussion in 'The Squawk Box' started by Lyle Caldwell, Jan 18, 2011.

  1. Lyle Caldwell

    Lyle Caldwell Senior Member

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    A client brought me his new Class 5.

    In stock form I found the amp almost unusable. Weird mids and lows for Strats, sounded pretty good at higher gain with humbuckers, but splatty with single coils, and the low gain sounds were pretty bad. No real treble, just mids and boomy lows - the Bass knob was useless with such a narrow range.

    But I tweaked it all out and I think the result is a damn good sounding amp with about $5-10 worth of different parts.

    Here's what I changed - feel free to try this if you like.

    Input - change the input impedance to 1M. This helped the Strat single coils out immensely.

    I put in a switch to go from full lows to tight "Plexi lead" lows. You can play around with the cathode caps if you don't want a switch. The 1uf is the "Plexi" value for tighter lows.

    I put a very subtle bright cap across the volume and removed the voltage divider after the volume control.

    Changed the slope resistor to get a more balanced sound (not as trebly/middy) and changed the bass cap to a 22n to get a more usable range out of the Bass knob.

    Changed the cap coming off the plate of V2 pin 1 to have more lows so the earlier switch would be apparent.

    Put in the "Marshall emphasis cap" across R12 (this is to taste).

    Bumped up the values of the voltage divider after the cap on the plate of V2 pin 6, again to make the earlier bass switch apparent.

    Put in a "fizz cap" here (68pf to ground) and made the grid to the EL84 a 220K - slightly warmer.

    Put a wire in the speaker jack section to make the "headphone switch" become an output attenuator for both speaker jacks - allows late night practice.

    We're going to be putting a Mercury Magnetics choke in, but the amp sounds really good as it is now.

    Maybe some of you will dig these simple mods.

    http://psionicaudio.net/images/Marshall-Class-5-Mods.jpg
     
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  2. LazyPaisley

    LazyPaisley Senior Member

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    sound please!
     
  3. Lyle Caldwell

    Lyle Caldwell Senior Member

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    I'd love to give you sound clips. I have a MOTU 1224 that doesn't get along with the PCI version in my new computer - trying to find a new AD converter that works with my new computer. All I can do for now is use the built in soundcard, which sucks.

    I'll see what I can do. I've got to get this straightened out as I need to do a lot of clips of my toys!
     
  4. b-squared

    b-squared Banned

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    Lyle you still hail from Memphis? You modded my AC30 CC2X awhile back.

    I drove all the way from STL to drop it off and pick it up. :D

    BB
     
  5. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Yeah man! Of course I remember you - I was sick as a dog that day.

    I'm sitting in my work shop right now with 4 AC30s undergoing mods, an AC4 awaiting MM transformers to arrive (changed it to use an EF86 - this little amp kills now), the just-modded Class 5 (waiting for the MM multichoke to arrive), 5 custom amps (my own Psionic Audio amps), and about 50 pedals under construction.

    So of course I'm goofing off on MLP...
     
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  6. b-squared

    b-squared Banned

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    Hey man...everyone has to take a break. :D

    Good to see you posting...you kinda went off the radar for awhile!

    BB
     
  7. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Ah yes, the joys of divorce...

    Back in the saddle again...
     
  8. b-squared

    b-squared Banned

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    Good to hear, man....good to hear!
     
  9. singlecut

    singlecut Senior Member

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    exactly why i don't buy the latest flavor of the month,mass produced cheap amps that are reviewed here with such gusto by members when they get them.
    so here we have an amp repair guy saying how bad they really sound .
    let me see, who do i believe.
    someone with a world of experience in the field? or someone who plays at home,maybe has never owned a decent amp in the first place to compare it to and is convinced their amp is the best ever right out of the box.but it's a marshall.
    i'll go with the amp guy every time.

    i had played one of these as well, and was not impressed. but, everyone so far here it seems thinks they are so good.i don't think i've read a negitive word on them here. until now.
    so where are all the class 5 owners ?
    glad to see the truth come out for a change. it's usualy just buyers gushing during the honeymoon period.it seems some are more concerned with the amps actual color.to each his own i guess
     
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  10. Barber

    Barber Senior Member

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    I think there are a lot of people out there with a not so good view on the marshall class 5 and other cheap amplifiers.

