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A Unicorn with Haunting Mids - Dumble ODS Analysis & Build Adventure

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Dec 1, 2020.

  1. kleydejong

    kleydejong Tele-Holic

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    Intro

    The Dumble Overdrive Special... Undoubtedly the most (insert adjective of choice) of all guitar amps in history. I've always considered it to be a bit of a unicorn or white whale or cork sniffery of sorts. However as I've grown more in the DIY world I often find that there are scientific and logical explanations for why bits of gear operate the way they do. Science can also be replicated. So my plan is to do some compiling and analysis of the ODS circuit as I build a clone.

    Circuit Overview

    To start I have been compiling and reviewing A LOT of different schematics. ampgarage.com is probably the best resource I found. I don't know how much I want to dive into the specifics of each nuanced schematic yet, but Dumble definitely made a lot of custom tweaks on each amp he built. Not two seem to be identical. But let's start with a fly over of the basics.

    Inputs - Most ODS have two inputs, a normal and a FET. The normal input is very typical of classic tube amps. The FET places a transistor in line. I would guess this would help provide some buffer to boost weak pickups like from an acoustic guitar or lap steel or something like that. The FET has a trimmer to set the level and runs into the normal input.

    The first 12AX7 triode gain stage comes next. For the plate resistor I have seen the standard 100k as well as the hotter 220k. The cathode bias resistor ranges from 1.5k to 3.3k. 4.7 to 10 uf cathode bypass cap is common. Nothing too crazy yet.

    Next it runs into a tone stack similar to a Blackface Fender. The tone stack is usually more complex though. The specific values of components used varies a fair bit here. Some things of note that I am seeing:

    - The mid control runs parallel with the bass control instead of in series like in a Fender.
    - The treble capacitor of 390pf is sometimes bypassed by a 'mid boost' switch.
    - There is a rock / jazz switch which appears to shift the tone stack from a typical FMV style to more of a James style as explained nicely by Merline B here:

    [​IMG]

    - PAB = pre amp boost = a tone stack bypass by lifting the ground control. The signal only goes through the treble cap and treble pot to the next stage. The mid and bass pots are bled to ground.
    - Deep switch engages an RC low pass filter (like a guitar tone control) so the treble and midrange frequencies are suppressed a bit. Looks like it goes from the bass control to the volume control.
    - The volume control is at the end of the tone stack and it has a switchable bright cap.

    Upon exiting the tone stack it runs into a second 12AX7 triode gain stage. Typical 100k plate and 1.5k / 5 uf cathode components are common. I have sometimes seen an interesting local negative feedback loop going from the plate back to the grid with upwards of 40M of resistance and a .05 uf cap.

    This next junction point either sends the signal to the phase inverter, or you can engage the drive channel and it sends to a second 12AX7.

    The drive channel utilizes some trim pots and or voltage dividers to dump some signal between each stage. The coupling caps tend to be a little lower value like .01uf to .005uf to cut a little bass. I have seen some snubbing caps sending treble from the plate to the cathode via a 270pf cap to smooth out the highs. Shielded cables run to a level or drive control in between the third and fourth gain stages. Then a ratio control serves as another volume control after the fourth overall gain stage. Think of that like a master volume for the drive channel.

    Some later ODS models named HRM (Hot Rodded Marshall or Hot Rubber Monkey ???) would include a full FMV style tone stack at the end of the drive channel. I have heard some complaints that the placement of the first tone stack so early makes it fairly useless to dial in the overall EQ of the drive channel. The HRM would fix that. The EQ controls were usually on trimpots inside the amp.

    Next the clean and drive channels converge and there is a global master volume.

    Next is a passive effects loop. Dumble made an aftermarket buffer called the Dumbleator to help make various effects work better in the loop without signal or EQ loss.

    All of these switches were controlled with relay switching. This is a mystery right now.

    Next comes a fairly straightforward long tail pair phase inverter. The global negative feedback and presence controls are tied here as well. I have seen the two plate resistors tied to their B+ source via a trimmer pot so you can manually balance the phase inverter with a scope.

