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JCM800 + oscilloscope = square wave
Old 30th July 2020
  #1
Gear Maniac
 
camacozie's Avatar
 

JCM800 + oscilloscope = square wave

Hey GS!

I have a buddy that bought a Marshall JCM800 years back. He claims someone once put an O scope on it and did something so that 5 minutes later the sign wave became either a square or a saw-tooth (unfortunately he can't recall). Be that as it may, the amp just roars wonderfully at high gain.

What is this process called? Is it something that is done during biasing? Are there any negative consequences? Besides sinusoidal, which wave form is "The One"?

And the real question I'd like answered is:
Should I pay someone to do this to my Mesa/Boogie Mark V?

Thanks!
Old 30th July 2020
  #2
Gear Maniac
 
Janne19691's Avatar
There are many ways to modify a classic Marshall amp for high gain. I know a few ways. Very popular was to cascade the two Superlead channels in series (instead of normal parallel configuration). Marshall Randy Rhoads model has that modification and it sounds pretty good. You can find the schematics online and try it with a suitable amp (Marshall 1959).

Everything is doable. One can rebuild a whole amp to do what one wants but maybe I would not modify a perfectly fine Mark V. Maybe buy a Marshall and try a few mods with it and see what sounds good.

(In the US a tech called Jose Arredondo did his famous mods for famous guitarists' Marshall amps. And there were other well known techs too.)

Btw, JCM800 is a series of amps that include several different models.
Old 30th July 2020
  #3
Gear Maniac
 
camacozie's Avatar
 

I'm reading online that the Mark V is a pain to service/mod. Perhaps it's not worth the labor considering she already kicks a$$.
Old 30th July 2020
  #4
Gear Maniac
 
Janne19691's Avatar
Quote:
Originally Posted by camacozie View Post
I'm reading online that the Mark V is a pain to service/mod. Perhaps it's not worth the labor considering she already kicks a$$.
Yes, modifying a PCB amp anything more than a few component values is not great.
Old 31st July 2020
  #5
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enorbet2's Avatar
Most JCM800s have a stage associated with the Hi gain input that are biased "wrong". If you read schematics you will see that instead of the common 1500 ohm (1.5K) cathode resistor for 12AX7s the hi gain stage on a JCM800 is almost twice that at 2700 ohms (2.7K) which drives the bias voltage at the input up. This reduces noise when not playing somewhat but mainly causes the attacks to "jump out". Because that bias resistor is bypassed by a .68uf cap, it accentuates high frequencies and the combination creates a very aggressive attack. It doesn't make for a sweet dynamic clean though so it's best to only clone that design in a dedicated dirty channel or amp switching rig.

The Orange OR 120 Overdrive Amp is similarly designed and was an important part of Dean DeLeo (Stone Temple Pilots lead) squeaky drive tone. Most players wouldn't want that as an "all the time tone" but it's pretty cool to switch to.

BTW at the onset of clipping, which is quite low threshold on that stage, the waveform is by no means Square. By that I mean it is not symmetrical. One side clips WAY before the other because of the skewed bias.
Old 31st July 2020
  #6
Quote:
Originally Posted by Janne19691 View Post
There are many ways to modify a classic Marshall amp for high gain. I know a few ways. Very popular was to cascade the two Superlead channels in series (instead of normal parallel configuration). Marshall Randy Rhoads model has that modification and it sounds pretty good. You can find the schematics online and try it with a suitable amp (Marshall 1959).

Everything is doable. One can rebuild a whole amp to do what one wants but maybe I would not modify a perfectly fine Mark V. Maybe buy a Marshall and try a few mods with it and see what sounds good.

(In the US a tech called Jose Arredondo did his famous mods for famous guitarists' Marshall amps. And there were other well known techs too.)

Btw, JCM800 is a series of amps that include several different models.
John "Dawk" Stillwell did a lot of amp stuff for guys like Iommi and Blackmore back in the day.

