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PostPosted: Wed Jan 05, 2011 6:54 pm 
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Apparently Dr.Z offers this mod on some of their amps including their 18? There's a foot switch that bypasses the E.Q. and gives extra gain!..? Anyone done this or heard the results of this mod?


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PostPosted: Wed Jan 05, 2011 7:06 pm 
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My Allen Old Flame does something similar to this via a pot called the Raw knob.

Bypassing the tone stack seems to incrementally beef up the gain, most noticably in the mids. Sort of takes a BF type amp and adds a Tweed vibe for a while (low knob settings). Higher settings are more all out grind.

I'm pretty sure there are a few current production amps that do this very thing to add a pseudo second "channel" tone.

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PostPosted: Wed Jan 05, 2011 8:49 pm 
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I just did this mod on a Tramp I built. I added a footswitch to turn the boost on and off. I have the boost set up a fixed resistor but you can make the amount of boost variable by using a rheostat. I'll post a pic later if you want.

Btw often referred to as the "raw mod".

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PostPosted: Wed Jan 05, 2011 11:12 pm 
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Yeah, there is a drop in gain thru the tone stack so bypassing will jack it up a bit. I'd like to hear it!

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PostPosted: Thu Jan 06, 2011 2:45 pm 
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Hmm, I still have an unused push pull pot switch on the gain knob of my TMB channel, maybe I should rig this up.


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PostPosted: Thu Jan 06, 2011 3:10 pm 
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Here is how I did it on a Tramp I just built. I used a 680K resistor....

Image

Very easy to implement. Could be done with switch too.

No cable plugged in no boost, amp is stock. Plug in a cable and you can toggle between boost and stock.

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PostPosted: Thu Jan 06, 2011 3:27 pm 
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would that be R13 on the Tramp schematic? Would you wie it the same way on the 18 tone stack?


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PostPosted: Thu Jan 06, 2011 3:48 pm 
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Hey Stoo, ya it would be R13 on the Tramp schem. Disconnect that guy and run it to the jack/switch.

This boost can be implemented on any amp with the classic Fender/Marshall tonestack (so a ton of amps). It can be implemented in various other ways on others tonestacks and areas of amps but that's something else...

I'm looking at the sIII schem here
viewtopic.php?f=12&t=689
See the mid pot VR6. Disconnect its connection to ground (lug 3). Run that lug 1 to the jack/switch instead of ground. Then implement as shown above.

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PostPosted: Thu Jan 06, 2011 4:57 pm 
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So instead of open circuit you're using a fixed resistor, how does it affect the tone pots on your build?


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PostPosted: Thu Jan 06, 2011 5:49 pm 
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You can use an open circuit but the increase in gain/boost is nuts.

Tone stack is rendered useless, treble pot still has a little affect. Your bypassing the TS...

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PostPosted: Thu Jan 06, 2011 9:33 pm 
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Okay cool. I think I'll get some alligator clips and try out some mods to see what I want to do with that extra switch I left for myself.


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PostPosted: Fri Jan 07, 2011 12:26 pm 
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ForcedFire wrote:
Okay cool. I think I'll get some alligator clips and try out some mods to see what I want to do with that extra switch I left for myself.


:D Keep us informed OK FF!?


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PostPosted: Fri Jan 07, 2011 9:02 pm 
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Stoo wrote:
ForcedFire wrote:
Okay cool. I think I'll get some alligator clips and try out some mods to see what I want to do with that extra switch I left for myself.


:D Keep us informed OK FF!?



Sure, I know everyone thought I was insane when I mentioned it in my thread but I kind of want to try silicon clipping after the first gain stage of the TMB channel too.

Anyone have the gain of that stage calculated? I just did some rough estimations that the signal would be around 12V after this stage. I'll have to look into it in more detail.


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PostPosted: Fri Jan 07, 2011 9:17 pm 
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^that sounds high. What was your numerical gain value you calculated and your estimated input signal strength?

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PostPosted: Sat Jan 08, 2011 2:43 pm 
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kurtlives wrote:
^that sounds high. What was your numerical gain value you calculated and your estimated input signal strength?


Estimating gain of 44, input signal ~300mV.


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PostPosted: Tue Jan 11, 2011 3:57 pm 
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I heard this mod on Sunday and it is worth trying it out. Almost like a boost switch. Quite cool if you dont mind the reduced Tone controls.

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PostPosted: Tue Jan 11, 2011 4:39 pm 
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coco wrote:
I heard this mod on Sunday and it is worth trying it out. Almost like a boost switch. Quite cool if you dont mind the reduced Tone controls.

What sort of amp Stephen?


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PostPosted: Tue Jan 11, 2011 5:11 pm 
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coco wrote:
I heard this mod on Sunday and it is worth trying it out. Almost like a boost switch. Quite cool if you dont mind the reduced Tone controls.



Cool, I'll definitely try it. I've done this on my champ and it's pretty useful.


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PostPosted: Tue Jan 11, 2011 9:18 pm 
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ForcedFire wrote:
coco wrote:
I heard this mod on Sunday and it is worth trying it out. Almost like a boost switch. Quite cool if you dont mind the reduced Tone controls.



Cool, I'll definitely try it. I've done this on my champ and it's pretty useful.


champ has no tone control. what did you do?

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PostPosted: Wed Jan 12, 2011 3:13 pm 
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coco wrote:
ForcedFire wrote:
coco wrote:
I heard this mod on Sunday and it is worth trying it out. Almost like a boost switch. Quite cool if you dont mind the reduced Tone controls.



Cool, I'll definitely try it. I've done this on my champ and it's pretty useful.


champ has no tone control. what did you do?


Vibro Champ sorry. Bypassing the tone-stack turns it into a regular Champ. I also added a dummy load and line-out to drive the front of the 18W for fuzz or tremolo. It gets pretty hot when I crank it into the dummy load. I used 2 5W 10ohm resistors in parallel, I think I might add another one for a 3.33ohm load (15W), the 4ohm output on the amp might be happier with that.


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