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Dumble 2nd tone stack useful?

5/19/2005 1:07 PM
Wild Bill
Dumble 2nd tone stack useful?
Been wanting to try something different so I figure I may as well play with some Dumble type overdrive circuits.  
Don't know if it will make me sound like Carlos Santana while I play Louie Louie in a 3 chord barre but what the heck! :)  
Anyhow, there's lots of schematics out there to use as a base platform. The "1997 Humble Overdraft Specimen Rev 1.2" looks like a good start. I've got an old combo 2x12 amp that will let me run a pair of 6L6's with lots of room across the front faceplate for controls.  
My immediate question is about using a tone stack for the OD section. Some circuits don't bother and the others I've seen use trim pots. I guess they set them to taste and leave them inside the amp.  
Has anyone tried mounting the second tone stack with regular front panel pots? I'm wondering if a player would appreciate having the control on the OD tone easily available or if it would make things too "busy".  
Hopefully someone has already tried this and can give me an educated opinion...  
---Wild Bill
5/19/2005 6:05 PM

Hello Bill,  
I'm already tweaking Twin Reverbs, Dual Showmans and Bassman 135 heads to Dumble clones with a completely new circuitry inside on an eyelet board since about 15 years - and I've also tweaked some orig. Dumbles after customers demands.  
It's already more than 10 years ago, that I'd buildt a Dumble clone with two tone stacks and 6 tone controls the first time - and for a while I've ended up with the circuitry with 2 tone stacks, but where in the first stack the treble control then was fixed, in the second stack the mid and the bass controls were fixed - so that I finally had only 3 tone controls as usual.  
Sometimes I've putted in 2- or 3- position switches, to alter the values of the "fixed" controls.  
But later I've skipped the second stack again, because all of these circuitries to my ears sounded too much "British". Of corse I've already tried out EL34's and 6550's in the power section, Dumbles with build in tube reverb, tube based effect's loops and alot other tweaks.  
But by my opinion, all these Dumbles were neither fish, nor meat. And by my opinion, a particular Dumble doesn't have a second tone stack, doesn't have EL34's or 6550's - and doesn't have a buildt in reverb or a loop. That's the extract out of all my Dumble experiences over the past 15 years.  
Your intend to start with the "Specimen Rev. 1.2" as a base platform isn't certainly a bad idea by my opinion. But I'd recommend to skip the second EQ completely and instead of it couple out with a .005 cap into a 150K, followed by a 100K pot as the "OD Level". And in parallel to the Level pot put a .022 cap in series with a 390K to ground.  
My further recommendations wouldt be:  
- to reduce the "Drive" to the orig. 100K value and also reduce the coupling cap before the 100K to a .01 cap.  
- to swap in chinese 6L6WGC instead the EL34's - or even better, if you couldt get orig. GE 6L6GC. Stay away from the 7581A or the small bottle 6L6GB. No need to say, that the grid (beam) resistors shouldt be only 470 ohms then.  
- to keep the .047 cap at the 2-nd plate, but increase the resistor to the 100KB trim pot to 220K - also keep the 68K grid stopper.  
- to put in a NFB loop from the 2-nd plate to the grig with a .047 cap and 2 x a 10 meg resistor in series. Then put a switch between the two resistors, to switch this point to ground (NFB on/off).  
- to put each a 250p cap from the plate to the cathode of the 3-th stage and the same on the 4-th stage.  
- to swap in following plate load resistors (from 1-st to 4-th stage): 220K/150K/180K/120K and the following cathode resistors (1-st - 4-th): 3K3/2K2/3K3/2K2, while cathode bypass caps all shouldt be 4µ7 with this application.  
- to increase the plate load resistors on the driver tube to 100K & 120K, but keep the whole rest as shown in your choosen diagram.  
I'd recommend you to supply the amp with NOS RCA or GE 12AX7's - the EI ECC83 also isn't bad in this application, if they are low microphonic - but all kind of Sovtek's here are... (arggrrr).  
B+3 shouldt be only around 400V, B+4 between 310-320V and B+5 between 290-300V.  
These all were only my recommendations of no consequences. But to me it seemed as you're one, who pretty exactly knows, what he wants - and that you won't have too much trial and error.  
And so my recommendation is that, what I've read out of your post that you're probably looking for. I finally hope, that you'd all understood, what I'd written, because sometimes it's hard for me, to bring it down in English, what's all in my head. Although my nickname is Larry, you still know, that I'm a German ;) ?  
I hope, this above can help you - and please let me know about it on this board, if you'd tried it out. If you'd have any further question, also let me know about in this thread.  
Much fun! Larry alias "novosibir"  
Btw: The above described Dumble clone also sounds great into a 4x12" greenbacks loaded cabinet or into a 2x12" Vintage 30 supplied VOX AC30 combo.
5/19/2005 7:46 PM
Michael Polutta

