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Re: But which schem is the correct one?


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1/11/2005 6:41 AM
Munk Re: But which schem is the correct one?
I have seen many posts about these humble amp schems over many years here but was wondering when some one builds this amp that which schem was the correct one?  
 
Kinda like all those trainwreck schems floating around but no one could figure out which was the correct one to build.  
 
 
Munk
 
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1/11/2005 10:08 AM
Wild Bill

Munk, there isn't any correct one!  
 
Amps like Trainwrecks were done by guys who built amps one at a time, usually as a commisioned project. This is much different than coming up wit a fixed design then churning out the "cookie-cutter" machine to fill all the store fronts.  
 
I'm sure that Mr. Fisher never built two of his early projects exactly the same and often did a lot of tweaking to make that one particular customer who had already spoken for the amp happy.  
 
So a reverse-engineered schematic is only good in general and tweaked for one specific player.  
 
That's why the player went to him in the first place! If he wanted a cookie-cutter amp he would have bought a standard off-the-shelf Fender or whatever.  
 
There's only a certain number of circuits or ways to make a guitar amp do certain things. All them affect the tone and a boutique designer is likely to pull any of them out of his bag of tricks, a bag he filled with years of learning and experience.  
 
Try thinking of that schematic as just a platform to get you into the ballpark - to start an amp that has the designer's "style". The rest will be up to you...  
 
.02  
 
---Wild Bill
 
1/11/2005 9:18 PM
Bob Ingram

I think it's been answered... but. there really is no "correct" one. The Dumble is more of a concept than an actual schematic. It includes things like the series resistors between stages to accent mids, many gain stages controlable with either trim pots or front panel pots. The goal is a silky smooth overdrive, with lots of sustain and not a lot of distortion.
 
1/11/2005 2:53 PM
Tommy
Re: Looking for that Magic Dumble overdrive
A newbie question. Where exactely is the negative feedback leads in the schematics? Is it the one with the 4,7K that goes from the output transformer back to the Phase inverter?
 
1/11/2005 9:05 PM
Wild Bill

Yeah, that sounds like it!  
 
It's not always the same value from amp to amp, Tommy. This resistor forms a voltage divider with another resistor at the PI point to ground. The ratio is usually about 10:1 but often is "diddled" to adjust the amount of feedback to get a different amp sound. Sometimes that resistor to ground is actually a presence pot.  
 
Fenders usually use a 820 ohm, with a 100 ohm or sometimes a 47 ohm to ground at the PI. Amps with multiple speaker impedance taps will use a different value if they pick the voltage from the 16 ohm tap instead of the 8 or 4 ohm. This is because the available voltage is higher on the 16 ohm tap than the 4 ohm.  
 
Whatever the value, since you know already it's the on coming back from the speaker winding to the PI you'll always be able to spot it.  
 
---Wild Bill
 
1/11/2005 9:31 PM
Bob Ingram

So I'm close to done now. I used an old dead Crate as a donor cabinet, installed it and connected the stock Crate speaker. For now I'll call it a "Crumble" (*laugh*)  
 
I have a Celestion G12H80 on order so I'll reserve the final adjustments for that speaker.  
 
http://www.patmedia.net/bob-ingram/Dumble/Crumble.jpg  
 
I think I can take that to a club without worrying about it being stolen. ;-)  
 
http://www.patmedia.net/bob-ingram/Dumble/Crumble.mp3  
 
Here's a quick recording. I'll tune up and check my recording level next time. I especally like the tone with the distortion channel set with just a little overdrive. I cranked up the gain and trim but it sounds better without it.  
 
I also installed a voltage divider before the PI to help tame the volume. I think after the PI would be better but that's for later.
 
1/13/2005 3:22 AM
BillD.
anyone build the 70's circuit???
You know, with the 100k plate resistor, and 1.5k/5uF cathode values. If anyone has, how does it sound compared to the '97 version? My first guess would be less gain but is there anything else? Anyways, this whole post and Bob's awesome last clip:), has inspired me to whip up a Dumble clone of my own just to see what the fuss is about.
 

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