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Re: my 7th amp - is a dumb bell clone worth the effort

11/1/2005 12:58 PM
pete c Re: my 7th amp - is a dumb bell clone worth the effort
thx norm - i like that clip  
ive seen a coloured hand drawn layout - is that Ben's ??  
Is there any more info around to help me on layout ?  
Any guidance on tweaking would be great.  
Who is Ben - does he have experience that could help me ? should I page him ?  
I have a very large chassis I can use ( ex-military rack mount 19x12x7 with loads of space so I can avoid coupling oscillations etc. )  
When Ive tweaked it I intend to build a really nice hardwood chassis and faceplate.  
thanks again  
11/1/2005 5:27 PM
Ben Shelton

My layout is at and the bias board layout is at  
Note that this is an old version of the layout. The tone stack is upside down, and some of the values in the OD section are not quite right. The eyelet board layout is correct, though.  
Look at the '70s ODS at  
There are schematics at Schematic Heaven, as well as at Blue Guitar. Should be fine to get you started. There are a couple more circulating.  
If you want reverb, there are several ways to do it (Glaswerks Amps' site has one of them), or I can send you a schematic with how Dumble did it in the OD Reverb (it takes 3 tubes, though).  
Once you get it working, there are plenty of things to experiment with:  
-The value of the small cap in the tonestack (330pF or so on the standard layout) is very critical. 250pF is the Fender value - sounds somewhat thin in the Dumble. 500pF is a significant boost in the mids - amazing sustain, but VERY middy tone.  
-The negative feedback loop in V1B is crucial to the sound. 0.047uF/44M works well there. 0.047uF/22M tightens up the sound and feel even further, but it's overkill for my ears. Removing this feedback loop gives the overdrive a more compressed, Boogie-ish character. Something you might want to make switcheable, but be sure to put the switch on the grid side of the capacitor or you might end up with high voltage on the switch.  
-A little bit of positive feedback in the input stage, V1A, goes a long way.  
-The overdrive section is a matter of taste. It depends on what Dumble tone you want. Tweak to perfection.  
-The power amp is fine how it is, but it would be cool to experiment with class AB2 (direct-coupled cathode follower or MOSFET drivers) or ultralinear.
11/2/2005 10:52 AM
Jack Koochin

Hi Ben,  
Would you mind sending me a copy of the Umble ODS Reverb schematic?  
Thank you kindly,  
11/2/2005 12:01 PM
Ben Shelton

Right now, it exists only as a blurry scanned copy of a layout, and the file is humongous. When I get the schematic drawn up, I'll send a copy your way.  
11/2/2005 3:31 PM
Ben Shelton
Be forewarned that this is VERY rough, and only the preamp and reverb stages are included. I haven't built anything based on it yet. Values with (?) meant that they were blurry and I picked the best guess.  
If you have any corrections to make, post them up here and I'll fix them as soon as I get the chance.  
Things to notice include the negative feedback loops in both the reverb driver and reverb recovery circuits, the lack of a negative feedback loop on V1B, and the wiring on the guitar/mic switch. Interestingly enough, it appears that on this model, the reverb circuitry comes before the overdrive.  
And to Mr. Dumble, if he by chance happens to read this, I'm putting these drawings up here so that we might learn as a group from what you've designed, not so that we might copy your amp and sell it for an exorbitant price. If you would like them taken down, just send me an e-mail.  
This is for personal use only.  
11/3/2005 3:00 PM
Very Interesting, Thanks Ben. My REV A modded bassman is close but still lacks the Robben Ford, Sonny Landreth mid sustain tone (for lack of an adjective) I'll give this a tweak and let you know. Thanks so much for the post/info. BobW
11/7/2005 6:30 AM
pete c
Many thanks for your really helpful layouts and commentary. I have most of my parts ordered/delivered so ready to start the build.  
there are a few things I wondered if you could help me out with as I plan this build. Or point me towards other info etc ?  
1) Effect loop. I have a circuit schem similar to a *umbleator, but dont have any details of the plate voltages required. Ive read somewhere that they were operated from a low voltage transformer.?!  
I have plenty of current headroom and space in the chassis to build the effect loop in, so I really want to incorporate this while I'm building. Do you have any experience of it ?  
2) I just wanted to seek guidance on your choice of values for bypass caps and plate / cathode resistors you seem to have gone with more fender type values on your layout rather than the Rev A values.  
3) Also as I'm trying to make a layout based on the Hybrid Rev A can you let me know which version your layout is based on ?  
I wanted to incorporate the front panel Boost switch . The only footswitchable option i need is the channel select so I'm using a DPDT relay for that.  
Any thoughts greatly appreciated.  
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