"THE ANDREW MOD" instructions for the Formula

Triaxis, Quad Preamp, Stereo Power series, Recto Preamp

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Bottle Rocket
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Joined: Wed Sep 13, 2006 2:43 pm
Location: Holland

"THE ANDREW MOD" instructions for the Formula

Post by Andyman » Sat Mar 17, 2007 6:31 am

For anyone who needs or wants to know, coming right from the source!
I regret to say i personaly don't own the Formula anymore because of portability reasons.
Have fun!

Andrew de Ronde

The Formula mod I did was to open up the overall sound in both lead-channels.
Now it is not so extreme difficult to dail in the sound you want.
Although I don't mind having to work a bit for my sound , I just did not get
what I wanted (expected) out of it.
I found the equalizer to be much more effective now, it's not necessary to use it
to make the sound but it's now used to fine-tune the basically already good sound.
Also, before the mod the EQ seemed to strangely shift some wrong frequencies in.
I should add that the .47 uF cathode bypass capacitor is optional to raise a bit more gain.
The 1 Meg resistor at the first gain-stage of the leadchannels is the main thing to do I think.
What ever the case, I'm more than happy with my Formula now, it really works great for me!
I think you will agree that when using both channels with the same settings for the tone control
one must compromise somewhat since they both share them.
However I'm using my Rocktron Replifex (tiny bit of presence) to dail in a
100% satisfying sounds for both (medium gain for lead1 and Boogie-overdrive for Lead2+EQ).
I was somewhat surprised to be able to dial in a Carlos Santana type of sound now,
with Lead2 without the equalizer.
Mind the treble setting above 12 o'clock though anything above will screw up the sound!

Here's the info on the modification i did:

First check the serialnumber of your unit!
Up until now i´ve recieved comments of some users with sr.nrs. below FP-000400 to be in a somewhat different state and can look pretty messy, with components soldered even in mid air!
Mesa clearly did some hasty last-minute modifications to these units.
However, they assured me that the circuitry is the same as the later "Standard" Formula´s!
On these units i would recommend to only try replacing the resistor first as this might be enough for you without too much hassle.
That said you can read on:

Be save when you work inside your amp!
There can be lethal voltages present even when the unit has been off for a while.
If you don't know which precautions to take poke around the Internet, there's lot's of information about this.
All the mods are to be done at the tube-printboard.
I did try several other things at the main-printboard but with no significant results.
Using common sense and care is required, don't force things!
I do not take any responsability regarding this mod, you do this at your own risk!

There is something you should check for in your Formula first, at the tube-printboard:

-Check if there's a .01uF (.01K) capacitor placed instead of a .047uF (.047K) near the
middle relay (#5) at the tubeboard (see Fig. 1)
If not: replace for the .01uF(500-600 Volts), use a Sprague "orange drop" or preferably a
Xicon Polypropylene capacitor (Boogie uses these too)
Chances are there is already a .01uF in there, mine did not!
I placed mine at the back of the printboard since there is little space left over, my old
capacitor was damaged by one of the nylon studs which hold the printboard.

-Replace the 475Kohm grid load resistor (colorcoded: yellow,purple,green,red )of the first
lead gain stage for a 1 Megaohm 0,5 Watt (or 0,33 Watt) resistor (See Fig. 1)
-Replace the cathode bypass capacitor for this same stage from 1 uF to .47 uF (See Fig. 1)
use a Sprague/Vishay 173D 35Volt tantalum capacitor for replacement, to preserve the Boogie layout.
You could use an ordinary electrolytic too but this could sound differently.
Make sure to place the capacitor in the right direction (+ or -)

I think This mod really opens up the unit and even the recording circuit sounds actualy very good now.

Here's the picture:

Mark I
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Post by blues57 » Sat Mar 17, 2007 4:46 pm

thanks for sharing. :D

Bottle Rocket
Posts: 1
Joined: Wed Feb 04, 2009 12:49 pm

Re: "THE ANDREW MOD" instructions for the Formula

Post by J D » Wed Feb 04, 2009 3:34 pm

I'm looking at your MOD for the Boogie Formula Preamp.... I would like for you to explain in detail how this mod works and what affects it has on the dirty channel more specifically.... My problem is that I use to use the Studio Preamp which was a Mark IIc style preamp.... Im severely missing from My Formula the last stage of the Preamp Gain... I need More gain from the preamp.. I lack the nice punchy attack and sustain that i use to have......I play Hard Rock,and Easy Metal...

Im just looking to improve that area of the preamp.....I currently use a TS9 Tube screamer in the loop to fix the missing drive issue.....

Jeff Danneskjold
Do u have a private email address by any chance...

Bottle Rocket
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Re: "THE ANDREW MOD" instructions for the Formula

Post by rylon » Thu Dec 24, 2009 3:06 am

http://www.mouser.com/ProjectManager/Pr ... fb4a6ade4f

Above is the link to Mouser electronics project manager with the specified parts loaded. For $1.88, and a bit of soldering skill you too can perform this mod.

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vitor gracie
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Re: "THE ANDREW MOD" instructions for the Formula

Post by vitor gracie » Sun Jan 17, 2010 2:55 pm

Getting my Formula Pre this week. WOOT! Paired with the 20/20 (deep modded) this should be a nice set up.
Now if only I can get Mosta's cell # again to do the Andrew Mod.... (*looks around for Monsta on the Board...) :wink:
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Bottle Rocket
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Re: "THE ANDREW MOD" instructions for the Formula

Post by Ioanq » Sat Jan 05, 2013 8:19 am

cool !
have you the schematics of the fromula pré ?

Mark II
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Re: "THE ANDREW MOD" instructions for the Formula

Post by okgb » Sun Jan 20, 2013 9:01 pm

Where in the circuit or on the schematic , is the .047 which gets changed to a .01 ?
not asking the physical location but what it does in the circuit ?
never knew the lead channal was more Marshallish

Mark II
Posts: 111
Joined: Sat Feb 07, 2009 7:38 pm

Re: "THE ANDREW MOD" instructions for the Formula

Post by okgb » Wed Mar 13, 2013 9:40 am

Hey , looking at the schematic and opening my formula preamp , there seems to be a conflict
where my pre has a dual ganged pot for the rhythm gain that the schematic does not show !
Anyone know what is going on here ? check that , the second section must be the switch just doesn't look like it

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