New Distortion Project - The Plexizer

[aron]

Please keep this thread re: pedal info.

[goosonique]

Yes Boss 

btw this circuit just reminds me of a Doug H's take on trainwreck   

[Gary]

What is it? I wanted to create a box that got a good Marshall 1959 (Plexi) sound - that's what the Plexizer does.

So the Thunderchief isn't a "good Marshall 1959 (Plexi) sound"?

Seriously though, nice work, mojotron.

I have spent the last several years studying Marshall amp circuits and have found many things.  "Plexi" can mean many things.  There was the Plexi JTM45 with KT66 tubes, which is a slightly modified Fender Tweed Bassman, or 5F6-A circuit right down to the shared cathodes of V1.  There was the "Plexi" Bass and Super Bass.  There was the "Plexi" Lead and Super Lead, the Tremolo, the Super PA, Organ and so on.  Each of these circuits varied wildly from batch to batch, even within the same year or month of production.  Marshall would often substitute parts and significantly different values.

It was not out of the ordinary to see three amps with sequential serial numbers that had different OTs, different cathode configurations, etc.  This is the root of the old "a good Marshall" story.  There are the ones that sound good stock, and there are the ones that you need to tweak a little to get them to sound as good.  This is how the whole clique of "Marshall blueprinters" have become popular.  They are trying to match their amps to the "magic Marshall" recipe.

My point is, "Plexi" means different things to different people.

BTW, your circuit, like the TC, is based on the popular circa 1969 schematic.  Atleast, the "standard" amp schematic.

Again, nice work.  I'm looking forward to hearing the sound files.

[idlechatterbox]

Impressive design/schematic, to say the least 


cool that you worked a v-amp into the chain too. Congratulations on what looks to have been a lot of hard work.

[mojotron]

What is it? I wanted to create a box that got a good Marshall 1959 (Plexi) sound - that's what the Plexizer does.

So the Thunderchief isn't a "good Marshall 1959 (Plexi) sound"?
...
My point is, "Plexi" means different things to different people.

BTW, your circuit, like the TC, is based on the popular circa 1969 schematic.  Atleast, the "standard" amp schematic.
...
I'm looking forward to hearing the sound files.

Very good point! On top of there not being a 'Plexi' model... I tried a lot of different Marshall designs from a lot of different schematics to shoot for the JTM100 Super Lead sound comparing it to several I had played through (and sadly - never owned  ) the years - my favorite was/is a SL Tremolo (T1959). I think I really got interested in these from the first time I got the chance to play one loud enough to feel it - at that point I was hooked. The TC got me pretty close to that sound, I started modding things, added in the tone stack, as well as the caps C2, C8 and C17 (based on the info in the Fetzer Valve design - the ROG site ROCKS btw!!!), then I came up with these subtle changes from this 'Marshall 1959' (which I do believe is circa late '60's/early '70's) schematic http://www.mojotronics.com/smf/1959.gif that I think recreate the sound I have in my head - everyone else's head will differ I'm sure - I was shooting for a very specific tone I know well. Finally, I swapped out a 4.7k for the 10k resistor I was using in R17 because I really liked that extra kick that it gives you on the TC - and bingo I finally had that sound I was looking for. Of, course the louder you play with this thing, the better it sounds - as anyone within a square mile of my house will tell you!!

I would agree that this project has a rather presumptive title - but I think it's a cool name for a project.

I posted a bunch of sound samples earlier in this thread. One of these days I'm going to get some more sane clips on the mojotronics site - but the one's above are fairly representative.

[mojotron]

Impressive design/schematic, to say the least 
cool that you worked a v-amp into the chain too. Congratulations on what looks to have been a lot of hard work.

Thanks, I REALLY need to get a new SM57 - but the VAMP does a good job for a 'generic amp' sound for stuff like sound clips using the line outs on the back.

[idlechatterbox]

The v-amp Pro even has a digital-out line in the back .

[Gary]

I tried a lot of different Marshall designs from a lot of different schematics to shoot for the JTM100 Super Lead...

You mean just a Super Lead.
The JTM100, sometimes called a JTM45/100 was identifiable by the chrome "100" on the upper left corner of the cabs. That amp was a 100W version of the JTM45.  I think they had four KT66s, similar to 6L6s, in the power amp and a shared cathode on the first preamp tube.  They also typically had a 220-330uF cathode bypass cap.  Those amps were the fathers of the Super Lead/Lead amps.

http://www.mojotronics.com/smf/1959.gif

Yes, that's a schematic from the days when I think Unicord was making Jim Marshall's amps.  After that contract ran out, he opened his own factory and launched the JMP amps.  The schematics from before this time are very hard to find and are often not accurate, due to the changes that happened in production.

I would agree that this project has a rather presumptive title - but I think it's a cool name for a project.

