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Thread: I have modded my Marshall Class 5!!!

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    I have modded my Marshall Class 5!!!

    OK, y'all are going to pissed...as this is a tease sort of.

    I could not get a good recorded clean sound out of this amp without the "fizz"...and dimed the amp was too muddy...but I found an easy (and cheap) fix for it.

    Change R1 to a 220K resistor, and R18 to a 2.2K resistor. Effectively reduces gain on the first stage. Amp stays pretty clean up to the halfway mark, and is just gobs smoother. It also helped the muddiness a bunch.

    I am waiting on parts, but there are 7 caps in the signal path I will be replacing, as well as changing c12 to 33uF from 47uF (this will allow the EL84 to reproduce more bass)

    Well why would I do that? Because the 1963 Jensen c12r sounds a little thin compared to the stock celestion.

    What's that?....Yes, I made a baffle for a 12" speaker...and found a way for it to fit. I'll post pics of the completed baffle in a few days, but have been playing with the mock up, and it just blows away the stocker in every possible way. THIS amp is no more a "one trick pony".

    Also...the stock speaker is nowhere near 16 ohms. Lots of room for improvement there. I measured 11.2ohms, and the Jensen I'm using measures 11.8ohms.

    Marshall was kind enough to float me a clear and detailed schematic to mod the amp...I'd be lost without it! It is a sound design, but room for improvement!
    Attached Thumbnails Attached Thumbnails 12fq8-005.jpg   12fq8-017.jpg   12fq8-018.jpg  

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    Wink

    I think you should sue those b*st*rds, they stole you 4 ohms each.
    They did never imagine you would actually *measure* them, did they?
    Now I would ask not only for what they stole, but, say, an extra 4 ohms *each* for damages.
    I forgot .... plus interest !!!













    Now on to serious stuff: what you are measuring is the DC resistance of the voice coils; he actual speaker impedance will always higher than that, and varying with frequency; so to make life easier, a "conventional" value is used, some 20% to 30% higher t5han VC resistance, so those almosd 12 ohms indicate a real-world speaker of around 16 ohms.
    The exact value is imprecise, or, better said, complex.

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    Yeah, it's tube electronics too...so it's like horseshoes and hand grenades where "close" is OK. Although the lower ohm load would lose headroom. I know the amp is just supposed to be rowdy, but the tone is good stock...so it's such a waste that 3/4 of the dial is distortion.

    I did find the PCB is VERY fragile, I may hardwire the tube mounts and use mounts that attach to the chassis. I'm always changing tubes, and will probably kill the stock mounts in no time. Plus (especially with the 12" speaker) the EL84 is almost right on the speaker...probably wouldn't hurt to relocate the tubes up a bit higher.

    To make chassis removal easier I disposed of the stock aluminum screws, and put in more durable hand screws. I personally don't care about the warranty...so I'm making the amp fit MY needs. I did find a nice combo of tubes for it. I have a pair of '62 RCA 12AX7's, and a late '50s Amperex/Holland EL84. The stock JJ's were totally microphonic in that amp.

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    Quote Originally Posted by ron vogel View Post
    Yeah, it's tube electronics too...so it's like horseshoes and hand grenades where "close" is OK. Although the lower ohm load would lose headroom. I know the amp is just supposed to be rowdy, but the tone is good stock...so it's such a waste that 3/4 of the dial is distortion.

    I did find the PCB is VERY fragile, I may hardwire the tube mounts and use mounts that attach to the chassis. I'm always changing tubes, and will probably kill the stock mounts in no time. Plus (especially with the 12" speaker) the EL84 is almost right on the speaker...probably wouldn't hurt to relocate the tubes up a bit higher.

    To make chassis removal easier I disposed of the stock aluminum screws, and put in more durable hand screws. I personally don't care about the warranty...so I'm making the amp fit MY needs. I did find a nice combo of tubes for it. I have a pair of '62 RCA 12AX7's, and a late '50s Amperex/Holland EL84. The stock JJ's were totally microphonic in that amp.

    Why don't you make a new turret board while you're at it and use ceramic tube sockets?

