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Author Topic: [HELP] modding ROG Eighteen Tone control  (Read 3613 times)

jogina111

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[HELP] modding ROG Eighteen Tone control
« on: May 29, 2013, 11:19:17 AM »
hi, I'm
planning
to mod the
runoffgroove
eighteen
tone control to a TMB
 tonestack.. Would
it be possible? If so,
how and where do I
wire it? Should it
go to the tone control's place or
at the end of the
circuit?
Help please...

J M Fahey

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Re: [HELP] modding ROG Eighteen Tone control
« Reply #1 on: May 29, 2013, 07:05:38 PM »
Start by posting the original circuit here and the TMB stack you plan to add, so ea ll talk about the same.

That said, the original Marshall 18W (which by the way was not designed by Marshall) is a killer amp as-is.

Probably kludging a TMB stack there will not improve it (that's the kind way of saying it will probably f*ck it).

I'd rather build the clone of a Plexi or something.

Not sure, I think it's called the "Thor" or something.

jogina111

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Re: [HELP] modding ROG Eighteen Tone control
« Reply #2 on: May 30, 2013, 01:01:48 AM »
I was planning of replacing the tone control from this: http://www.runoffgroove.com/eighteen.html with the tonestack from this: http://gaussmarkov.net/layouts/drboo/drboo-schem.png .. what difference would it make? any tips? what about putting the tone stack at the end of the circuit and/or adding an output buffer to drive the tonestack??
« Last Edit: May 30, 2013, 01:13:19 AM by jogina111 »

Roly

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Re: [HELP] modding ROG Eighteen Tone control
« Reply #3 on: May 30, 2013, 07:40:56 PM »
Hi and welcome jogina111.

I'm starting to get a dose of the queeblies every time I see "runoffgroove" now - I do wish they would take this bloody abomination of a preamp "design" down and do us all a favor.   :grr

The second Dr Boogey version isn't any better.  Just for example, it contains a bunch of tone conditioning, much of which doesn't make a lot of sense (e.g. C2 to C18 which contain contradictory elements), but at C16/TRIM3 there is an element that is totally dependent on the specific  device used at Q3 - in other words every build is a deliberate victim of component spread rather than eliminating any variation on component spread - the essence of good design.  It is hard to make any observation about these "designs" without being offensive.


Here is a real design, no trim pots.  This uses a conventional tonestack and zener regulated supply.




This is a later development of the above which uses an experimental "quasi-parametric" tonestack, and an improved power supply with lower hum.  The improved power supply arrangement should be used in all cases.  Note also that R14 has been reduced in value to produce more valve/tube-like "soft clipping", the earlier higher value giving more abrupt hard clipping on overload.




Where the tonestack is placed with respect to the circuit gain determines how easily you will get preamp overload, and what control the tonestack will have over that overload.  By placing the tonestack after much of the preamp gain you are more likely to get overloading of the later gain stages.

Tonestacks generally work best if driven from a low impedance and are followed by a high impedance.  In the case of the Dr Boogey circuit there is no buffering at all following the tonestack so its actual operation in practice will depend a lot on the power amp that follows and what sort of input impedance that has - and these are typically not high.

Overall the Runoffgroove and Dr Boogey circuit(s) are very influenced by their exact operating situation and therefore quite hard to predict.  A good design is one that will tolerate a wide spread of component values and operating conditions such as supply voltage, source and load impedances, temperature, and so on, yet deliver consistent operation under all those variations.
If you say theory and practice don't agree you haven't applied enough theory.

 

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