It sounds like Larry is suggesting you follow the Hybrid-A schematic pretty closely. Perhaps check that one out as well...
5/19/2005 9:14 PM
Wild Bill

Thanks, gentlemen!  
I was hoping to get some experienced input. I don't want to make the same mistakes as everyone else - I want to make my own unique and individual errors! :)  
Especially thanks to you, Larry! I followed your English just fine but by the end I had a strong desire for a Staten Pils or a Dortmunder Union.  
You could appreciate better than most my story about how I chose my company name. Until a few years ago amps had been only an occasional hobby. I had always been a builder but mostly ham radio transmitters and the like. I did repair guitar amps for friends and family, having been a "roadie" since my teens and although I had settled down into a career as a salesman for electronic parts I would often get a friend at my door asking if I would fix his amp.  
Then I got given an old PA amp with a pair of EL34s for the output and a little voice inside my head said "Why don't you make this into a Marshall?"  
So I did but in the process I found I needed help with the cosmetic aspects - to make it beautiful!  
I had a long and close friend whose parents were German. He's a great and precise craftsman, whether it's carpentry, electronics, metal working or whatever. Soon we were in his basement workshop hacking out the chassis and spraying paint in all directions.  
As we were working he got a peculiar look in his eye and asked me if I had chosen a name for my amplifier projects yet. I told him I hadn't given it much thought but he insisted that I might wind up making a lot of amps and perhaps go "professional" some day.  
He suggested "Busen Amps", saying it was german slang for "balls". Of course I thought it was a great idea to have my amps named for a rock and roll "ballsy" sound and he helped me make a nameplate.  
Over the next year and a half I wound up making nearly a dozen amplifier projects, each with a Busen nameplate. My friend gave me a lot of help making each one beautiful and teaching me valuable craftsmanship techniques. Then one time as I was watching him etch some lettering into a piece of acrylic I happened to open up a German/English dictionary which was lying on a table and thumbed to the entry for "Busen".  
I looked over at my friend and said "Hey! Busen doesn't mean balls!" He cracked up and fell off his stool laughing!  
Only a close and dear friend would patiently wait over 18 months for me to realise the joke! By then I had made so many projects I figured I might as well keep the name!  
Fortunately here in Canada and with my American friends few customers get the joke that Busen Amps don't have balls but rather boobs!  
Maybe I could get Bonnie Raitt to try one... :)  
---Wild Bill
5/19/2005 11:50 PM
NFB Ignoramus
It is Spaten Pils.  
Be lucky he didn“t tell you to name it "SCHWANZ Amplifiers"  
greetings from München,  
5/20/2005 10:45 AM
Wild Bill

"Spaten Pils", not "Staten..."  
That's what happens when you hear a new beer name after you've already had a few... :)  
---Wild Bill
5/20/2005 4:20 AM
Michael Polutta

Bill, that is truly funny! You know you have a great friend when HE is putting that name on YOUR products!
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