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    Ehh, maybe someday, sounds like a lot of effort!

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    Hi Ron.
    You will, eventually, because as you correctly guessed, those relatively thin/narrow tracks have the nasty habit of cracking just where they join the soldered pad .
    Removing tubes often won't exactly make them last longer, so someday in a not too distant future, you'll find rebuilding on turret or eyelet board the "easy"solution, relative to troubleshooting repetitive so called "cold solder" problems.

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    Senior Member Enzo's Avatar
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    As JM points out, your ohm meter is not measuring the impedance of the speaker, only the DC resistance. If you want to know the impedance for real, use a 1kHz test signal and AC measuring meters and determine the impedance with Ohm's Law.

    It might be instructive to measure a lot of your speakers. You will find that the DC resistance of most 8 ohm speakers is around 6 ohms, that of 4 ohm speakers is around 3 ohms, and that of 16 ohm speakers is... about 12 ohms. The amps do not produce DC output, at least we hope not. SO the amp does not face whatever resistance your meter shows.

    You cannot measure speaker impedance with a regular ohm meter.
    Education is what you're left with after you have forgotten what you have learned.

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    Just wanted to show off the final product...stuffed a 12" speaker in there with a custom baffle. Just got the grill fabric on today, put on the piping, and put it all back together:



    Last edited by ron vogel; 05-12-2010 at 01:36 AM.

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    Hey, It's nice.
    That grill cloth and piping sure look professionally applied.
    Wouldn't have believed that your C12R would fit there, and even with a little room to spare.
    Now we deserve some MP3's.
    Can't you place the original speaker side by side or close by to the new one?
    I'm curious about what they fit as an original.
    A couple sideways, back and front pictures wouldn't hurt either.
    The Jensen, I know by heart and don't specially need more info about it.
    What difference do you hear, in actual use, between them?
    Thanks.

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    Quote Originally Posted by J M Fahey View Post
    Hey, It's nice.
    That grill cloth and piping sure look professionally applied.
    Wouldn't have believed that your C12R would fit there, and even with a little room to spare.
    Now we deserve some MP3's.
    Can't you place the original speaker side by side or close by to the new one?
    I'm curious about what they fit as an original.
    A couple sideways, back and front pictures wouldn't hurt either.
    The Jensen, I know by heart and don't specially need more info about it.
    What difference do you hear, in actual use, between them?
    Thanks.
    Thanks!
    The grill cloth and piping are original Marshall. The grill cloth is $$$ ($70 a yard!), I put the unused pieces in my ebay store. The Marshall logo was pretty pricey too, I should have just bought the plain white 6" logo. I have done a few cabs, so I have a little experience under my belt. The fabric is very heavy duty, and not easy to work with. I probably removed more staples than I put on when I was working with it!

    I wouldn't want to put in the original speaker in there with it, I'm already pushing the cab volume with the larger speaker. I made the new baffle like a soundboard so I can still get a nice "bloom" out of the amp...which fortunately worked out very well.

    The magnet has about 3/4" to spare installed...so it is a very tight fit.

    I also forgot to mention I replaced the hard plastic feet with large rubber ones.

    I'm really not trying to whore out my ebay store, but I added several of the items (like the gold corners, fabric, rubber feet, and felt) I used that were hard to find.
    eBay Stores - Fogelsang Studio items on eBay.com

    I have a recently done clip with the Marshall BEFORE the mods:
    Start Player

    I am starting a new tune this week that has to have a very defined tone, and will be using this amp. I wanted to already start it, but I have 4 kids...so recording time is a bit limited!

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    You are right Enzo, ron vogel dind't have a 16 ohms but a 8 ohms.

    I knew a guy who is testing is ouput trans, at the secondary if he has 8 ohms.
    Of course not, after mesuring with an ohnmeter he was sure that is transf was bad.

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    Ron,

    Would you mind sharing that schematic? I can only get hold of the blurry copy thats been doing the rounds.

    It seems like you are aiming for the same goal as me, cleaner/less muddy, so I will give the mods you've documented a go. I will however be keeping the stock speaker as I like the bass and punch it gives (unless I come into a Celestion G10 Gold by some stroke of luck).

