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Dumble Clone Parts: Trannies etc...

5/22/2000 11:23 PM
Dave B.
Dumble Clone Parts: Trannies etc...
A few ampagers talked me out of Modding my Princton Reverb into a Dumble clone so I'm starting down the clone trail.  
I've got someone working on the chassis but I need some other parts.  
1) transformers: I've talked to other cloners who've used Hoffman trannies for their clones. Are those the same trannies that new sensor sells or does Hoffman have his special made? I looked at the new sensor trannies and the OT for a twin only has a 4 ohm tap. I think the Hoffman has a 4 & 8 ohm tap. They're sure alot more expensive though.  
What's the best trannies for a Dumble clone?  
2)Speaker: a dumble cloner is telling me a 12" EV, I'm thinking 15" EV. What's best for that Robben Ford sound?  
3)If you were going to add reverb, what circuit would you use?  
I've got a handle on the rest of the parts but any insight would be truely apriciated.  
Thanks for the help, another Fender has been saved.  
Dave B.
5/23/2000 12:30 AM
Carl Z

From what I can gather Dumble himself just used stock Fender replacement transformers. That'll put you at 4200 ohms primary impedance into either a 4 or 2 ohm load.  
If you're looking for the Robben Ford sound don't deviate from the package. You change your speakers and the entire amps dymamics will change and you'll have to redesign the entire thing! Stick with what's been proven.  
As for adding reverb, look at the AB763 circuit. This isn't the same reverb that Dumble used, however if you look at the layout and the schematic long enough you'll start to see the overdrive special with the skyliner eq start to emerge from the forest. It'll just require rerouting a few wires and adding a jumer or two. Hint: (Use the normal channel as the OD)  
Carl Z  
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5/23/2000 1:53 AM

I've used Hoffman trannies for a *similar* type amp ;-) They sound great. I also used the blackface style reverb, but omitted the cathode bypass cap on the 12AT7 driver tube. This worked out well, also. Stick with the 12" speakers and you should be there.  
5/23/2000 3:23 AM
Gil Ayan

Dave, my two dollars worth (inflation!). Dumble has apparently used a number of different things to build his amps over the years... First ones were probably very close to Fender knock offs, and therefore it is reasonable to assume they were Fender parts. Older Dumbles didn't have impedance selector switches and were rated at 4 ohms, so either he didn't bother with other taps, or his transformers were Twin replacements.  
More modern Dumbles reportedly use Fender Twi laydown mains transformer from Magic Parts, Fender Twin choke, and a Magic Parts TF 170 Marshall Plexi otuput transformer. I am a Magic Parts customer and this information was verified by them.  
The more modern amps have an impedance selector switch for 4, 8 and 16 ohms (all of the taps in the TF 170).  
Regarding the Hoffman transformers, the OT sounds very good, but the mains transformer is mechanically noisy (I have to of them in two amps). They do work well, but they are not as beefy as I would like them to be, structurally speaking. Knowing that the rela thing uses Fender Twin mains, you may want to go that route. I have used the New Sensor MPT-100 (plexi 100W) and that thing is great. It has a slightly higher B+ than then Hoffman, but it is built very nicely and is dead quiet -- again, mechanically.  
Regarding the addition of Reverb, anything can be done to any amp and you may end up with a better sound. However, I can guarantee you that the sound will change. Part of the thing about the Dumble circuit is that it is extremely simple, with only four stages before the PI in overdrive mode. If you add reverb to the amp, a la AB 763, you will add another stage to the clean sound and there will be a number of differences:  
1. The dry signal will pass through the 3.3Meg//10pF network, and that may not agree with the lead tone.  
2. AS someone already poited out in another thread today, THAT Fender network allows for the output of the overdrive to travel back to the input of the overdrive via the 3.3Meg resistor. I had built a agin amp (much like a Boogie or Dumble) on a Fender circuit before, and while it ended up singing great, I had a lot of problems fighting oscillations.  
3. Having more stages will inevitably compress the amp some more. Part of the Dumble thing is that it is a very dynamic amp, which kind of sounds like a power section pushed hard, yet, the effect is achieved by using preamp overdrive, to a great extent.  
If I were you, I would wire up the real thing first so that you know what it's supposed to sound like. Then, if you want to "make it better," at least you will have the proper reference to compare the nerw sound to.  
Good luck with your project,  
5/23/2000 5:03 AM
Peter S

I agree with Gil......all of my early amps had reverb built....I used the Fender circuits. Then I came across this really fabulous sounding amp that had no reverb, but it sounded so great, it was breathtaking. I decided to try a few like that and lo and behold I learned the secret to building really great sounding amps. Simplicity. I haven't built an amp with reverb in over twenty years, however I still like the sound of reverb sometimes so when I want it, I use an outboard unit. Try building the amp without it first, and add reverb with an outboard unit. I always build 'em with a four input preamp design these days like a tweed bassman....I usually just jumper into the outboard reverb unit and plug the reverb output into the other channel. Then I just mix it into the signal using the volume knob on the second channel. This way I get the dry unadulterated signal and the reverb laden signal both.
5/23/2000 5:35 AM
Steve Ahola
How about the Dumbleator?
    I'm surprised that you didn't mention the Dumbleator (tube buffered FX loop) as one option for adding in reverb... especially since you have given it high marks before.  
    As for the Fender transformers, I believe that it was Rob Livesleys's site that explained that in the 70's Dumble shipped many of his amps to Europe without the transformers (to save on shipping costs- that guy sure knows how to pinch a penny! I guess that was before he figured out that he could save even more by not paying his suppliers! < grin > ) Once overseas I believe that it was Fender Twin Reverb trannies that were installed in the 100 watt amps. And maybe Bassman or SR trannies for the 50 watters.  
Steve Ahola  
P.S. As Peter S. said in another post here, with a really hot amp you don't miss reverb at all! IMHO reverb was just an early method to allow single notes to sustain longer... although running the Dumble clean channel through a nice plate reverb would sure be fine!  
    For recording, FX like reverb can be added in after the fact, and for live shows, they could mike your amp and add in your own dedicated reverb for the PA mix— that way you could have the pure unadulterated signal coming out of your amp... Just my "Humble" opinion!
5/23/2000 5:57 AM
Dave B.

Hi Steve,  
I threw that reverb question out there just to get some opinions. I'm planing to install the Dumbleator internally. I'm having the chassis made a bit larger to accomadate the extra tube and the controls.  
Some one commented the amp sounds warmer and fatter with the Dumbleator in there (maybe it was you)so shall it be.  
Dave B.  
P.S. got any 12" EV's laying around?
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