18 Watt MojoTone Kit Build With Conversion To JCM800 Circuitry:

Cadorman said:
View attachment 48153
Robert, this is the diagram I used for my 1987 build. Especially note the 10uf cap I used on the first stage per discussion with Ivan. The bass is very tight and I love the fullness when I blend the two channels together. Works great with my RAT and Soul Food. The build was based on a 1968 schematic someone at Metro did from a sock amp. Evidently there is some variation on the stock amps.
Click to expand...

That's awesome information to have in this thread!!!! Thank You!!!!
 
Amp Mad Scientist said:
The huge 250µF (often 330µF) cathode bypass cap on the 1st tube in the JTM45....

The 250uF cap extends the frequency response down to subsonic frequencies.
The subsonic, in turn overloads the next stage (V2 compression stage) resulting in a muddy sort of farting sound quality.
Changing this cap to .68uF or 1 uF (for example) rolls off the subsonic overload.
* If you like to see the frequency response based on the bypass cap, and compare different bypass caps, use this calculator:

Cathode Bypass Capacitor Calculator

www.ampbooks.com www.ampbooks.com
This calculator does not require deep technical knowledge. It's simplified.

- The 1987 adds a 0.68µF bypass cap to the 1st gain stage of V2. This has the effect of boosting gain of this stage compared to the JTM45, but again focuses the extra signal gain towards the mids/highs.

V2: is not a gain stage.
It is a compression stage.
The sensitivity is boosted, but the gain is not increased, it's decreased.
The output gain of V2 is always LESS than the input of V2. This is because the output of V2 is a cathode follower.

There is a difference between "sensitivity" and "gain." This is really important.
Although people frequently mix the 2 terms up.
More explanation of this to follow.

Adding the bypass cap to V2A increases the frequency response in a certain range. It also causes more subsonic lows to be rolled off.
Again, use the calculator to see this - on a graph:

Cathode Bypass Capacitor Calculator

www.ampbooks.com www.ampbooks.com

A "gain" stage works like this:
Input = s
Output = S
The stage added gain.
---------------------------------------------------------
But, the V2 stage in a Marshall adds only sensitivity and compression, not gain. The cahode follower subtracts gain.
Like this:
input = S
output = s

Compression: the input reaches a certain level, then the output is limited to a certain level. It cannot go any further.
No matter how hard the input is pushed, the output does not increase.

input is this = s
Or, input is this = S

But, the V2 output is always this: S
No matter how hard or soft the input of V2 stage is pushed,
the output can only be this = S
It never increases beyond that level.
This is "limiting." Just like a compressor / limiter in a recording studio.

The result:
loud passages or soft passages come out at the same level. That constant audio level is maintained.
It makes very soft passages louder.
It keeps very loud passages from exceeding that same certain limit.

The output of the preamp is "smoothed." Loud and soft comes out at a uniform audio level which never increases or decreases.

You can also call this type of circuit "a compander."
It is a expander, compressor, and a limiter, all at the same time.

A. This is why a Marshall amp is always loud ------ no matter how hard- or soft - you are playing.
B. It makes soft passages louder.
C. It limits loud passages to the same limit as the soft passages.

D. The dynamic range is removed.
Loud and soft come out at ==== the same level.
Click to expand...

Awesome response, and much appreciated!!!!!
 
Robert Herndon said:
My goal is super-tight bass response.


I want an amp that will deliver clear and defined high-gain tone, even when being pounded by a TS-9 with heavy bass frequency dialed in while simultaneously offset with dimed mid range frequencies...
Click to expand...

If you are looking for a super-tight, stop-on-a-dime but big and well defined low end (modern metal style) I respectfully submit that you won't get it from a Marshall (well, maybe a JVM but not from an 800-style circuit with low power). I say that as someone that gigged with a 50-watt 800 for 6 or 7 years. I ran it with the bass essentially at 1, with a TS-9 in front. Anything higher than that and it was a giant wet stinker.

Maybe try that Engl you were talking about?...that'll take your insides apart without breaking a sweat or needing a mod of any kind.
 
Buy this and be done with it.

reverb.com

1985 Ken Fischer Trainwreck Express "Donna" Amp Head | Reverb

This is a real deal, a Ken Fischer built Trainwreck Express "Donna" from 1985. This amp is 110% tone. Absolutely insane overtones and expressiveness. Every note blossoms.Just like a lot of Ken's amp it has a nice and storied history. Once owned by Lenny Kravitz, this amp was given to his tech, wh...
reverb.com reverb.com
 
lovehow "legend" has it-- they dont have documented proof "just legend"


OH BY THE WAY -----ROBERT Ill make a you a SWEET deal --- I have this ...........
MArshalll1.jpg
for sale-- which "legend has it" was owned by SIR PAUL McARTNEY who used it when visiting his son and staying at his place here in Winter Haven Fl. --- Weekend jams at the swamp ranch by Sir PAUL --- I will let it go with all that mojo for a VALUE PRICED 25,000.00
you got my pay pal buddy Ill ship it out tomorrow ;)
 
67plexi said:
I did a 18 watt Vox AC-15 with a Marshall second channel master volume end of the day the Vox EF86 eats it lunch
I never play the Marshall channel. If I did it over I would omit the Marshall channel

Looks like a Marshall

View attachment 48145
Click to expand...


I'd like to know more about this. I was thinking of doing an 18 watt Marshall because it's the same circuit as a Watkins Dominator. I'd make up a chassis and box so it would look like one, and cover it in the two tone blue. But, an AC-15 with an EF86 sounds like much more fun.
 
gball said:
If you are looking for a super-tight, stop-on-a-dime but big and well defined low end (modern metal style) I respectfully submit that you won't get it from a Marshall (well, maybe a JVM but not from an 800-style circuit with low power). I say that as someone that gigged with a 50-watt 800 for 6 or 7 years. I ran it with the bass essentially at 1, with a TS-9 in front. Anything higher than that and it was a giant wet stinker.

Maybe try that Engl you were talking about?...that'll take your insides apart without breaking a sweat or needing a mod of any kind.
Click to expand...

I agree with you, but this is something that I have been wanting to build anyways...
 
67plexi said:
It's just a tax deduction why pay I Really Steal or IRS. As far as I know the two most expensive amps ever sold
were Dumbles Kirk Hammett SSS-150 made for SRV and it was stolen the other is SSS-100 #3 Nicole Kidman got for her husband
Keith Urban. Total Amp Porn.

View attachment 48174
Click to expand...

I would never spend money - to that degree - on an amplifier, when we have so many humanitarian programs that could really use that money to improve people's lives...IMHO
 
You must log in or register to reply here.
Back
Top