    One mod I've started, is replacing LK23 (Link between mid pot and ground) with a foot switch to bypass the tone stack and provide a boost. Sound like a good Idea? I've got the jack socket in place just need to wire up a footswitch. Using a switching jack so the amp is stock (with tonestack) when the footswitch is unplugged.

    Also had thoughts about removing C8 (10uF). I believe it will increase headroom and increase upper mid/treble response. Am I on the right track?

    Cheers,

    James
    Last edited by dido__15; 05-20-2010 at 11:46 AM.

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    Screw it...here's the schematic!

    I don't know about the boost switch, but I am still making changes as I go (when I have time to).

    I put the 100K resistor back in r1 (it dropped too much bass), but kept my change of 2.2K in r18. I don't think removing c8 is a good idea, but upping the value a touch will lower bass.

    The design is a fine balance, and the trick is to cascade the gain to retain the touch sensitivity, and still drive the output tube. I still can't get a good overdrive sound out of the amp recorded. It just sonds fizzy, and I think the output tube is not being driven hard enough, and the preamp tubes are driven too much...although it's much better.

    I find myself turning the mid pot down to get a tone that isn't too muddy. I'm going to swap in a lower value in to see if it works.

    I'm no expert, just an eager tinkerer. So if anyone more knowledgable can chime in about how to increase headroom in the preamp, and either lower the voltage to the EL84, or push the input (el84) on it harder that would be great!

    (and help me get the mid pot frequency to a higher range)
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    Last edited by ron vogel; 05-22-2010 at 10:48 PM.

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    Going step by step.
    1) Ron, thanks for posting the schematic, it's very useful and it answers a lot of questions.
    2) ¡¡¡ Oh My God !!!!!!!!!!!!!!!!! ¿What's that? ¿What were they trying to do?
    That's not a Marshall !!!!!!!!!!!!!!!! Totally amateur design, no wonder it's muddy and fizzy.
    Drop that board in the nearest trashbin and build some of the AX84 versions. Any of them will sound much better. Do not waste your time polishing a turd.
    Just the simple fact that a track is broken in up to 6 pieces joined by jumpers shows the amateurishness.
    The headphone out is incredible. That bass lifting net they use is too high impedance for headphone use and too poor to compensate for fizziness if used as a direct recording line out.
    3) Consider building something else there, an eyelet or turret board will allow you mod it to death at some later date.
    Sorry and good luck.

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    I jumped the jumpers with push-back wire...and I gotcha on the design...might be why Marshall was reluctant in letting it get public.

    I truthfully have had a lot of issues with this amp...mostly from trying to record it. It sounds absolutely great in person, nice cleans, and the amp dimed is unbelievable. However, I think the lack of 2 output tubes hurts it for harmonic content...maybe I just like the 6V6 sound better I dunno.
    For recording, I put a modded DS1 in front of it and it helps a bunch for tone. I also have good luck with running a wah in a set position to bump the mids.

    I like the amp, but I can only have one...so this may go in favor of a nice little champ...I made up my own design (a ripoff cross between a Champ/Supro)...maybe I'll just build from scratch and be done with it!

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    Quote Originally Posted by ron vogel View Post
    I'm no expert, just an eager tinkerer. So if anyone more knowledgable can chime in about how to increase headroom in the preamp, and either lower the voltage to the EL84, or push the input (el84) on it harder that would be great!

    (and help me get the mid pot frequency to a higher range)
    Well....the changes you did, or plan on doing, actually have opposite effect to what you intended. So to get the amp where you want it go in opposite direction with component value changes.
    tip: change R11 & R13 to half their present values, change C8 to 1/10th it's value.
    Aleksander Niemand
    Zagray! amp- PG review Aug 2011
    Without the freedom to criticize, there is no true praise. -Pierre Beaumarchais, playwright (1732-1799)

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    Quote Originally Posted by Alex/Tubewonder View Post
    Well....the changes you did, or plan on doing, actually have opposite effect to what you intended. So to get the amp where you want it go in opposite direction with component value changes.
    tip: change R11 & R13 to half their present values, change C8 to 1/10th it's value.
    I thought C8 seemed high, hence why I had thoughts of removing it. I will definately try 1/10th the value if I have it in my parts drawer.

