Need info on modding my own Marshall ? High gain stuff

[screamindemon]

So I have been into the modded marshalls for a long time. I have had or still have amps modded by Cameron, Lee Jackson, Fortin, Langner, Metal head and so on.

I have been biasing my own amps for a while(weber Bias right) and have done some small mods and things with Marks guidance.

I am pretty good with the iron.

Any way I have stock 800 and a 78 Jmp that would be great to experiment with. I have the Jose mod schematic and would like to add more gain. But don't know how to read it.

Where would be the best source for preamp building info?

Any help would be greatly appreciated!

Thanks,
Brian

 

[tehuk]

Try the Metro amp forums. The guys there are full of knowledge and will help you hot rod your Marshall in a heartbeat.

 

[GuitarGoat]

Read through this site, it's got some pretty good info: http://www.freewebs.com/valvewizard/

 

[tweed]

http://forum.metroamp.com/

 

[jerrydyer]

Ill help you read it als o
pm on the CCV.

 

[ericsabbath]

I'm having my '73 JMP rebuilt by a friend to a naylor/friedman/budda circuit plus another different drive channel
mine has 2 extra preamp tubes, though

what exactly are you looking for?
just a regular jose mod?

 

[screamindemon]

Thanks for all the replies.

I really like the sound of the Jose mods. I would really like to gain an understanding on how the basic preamp section works so I could make changes to my own taste.

Thanks Jerry I will be in touch.

 

[screamindemon]

Read through this site, it's got some pretty good info: http://www.freewebs.com/valvewizard/

Thanks, great link. That is the book that Mike Fortin recomended I get.

I will be ordering it tonight.

Thanks

 

[jerrydyer]

Thanks for all the replies.

I really like the sound of the Jose mods. I would really like to gain an understanding on how the basic preamp section works so I could make changes to my own taste.

Thanks Jerry I will be in touch.

schems are actually to read once you know some basics. for example the bottom of that page is the ground ref. the line on the bottom is ground. On that schem though, everyone misreads the 1meg volume pot with the "diodes" on the back.

another great thing for learnig is a simple test light. you can make on really quick. 9 volt battery and an LED. YOu can use this to test connections, and find grounds on tricky boards.

on that schem the front left of each half a triode is the input grid. the top is the plate and the bottome is the cathode. A lot of the tone and gain changes you make are on the cathode. You can play around with this area a lot yourself and see what you like. Even mistakes are good because you get to hear how it changes the tone and gain, for better or worse. On the jose schem, all he did was cascade the front and duplicate the cathode values. some minor changes between the first two gain stages as well.

 

[ericsabbath]

basics for hot rodded marshalls:
- cascade the two first gain stages if you have a 4 holer, and add a regular master or post phase inverter master volume;
- tweak the second stage cathode if you already have a master volume model;
- lower cathode resistor = more gain;
- lower cathode bypass capacitor = more midrange bite, less bass;
- bigger plate resistors = less voltage = more saturation;
- lower input resistance = thicker tone and better dynamics/picking response;
- lower gain pot or fixed voltage dividers = less saturation, more open tone, less fizz;
- keeping the cathode follower = subtle natural compression and open mids;
- removing cathode follower = dense mids, dry response, less volume dependent tone;
- small capacitors from signal path to ground or bypassing plate resistors = less fizz;
- bigger "fizz cap" linking the phase inverter plates = less fizz;
- bigger slope resistor and small treble cap in the tonestack = scooped mids;
- smaller coupling capacitors = more mid focus and clarity, less low end;
- bigger coupling capacitors for the phase inverter = bigger low end;
- lower negative feedback resistance = more negative feedback = better presence/depth control, less fuzzy response;
- higher negative feedback resistance = less negative feedback = louder and more "wild", with more abrasive high end and uncontrolled bass (like a recto on modern mode);
- too much gain = squished mids = less tone; do not put more gain than you'll ever use.


most boutique hot rodded Marshalls are just Jose mods with tweaked coupling capacitors and minor changes
common values are:
- first coupling cap 2.2n-4.7n
- second coupling cap 2.2n-10n
- first cathodes 2k7//.68 or 1k8//1u
- first plate resistor anywhere between 100k and 220k
- last stage cap .68 or 1u
- tonestack slope 33k-68k and treble cap 220p-500p
- fizz cap 100p-120p instead of Marshall's 47p

99% of our loved 3-stage british hot rodded amps are pretty much within these values
the higher gainers tend to have an extra stage after the second stage, usually with a flat voicing (no cathode bypass cap)