    It's just we don't always like to piss on other peoples chips when they have just saved up and bought something that they think is the bees knees and probably perfectly suits their needs.

    I have played a class 5 for long enough to get my own opinion of it, and whilst I agree with the original poster on some of its 'issues', when I played it side by side with the Orange equivalent, the vox equivalent, blackstar, epiphone etc etc.... a lot of the off the shelf 4 - 5 watt amps that have come out recently - the marshall was the best (of a bad bunch, some might say).

    I find it muddy, boomy and fizzy. I would be willing to bet that a full tube change and a speaker change would help it loads, but then its not a £300 amp anymore....
     
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  11. andjar

    andjar Senior Member

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    i thought it was only me finding the class 5 sounding like crap after listening to so many youtube videos.It has a plexilike crunch with an early ac/dc flavour but much more raspy and fizzy than that.I do hope some 12" speaker's will make the head version sound better but it also obviouslly needs some mods.
    I really want to like the coming little class 5 head.I think many people including me have waited and dreamed about a fulltube ,low wattge,low end marshall plexi head.But,right out of the box,a blackheart(both the 5 and the 15 watters) is killing the class 5 both in terms of classic marshall tone and circuit design.
     
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  12. Barber

    Barber Senior Member

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    I can Imagine the head sounding good through a 1 x 12 cabinet, celestion G12H30 anni, or greenback,

    a 5751 and a nice power tube, get rid of some of the fizzy 12ax7 sound
     
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  13. BrianGT

    BrianGT Senior Member

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    I can't really agree with all of that!:hmm:

    I have a Class 5 and after sorting the rattle, which I should not have had to do, it sounds good to me. The build is cheap........the chassis is crap...I don't think anyone is disputing that.

    I am SURE it can SOUND better and I am not disputing the mods carried out to it by Lyle...I just haven't heard them yet... but in the meantime I am enjoying the amp.
    I don't need to gig it, I have serious amps for that, I am just using it at home to amplify my guitar and it sounds cool to me.

    Sorry about the low volume..........

    [ame=http://www.youtube.com/watch?v=FwoSX0Ir0fI]YouTube - Clean demo of OTPG pickups thru marshall Class 5.[/ame] video...excuse the playing.
     
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  14. Lyle Caldwell

    Lyle Caldwell Senior Member

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    First, thanks. It can be depressing swimming in the internet sea where every opinion is interchangeable with another, no matter the experience.

    Second, what bugs me about the stock Class 5 is not that it is bad sounding because it's cheap, but because the circuit itself is badly thought out. They could have made it sound great at the same price point. I didn't do that much - just changed some values and moved one or two thinigs (I removed more than I added I think).

    For example, the input impedance of the stock amp is 470K. That's way too low to sound good with a Strat or Tele or other single coil guitar, and it's not ideal for Gibsons or other humbuckers either. No cost difference between a 470K resistor and a 1M resistor.

    Then they have a 470pf cap to ground at the input that serves no purpose but to roll off highs. But there is no predicting at what frequency this rolloff occurs because it will vary based on the player's volume control settings. Stupid, and it serves no real purpose.

    Then they have a fixed highpass on the first preamp stage at 21Hz, followed by a tone stack that has way too many lows, THEN the real gain stages where the highpass is set at 127Hz.

    So while there aren't any big deep lows coming out of the speaker, there are a lot of lows muddying up the gain stages, and the Bass knob has a really narrow window of effectiveness - below 9:00 not enough lows, over noon way too many lows.

    Then there is no bright cap or "Marshall emphasis" (most Plexis have an emphasis over 720Hz that follows a high pass set at 232Hz). So at lower volumes the sound is dark and muddy (remember the low input impedance and random high freq loss makes this worse) but then as you turn the amp up it gets too bright and fizzy.

    Lots of YouTube clips out there where people have the Treble and Mids dimed but the Bass is at 10:00.

    The mods I shared let you get a good balanced sound with all the controls at noon - and then you can shape from there.

    My mods let you choose between an initial (pre-gain) highpass at 187Hz or (switched) 33Hz. Then the tone stack has a useful bass control which also makes the midrange control more musically useful.

    The subsequent gain stages have highpasses of 7.7Hz and 59.33Hz, so the intial "low cut" switch is effective and the fixed 59.33hz highpass will never fart out the stock speaker but has enough lows to sound authoritative through an external 4x12 (and you can switch to the 187Hz "low cut" setting to really tighten things up without being thin).