    One thing though, will halving R11 and R13 not increase the gain of the last 12AX7 stage? I know this isn't a high gain stage but will it not add distortion?

    Also, increasing R1 to 220K seems to have lowered the headroom. Am I right in thinking that increasing the value of this resistor lowers the voltage heading to V1 causing it to distort earlier? I didn't think about it before the mod. I'm a tinkerer too but trying to learn the theory behind it. If I am correct, and this increase in R1 has increased the distortion, I will put it back. Would it be safer to go lower than 100K?

    There is a lot of potential in this amp, and I used to love it at my folks place where I could crank it, but since getting my own place and using it with a weber minimass you can notice it gets very fizzy when the gain is turned up, this fizz was filtered out by speaker breakup before so not as bad, but without the speaker breakup at low volumes it's very prominent. I think pushing the EL84 harder should go some way to improve it as ron vogel suggested.

    I think that lowering R25 from 2.2K to 1.5K will also increase gain going into the EL84 and bring a little more "sparkle" into the power section.

    Anyone got any other ideas?

    And Ron thanks very very much for the schematic, certainly much easier than squinting at the fuzzy version, you've made it much easier for everyone.

    Quote Originally Posted by ron vogel
    I find myself turning the mid pot down to get a tone that isn't too muddy. I'm going to swap in a lower value in to see if it works.
    Decreasing the value of the mid pot will mean more mids I believe, I've heard that changing the mid pot from 20K linear to 100K LOG A will get rid of a lot of the mud, and give more range on the pot. I might be wrong though, just passing on my research, haven't tried it yet. Does anyone know where I can get marshall pots like the stock ones to replace it?

    James
    Last edited by dido__15; 05-24-2010 at 12:57 PM.

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    Thanks for posting the schematic, I wonder what Marshall were thinking too, with all the tubes they could easily have made a mini jcm800 or mini 1959 type amp…. Which his probably what more people wanted..

    Here’s some of my thoughts,

    I don’t think blasting a SE amp power section super hard is a great idea, particularly if you want to get rid of fizzy distortion.
    I would lower the grid leak resistor on the power valve to 220k (R26) and raise grid stopper (r25) to 220k-330k too which will help smooth the high end a little.
    The input capacitance of power valves is really low so you can raise the grid stopper with out loosing high end, but it seems to help performance.
    Lowering R26 will also increase your headroom…

    I am surprised at how much bass they are putting through the amp, the only bass limiter is the tone stock and the coupling cap before the power valve, your muddiness may be attributed to the power section farting out with all the bass. The small speaker probably helped with this and having upgraded to a bigger speaker might have made it more evident? Anyway you can re-enforce this somewhat if you want to help it handle the bass better by increasing C12 to 1000uf – 22000 uf
    Also increasing C4 to 100uf may help the bass response too a little.

    If you want to loose the low mids you can change R28 to 82k or 100k, you will also loose some drive by doing this but the low mids will be reduced..
    Increasing the mid pot size will not reduce the amount of mids.. it will only increase them.

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    OK seems like I was quite a way out with my thinking. I'm obviously not quite ready to be let loose on my own yet. Thanks a bunch guitarmike that's a very detailed explanation, I will do some more studying then give it a go.

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    with all the tubes they could easily have made a mini jcm800 or mini 1959 type amp…. Which his probably what more people wanted..
    Fully agree.
    Yet it's not too late to make a PTP/eyelet/turret board with , say, a JCM800 preamp, driving that single power tube.
    Look around for the layout, it's really simple, and you need nothing special, toolwise.
    The sound will be killer.
    Someday you can also make a speaker emulator board, to record straight into your PC soundcard with no-fizz killer sound.

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    I'm sure Mouser or Digikey would...I didn't look at it real close, but it looks a lot like a bourns pot.

    Good info on here.

    Also, the fizziness really shows up during recording. I did change the 220K back to 100K, but left the 2.2K in the V1 stage. My intention was to lower supply voltage to V1.