He did some 300 watt Marshals for Iommi that could make your pants flap in the breeze....
Old 31st July 2020
  #7
Gear Maniac
 
camacozie's Avatar
 

Quote:
Originally Posted by enorbet2 View Post
This reduces noise when not playing somewhat but mainly causes the attacks to "jump out". Because that bias resistor is bypassed by a .68uf cap, it accentuates high frequencies and the combination creates a very aggressive attack. It doesn't make for a sweet dynamic clean though so it's best to only clone that design in a dedicated dirty channel or amp switching rig.
This is precisely how I would describe the sound; the clean channel is basically a dirt channel. CH2 is punchy and dynamic, like it's alive.

The Mark V by itself sounds super awesome, but right next to his JCM800, the Mesa sounds compressed and flat.
Old 31st July 2020
  #8
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enorbet2's Avatar
Hey camacozie, You should be able to get less compression on the Mesa by backing off a bit on the drive and adjusting for more high frequencies. Also be sure to play with Master Volume levels because too much preamp gain tends to cause turning down the Master which does two bad things - 1) Reduces high end response 2) Greatly increases compression. It's the Power Section (after the Master) that "breathes".
Old 3rd August 2020
  #9
Gear Head
 

To the OP: an oscilloscope is used for measuring signals, not creating them. If it took 5 min to get the signal, it must've been some kind of feedback loop, all of which is a terribly innefficient way to get a square wave out of a marshall, not to mention dangerous.
If you don't know what you're doing, you could fry the scope, the marshall, or you could die. There are lethal voltages present in a Marshall JCm800.

You could use a signal generator if you want to get techie, but that would be very limiting. I would recommend using a good ol synth to come up with the wave, and throw it in marshall for further tweaking.
Old 3rd August 2020
  #10
Gear Addict
 

The oscilloscope measures and displays the wave form.

When you say the sine becomes a saw or square, that's what distortion is, every high gain amp will do that.
Old 3rd August 2020
  #11
Gear Addict
 
chipss36's Avatar
 

Op ...
The o-scope displays the waveform.
Also a signal generator is used, and more than likely a dummy load.
The signal generator is what would send signal to the amp, it could be sine, square,sawtooth,
And many others,Like sweeps....they all have a purpose, for amp analyzing.
The sig-gen , or function gen, sets the waveform...not the amp.
It may clipped, but it’s sill a sigh wave...takes some huge clipping to make a true square.
Pretty sure he meant square wave...had he zoomed into what he saw as a square wave, he would see the classic tube amp rounding happing...it’s not square, and it sounds bad ass...
A triangle I have never saw with guitar tube amp clipping.
The average engineer, or tech, flat out does not understand guitar tube amps, or desirable clipping, odd vrs even order inter modulated distortion and the crafting of it.
it goes against everything they have learned.
It’s a special bunch that gets into all this...




As for a simple bias, only a sigh wave would be used, if using a scope at all, it’s not so much needed to bias, unless you have some issue and need to dig deeper. Or a few other things,..like amp design.

Test gear used properly will not damage anything,
Used improperly, lots Of bad can happen..


I do understand the idea of blueprinting an amp for
Analysis sake of analysis....I do it all the time, and it’s an expensive hobby..

regardless of test gear.

If it sounds good, it is good...
And that’s the bottom line.
Play it.
If it sounds bad, find a good tech.

Last edited by chipss36; 3rd August 2020 at 10:51 AM..
Old 4th August 2020
  #12
Quote:
Originally Posted by chipss36 View Post
Op ...
The o-scope displays the waveform.
Also a signal generator is used, and more than likely a dummy load.
The signal generator is what would send signal to the amp, it could be sine, square,sawtooth,
And many others,Like sweeps....they all have a purpose, for amp analyzing.
The sig-gen , or function gen, sets the waveform...not the amp.
It may clipped, but it’s sill a sigh wave...takes some huge clipping to make a true square.
Pretty sure he meant square wave...had he zoomed into what he saw as a square wave, he would see the classic tube amp rounding happing...it’s not square, and it sounds bad ass...
A triangle I have never saw with guitar tube amp clipping.
The average engineer, or tech, flat out does not understand guitar tube amps, or desirable clipping, odd vrs even order inter modulated distortion and the crafting of it.
it goes against everything they have learned.
It’s a special bunch that gets into all this...