    Aside from the switch (and I added a second switch which lets you go from a single 100pf bright cap to adding a second in parallel for a slight brighter sound) nothing I did would make the amp cost more if Marshall did it this way to begin with.

    Yes, I used Mallory 150s for the coupling cap changes, as I don't have box caps here in 400V+ values, but there's nothing wrong with a box cap for an amp in this price range (and nothing wrong at all with a box cap if it's a Wima).

     
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  15. BrianGT

    BrianGT Senior Member

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    Lyle...just so you know.......I can totally see where you are coming from with your mods because I can recognise the "symptoms" in my amp!

    All I am saying is that not everyone who has a Class 5 is wrong about it and we are all lying to ourselves about the sound.........

    It does peeve me that resistors and caps are so cheap that it makes no sense to get it "wrong".....

    When you crank it you have to work hard with the eq to make it sound great.
     
  16. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Brian,

    I'm not saying you're wrong to like what the stock amp can do. There are some settings that sound quite good, or at least have a certain usable character.

    The modded amp can still get those sounds, but isn't limited to them.

    And anyone with a little bit of soldering experience can make this amp sound great for about $5 worth of parts and an hour or two's time.

    Resistors (all 1/2W or 1W):
    1M (x3)
    1.8K (can really be anything between 1.5K and 2.7K)
    470K
    47K
    220K (x2)

    Capacitors:
    1 uf
    4.7-10uf (you can use the old C8)
    120pf-500pf (bright cap)
    470pf/500pf (you can reuse C10 for this if needed)
    22n 400v or higher (x2)
    68pf

    On those 22n caps, you can reuse C13 for C20 and you can move the original C20 to C2, and then you don't have to buy any caps for these places.

    A little bit of 22 gauge wire for jumpering C13 and for connecting the optional switches.

    A little bit of 18 gauge wire for the headphone jack mod.

    Two SPDT mini toggle switches if you want all the options (can be skipped)
     
  17. BrianGT

    BrianGT Senior Member

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    Cheers Lyle! Bookmarked! :thumb:

    I love tweaking once I know what I should be tweaking!

    I wasn't refering to you Lyle...it was Singlecut I quoted. I just don't believe that we are all lemmings........:D...and I know Singlecut doesn't either!:D
     
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  18. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Yes, we are all individuals!!!
     
  19. Lyle Caldwell

    Lyle Caldwell Senior Member

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    And, as I am a geek for this stuff and I think I'm not alone here, the 47K resistor I suggest for R28 can be played with.

    That is the slope resistor in the tone stack. You can play aound with values from about 22K up to 100K. The larger the value of this resistor the more separation between the highs and the lows/mids there are (and the more the treble pot is effective and can in fact dominate).

    100K is the "Fender" value and is way too bright IMO for a Marshall.

    68K is stock for the Class 5.

    Most Plexis use 56K though I've seen lower.

    I tend to prefer 47K, just personal preference.

    If you're a Blackmore fan, you might prefer 56K or the stock 68K. If you are an Allman Bros fan, you might want to go as low as 33K.

    In some amps I'll put a 33K resistor in series with a 25K pot and let the customer tweak to their heart's content.

    Note: there is high voltage across this resistor - if you try the pot make sure to be careful with insulation. And don't touch the lugs if you fly in a temporary pot to try different values here.

    On that note, please research how to discharge the filter caps on an amplifier before trying any of these mods. Any amp, even one this small, can kill you if you do not safely discharge the voltage stored in the filter caps. Lots out there on Google.
     
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  20. NSCS

    NSCS Premium Member

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    The mass of inexperienced reviewers or a guy who's business it is modding amps...

    Just kidding of course Lyle :thumb:

    It always amazes me the money people will spend on tubes, speakers and those ridiculous mercury transformers to tweak the tone of an amp. Little do they know swapping some 10 cent resistors and $1 caps gives worlds more options.

    When I first saw the schem of the class 5 I was surprised at how far of a departure it was from any classic marshall, or any other guitar amp. An AC coupled cathode follower? Really, in a guitar amp??? Talk about a wasted opportunity.

    Seems like they turned a fresh EE loose to try and come up with something new and different, but they ended up making some mistakes that a more experienced guy would know to avoid.
     

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