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    Can you detail how do you record it?
    Thanks.

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    Quote Originally Posted by J M Fahey View Post
    Can you detail how do you record it?
    Thanks.
    yes that would be helpful.

    are you attenuating for the recording? if so speakers ( and your ears) appear to react differently at different drive levels, i.e. a g12h30 celestion is very fizzy at low levels.

    There are some recordings of this amp on youtube that are not overly fizzy....??

    BTW changing the first stage will not really effect the fizz, it will effect the drive to the rest of the amp, but if you end up driving it harder it will just fizz more

    Mike

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    Mike, I think I am going to have to concede that using an attenuator is what is giving me the fizz. But I could still do with some more headroom anyway, and just cant resist tinkering so I'm gonne have a crack at what you suggested. Any improvement is still an improvement right?

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    I'm curious if you tried out any of your own mods yet. I just sold my Class 5 (I put it back to mostly stock)

    I really was spending too much time trying to get my guitar tones with it...and would rather be composing!

    Anyway...I got lucky and picked up a '72 Champ for $200 locally. It was really poorly taken care of, and the seller probably thought he was selling me junk...but an evening of scrubbing has it looking pretty good...ordered all the caps today for it even though it works.

    I know the Champ will work for me, I got the class 5 on a bit of a whim...when I really was going to buy a champ originally...so it was a bit of a detour.

    It's still a nice little amp, just not for me.

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    I don't use anntenuators. I had a Princeton Recording amp with the built in one last year...it sucked the tone dry while it was in use. It really turned me off of those things.

    I have tried all sorts of micing set-ups with the amp. Mostly an SM57 or SM7B on the grill, and a condenser a little out in the room. I have also made several mics out of old vintage speakers...I got a nice big sound out of one of them on the grill, and a condenser back in the room.
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  27. #27
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    Quote Originally Posted by guitarmike2107 View Post
    I wonder what Marshall were thinking too, with all the tubes they could easily have made a mini jcm800 or mini 1959 type amp…. Which his probably what more people wanted..
    Thats what I thought too - I hear people asking for low-watt JCM800/plexi amps all the time. I wouldn't mod this amp, I'd just gut it and convert it to a PTP JCM800/plexi.
    HTH - Heavier Than Hell

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    The story so far.

    OK, so I've now done the following changes.

    R1 back to 100k
    R18 from 1.5k to 2.2k
    C8 from 10uF to 1uF
    R26 from 470k to 220k
    R25 from 2.2k to 260k

    The tone is getting much closer to where I want it. The fizz is all but gone now, I'm getting a much better overdriven tone, even with the attenuator on.

    However I've now reached a problem whereby if I play a chord with a loud attack (Particularly at the lower end of the neck) the amp suddenly drops volume. Happens occasionally with notes at the bottom end of the low E as well. Would this be the power amp "farting out from the bass" as you described? I haven't tried increasing the bass handling yet but is this likely to be the resolution to my problem?

    Any help is greatly appreciated

    James

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    Unless PT really sounds better then why waste a well designed PCB? This baby has start grounding and DC filamants, which is rare. It is close to optimized and just needs a few more tweaks, mainly de-"squashing". Kudos to Marshall!

    MV at R12 and R14 right? Or might it be better after the PI subbing out R24 and R26?

    Also with the four 470K resistors around V2 it might be an idea to drop in a 12ay7 and drop these all to 150K (ish). So as not to drive with the parking brake on. You'd think the distortion would disappear but it is actually a revealing of quality over/and quantity.

    A: If you're getting a drop in sound, add some beef to the filter caps, anywhere.

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    I tried putting a 12" Eminence Tonkerlite 16 into my Class 5 by borrowing from Ron's baffle design. It sounded great until I turned the volume to about 3 o'clock. After a few moments of playing at the higher volume level, the amp made a crunchy sound and lost all of it's volume. I immediately turned the volume down on my guitar and waited to see if it would recover. After about 30 seconds, the sound returned. It does this whenever I have the amp turned past 12 o'clock. Has anyone run into similar issues or knows what may be causing the problem?

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