As for a simple bias, only a sigh wave would be used, if using a scope at all, it’s not so much needed to bias, unless you have some issue and need to dig deeper. Or a few other things,..like amp design.

Test gear used properly will not damage anything,
Used improperly, lots Of bad can happen..


I do understand the idea of blueprinting an amp for
Analysis sake of analysis....I do it all the time, and it’s an expensive hobby..

regardless of test gear.

If it sounds good, it is good...
And that’s the bottom line.
Play it.
If it sounds bad, find a good tech.
Pretty good post, BUT...............

The word is "SINE", not "sigh".

Repetez-vous avec mois: Sine, Sine, SINE............

BTW, I have seen something approximating a very rough triangle coming out of a badly misbehaving Fender Twin circuit built by other than Fender using Fender parts......
Old 4th August 2020
  #13
Gear Addict
 
chipss36's Avatar
 

Well John feel free to discount my whole
Post over a misspelling....???
And miss the point.


Rather petty dude.

Interesting about a triangle wave in a tube amp.
Never seen that...

An lfo in a phasers sure. But they were build to do such a thing.
Old 4th August 2020
  #14
Gear Maniac
 
camacozie's Avatar
 

Quote:
Originally Posted by chipss36 View Post
Well John feel free to discount my whole
Post over a misspelling....???
And miss the point.


Rather petty dude.

Interesting about a triangle wave in a tube amp.
Never seen that...

An lfo in a phasers sure. But they were build to do such a thing.
I believe you meant "built". Haha jk
Old 4th August 2020
  #15
Lives for gear
 
enorbet2's Avatar
Quote:
Originally Posted by chipss36 View Post
Interesting about a triangle wave in a tube amp.
Never seen that...
Just FTR tubes have no difficulties producing a triangle response since response all about bias (EQ helps) but pure triangles just sound crappy and feel worse. Modified "curvy" triangle response can be interesting but not widely useful. The most flexible response comes from stages biased for symmetrical output with a gradual progression into clipping/cutoff.
Old 5th August 2020
  #16
Gear Addict
 
chipss36's Avatar
 

What is an ftr tube? What amp uses ftr tubes exactly?
Cut off? What? Explain this?
Old 5th August 2020
  #17
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enorbet2's Avatar
??? FTR is "For The Record", or were you making a funny?
Old 6th August 2020
  #18
Gear Addict
 
chipss36's Avatar
 

Quote:
Originally Posted by enorbet2 View Post
??? FTR is "For The Record", or were you making a funny?
Ftr this is absolutely false.....guitar amps do not put out triangles....
Pure bs pal, and I know better, thy that on someone else ... best post a screen grab of said triangle and prove me wrong hu pal?



And again exactly what guitar amp uses a tube at cut off? Rhetorical, it’s not.
Biasing a tube outside it’s load line to cut off is not something good...ever

No idea why you went saying all this...it’s all bs

It’s also silly, so off of what the op asked.
Funny how no one answered him, but many standing by to sharpshooter or grammar **** the one guy that did...
And why I don’t hang around this place....
You should be ashamed
Old 6th August 2020
  #19
Lives for gear
 
enorbet2's Avatar
Yo chipps! Jump to conclusions much? I never said guitar amps produce triangle waves but they can be made to do so with modification but why would anyone want to? The point was simply that tubes can be configured to deliver any manner of waveform as they have for many decades in the earliest signal generators. Apparently you have cherry picked 'cuz ur butthurt over a single picayune conflict that didn't involve me. I posted specific help twice and the first one got 3 thumbs up including 1 from OP. Apparently I answered appropriately which you just ignored for your own personal cause.. Lighten up, Bro.
Old 7th August 2020
  #20
Gear Maniac
 
camacozie's Avatar
 

I appreciate all the feedback. I realize GS is chalk full of in-fighting, but as someone with no dog in the fight - and as someone with no EE training - it's part of what makes GS such a great resource. Even as I pickup little tidbits that may or may not be true, I am still growing purely from the shared experiences. I don't believe that anyone has a reason to lie, and that while there is a certain chance that the information shared may not be totally correct, the opinions shared are real for the poster. So, on behalf of all us living on the ignorant side, thanks for contributing!
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