Calling this a Plexi is as justified as anything else.  I hope you didn't take what I said as aggressive.  I wanted to point out that there are many things that are called Plexi, right or wrong.  For that matter, the circuit you built from was the same basic schematic that was used for the metal panel amps.

I posted a bunch of sound samples earlier in this thread.

I've D/Led the files.  I will listen to them when I get home from work.  I'm really looking forward to it.

As someone else said, the JFET as amp-sim thing all started with Doug H's Meteor.  We spoke with Doug after the Meteor came out and asked if we could explore this further.  Doug said it was cool, so off we went.  I still have that e-mail printed out somewhere.
About 4 years ago, RoG posted the original "Grace" circuit.  That was the first attempt at series CS JFET stages we tried.  That was in April 2002.  Doug's Meteor showed us how to refine that first idea into something much cooler.

Again, nice job.  See the RoG FAQ for something very similar.

[mojotron]

You mean just a Super Lead.
...

Hmmmmm,  you are probably correct about that.

Yes, that's a schematic from the days when I think Unicord was making Jim Marshall's amps.  After that contract ran out, he opened his own factory and launched the JMP amps.  The schematics from before this time are very hard to find and are often not accurate, due to the changes that happened in production.

This is about the only schematic I could find that was from the right period, that would explain a few things.

I would agree that this project has a rather presumptive title - but I think it's a cool name for a project.

Calling this a Plexi is as justified as anything else.  I hope you didn't take what I said as aggressive.  I wanted to point out that there are many things that are called Plexi, right or wrong.  For that matter, the circuit you built from was the same basic schematic that was used for the metal panel amps.

Thanks for the clarifications, I appreciate the info! Correct me if I'm wrong, during that period in time didn't they mix the panels depending on what they had?

As someone else said, the JFET as amp-sim thing all started with Doug H's Meteor.  We spoke with Doug after the Meteor came out and asked if we could explore this further.  Doug said it was cool, so off we went.  I still have that e-mail printed out somewhere.
About 4 years ago, RoG posted the original "Grace" circuit.  That was the first attempt at series CS JFET stages we tried.  That was in April 2002.  Doug's Meteor showed us how to refine that first idea into something much cooler.

Again, nice job.  See the RoG FAQ for something very similar.

Yes, quite similar, it's obvious this and the TC were shooting for the same Marshall 1959 design. I don't have a schematic of the Medeor - but I know this sort of thing all started with Doug and we owe him a lot.

I did not very too much from the 1959 schematic, on this project, as I really liked the sound. The only exceptions were the presence circuit and where I put R17, 10k sounded kind of weak - I noticed that this was half that on the TC and Electric's JCM emu - and that sounds great. I'm still working to perfect the presence circuit using yet another stage, however I'm coming across diminishing returns on my last few ideas.... The simple 100n bypass works pretty well I think.

[Gary]

The early metal panels were, for the most part, identical to the schematic you used.  By mid to late '69, the basic design had gelled into what you see on the Unicord schematic.  There were deviations from that in production, as that appeared to be the Marshall philosophy.  Their attitude seemed to be to use whatever they had on hand when they ran out of a certain part.

Something else I've noticed on your schematic:
The second stage cathode (source) bypass cap.  This only appeared on a few amps that I've seen.  Those were mostly from the earlier '68 period, which was when they began to transition to the values on the Unicord schematic you worked from.  Coincidentally, we used the Unicord schematic for the TC.  The new HW 1959s use this same schematic, despite the marketing line of being based on a pre-July '69 schematic.

I have spent many years trying to unravel the Marshall history.  The "History of Marshall" is a good starting place to learn more, but even that book will not prepare you for all the variations in production you will see in real-world analysis of the amps.  90% of the vintage amps you'll see are from the Unicord schematic.  The earlier ones are sometimes very different.  Another good source for Marshall information is from Velvet George at Metroamps.com.  Unfortunately, they have taken their board down, but you may be able to contact them directly for information.  they have a nice CD of schematics that comes with their amp kits.  It has som pretty good information that details many variations of Lead and Super Lead amps from around '67 on up.

On your presence control, look at the Full TC schematic on the RoG FAQ.  See the 4k7 on the tail of the PI stage?  Change that to a 5kL pot and tie the 100n to the pot's lug 2.  Lug 3 will go where the top of the 4k7 was.  Lug 1 goes to ground, of course.  There you go.  That's how the amp was wired.  It will work here, too.  Most classic use of the 1959 was "dime all the controls and let her rip."  That's why the TC omitted the tone stack and wired the presence pot wide open.  Besides, the Marshall tone stack doesn't have a wide range of adjustment.

Hope this helps.

BTW, I listened to your clips.  They sound nice.  I like that Yngwie-esque scale up you do.
Do you find the Vamp speaker sim adds a lot of bass back to the sound?  You used the analog one, right?  You bypassed everything and took the signal out from the XLR connectors?

[mojotron]

The early metal panels were, for the most part, identical to the schematic you used.  By mid to late '69, the basic design had gelled into what you see on the Unicord schematic.  There were deviations from that in production, as that appeared to be the Marshall philosophy.  Their attitude seemed to be to use whatever they had on hand when they ran out of a certain part.

Something else I've noticed on your schematic:
The second stage cathode (source) bypass cap.  This only appeared on a few amps that I've seen.  Those were mostly from the earlier '68 period, which was when they began to transition to the values on the Unicord schematic you worked from.  Coincidentally, we used the Unicord schematic for the TC.  The new HW 1959s use this same schematic, despite the marketing line of being based on a pre-July '69 schematic.

I have spent many years trying to unravel the Marshall history.  The "History of Marshall" is a good starting place to learn more, but even that book will not prepare you for all the variations in production you will see in real-world analysis of the amps.  90% of the vintage amps you'll see are from the Unicord schematic.  The earlier ones are sometimes very different.  Another good source for Marshall information is from Velvet George at Metroamps.com.  Unfortunately, they have taken their board down, but you may be able to contact them directly for information.  they have a nice CD of schematics that comes with their amp kits.  It has som pretty good information that details many variations of Lead and Super Lead amps from around '67 on up.

That's awesome! Gary - I appreciate it - thanks for the info! I have a lot of homework to do.

On your presence control, look at the Full TC schematic on the RoG FAQ.  See the 4k7 on the tail of the PI stage?  Change that to a 5kL pot and tie the 100n to the pot's lug 2.  Lug 3 will go where the top of the 4k7 was.  Lug 1 goes to ground, of course.  There you go.  That's how the amp was wired.  It will work here, too.  Most classic use of the 1959 was "dime all the controls and let her rip."  That's why the TC omitted the tone stack and wired the presence pot wide open.  Besides, the Marshall tone stack doesn't have a wide range of adjustment.

Hope this helps.

That's pretty close to the presence control I have on there - minus the 2.2nF/100k feedback - and I do like that. As I remember that presence control was not all that significant on the actual amp - infact, IMO the Marshall tone stack is not really that affective for controlling 'tone', but rather significant to getting the right tone for anything that was not a 'dimed' tone. I think the Miller caps C2 and C8 really make the tonestack work - at least to my ears.

The caps C6, C9 and C17, I just put them in there to see if I liked the sound any better: I think the circuit retains just a bit more bite with C6 (came from the 1959 sch) and C9 in there and C17 cut some of the shrill of my Strat when I used a booster in front of it. C9 and C17 were actually mistakes, putting them on the wrong Qs at 2am (happens...), but I liked the subtle changes so I left them on there. I found that C6, with the treble and presence rolled back - T@50% P@0%  and roll the mids back to about 80-90% - gave me a great early Eric Johnson sound - add a little FF and TubeDriver in front of the plexizer with a generous amount of delay in the loop and you get the right EJ tone when going pentatonic at light speed - IMO. This is all very subjective, but there is an upper mid emphasis with these 3 caps in there that I really like. I spared you guys my attempts at 'Cliffs of Dover' on the clips  , but getting a dimed sound as well as that Ah Via/Venus EJ sound (with the right supporting boxes) was what I was shooting for - equally. People can leave off C9/17 if they want to save 5 cents, or experiment and put C17 from gate to ground which is where it was supposed to go initially.

BTW, I listened to your clips.  They sound nice.  I like that Yngwie-esque scale up you do.
Do you find the Vamp speaker sim adds a lot of bass back to the sound?  You used the analog one, right?  You bypassed everything and took the signal out from the XLR connectors?

Thanks - I like Yngwie's playing on 'Fire and Ice' a lot - I'm not sure if I think I'm a big fan of some of his stuff, but he's the godfather of that sort of thing.

I think the VAMP does adds some bass, but I can dial it out on it's 1x12 speaker sim. I never really look at that box as a 'dead-on' set of sims, rather not a purist, I lower my expectations to just getting about the same tone as my TubeWorks gear - and it does that well for testing effects. I just plug into the font, use a small combo sim with it's 1x12 cab sim then take the line outs off of the back and run them into my computer. It works, usually my biggest problem is overloading the front end of the VAMP, but I have been using a Morley volume pedal to adjust for that and it works for what I need it for.

Using the VAMP I still wake up my kids a night when I scream "Thankyou Peoria!!" at the end of a session 

[Gary]

I think the Presence control on the amp is pretty effective, although I have not had the chance to play one with the more rare 0.68uF cap.  My experiences have always been with ones that used a 0.1 cap.

My point was, with the negative feedback, the Presence control has a more dramatic effect.

I have a Vamp Pro.  I use it fairly regularly, although I do not like the Brit Classic setting.