    This leads to the power section. It is generally fixed bias. I have seen 6L6's and EL34's both as pairs or quads.

    Transformers are quite beefy.

    My Build

    I'm going to try and build a Dumble ODS inspired amp. In my studying I have thrown together this schematic. It is a bit of an amalgamation of all the ODS's I studied. I am cherry picking some of what I think I will want and throwing out some of the fluff. I'm sure things will change before the final product is completed.

    [​IMG]

    I am making use of transformers I have on hand. Specifically a Classictone 40-18056 PT meant for a Marshall 18w. Then I grabbed a Hammond 1750pa OT rated for 8.4k primary impedance and 18 watts. So it looks like my power section will be push pull EL84's. I'm leaning towards adjustable fixed bias.

    I'm going to use a Hammond P-H1444-1372 chassis. I do all my own drilling by hand, so aluminum will be welcome compared to the thick still I had been using recently. I'm mildly concerned I should go for the thicker P-H1444-22, but we'll find that out.

    Chassis Fabrication

    I have had a chance to get cranking on the chassis.

    I use a ruler to measure and make straight lines as best as I can. Spacing is a bit relative. I try to follow good layout principles. V1 is as far away from the PT as I can get. OT and PT are oriented 180* from one another. I will be using PTP and terminal strips for wiring.

    Once I draw my lines I use a nail and a hammer to hole punch. This helps center the drill bit from wobbling. Then I drill with a small bit and gradually step up as needed. Larger holes use a stepped bit. Aluminum is so nice work with. Some of the odd shapes are done with a dremel. After drilling the hole I flip it over and debur using a larger sized bit.

    [​IMG]

    [​IMG]

    After drilling most every hole I did some sanding on the aluminum. This helped clean up some of the marring from my work and it gives a brushed look. This is after one pass. I think I started with 320 grit and moved up to 440 and 600. I also did some smoothing over all the holes where wires would pass. I will use grommets as well.

    [​IMG]

    Then I mount the components.

    [​IMG]

    My OT has taps for 4, 8, and 16 ohms. I need to get a selector switch.

    Onward!
     
    Piotr, simoncroft, Antoon and 7 others like this.
  2. JuneauMike

    JuneauMike Poster Extraordinaire

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    Very cool. Best of luck. The problem with building Dumble amps is that they are so rare and each being apparently unique that you may never know if you actually "nailed it." But at worst, you will end up with a cool amp.
     
    scook likes this.
  3. dogmeat

    dogmeat Friend of Leo's

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    I'm in...
     
  4. James Knox

    James Knox Tele-Holic Silver Supporter

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    Looks like a fun build Bro!

    I be watchin’...
     
  5. surfco

    surfco Tele-Afflicted

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    Looks spectacular!
     
  6. jmp81sc

    jmp81sc Tele-Meister

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    Looks great. That is the same procedure I have been using to do my layouts. On my latest build I am using Inkskape to mock up my face plate graphics, but I also used it to make a hole drilling template, which worked out really well.

    I am adding a foot switchable "gain" setting to a 6G3 preamp section and I will be using a Hoffman relay and power supply kit.

    A bit off topic, but you have a much greater understanding of amps than I do, any thoughts on preamp design with regard to where to place the tone stack relative to the gain stages?
    I copied the Mesa Boogie Mark I which places it before the tone stack, while your design shows the 2 gain stages after the typical preamp design. Reading the Merlin book it seems that the way I am planning on might not be ideal.a

    thanks
    John
     
  7. sds1

    sds1 Tele-Afflicted

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    Yo KDJ it looks like your power supply is just copy/pasted so far? I was looking for the choke and 220uF at the first node.

    That PT is convenient because you can give your relay power supply their own winding. Dumble used a dedicated 12V transformer. In general you'll see a lot of relay power supplies tapped off the filament secondary; ideally, the relays are isolated from the filament so in case of filament short the relays are out of harm's way.

    There are man many different ways to do it (recent discussion on TDPRI to this effect), my recommendation will be to simply use a bridge rectifier on the "extra" 6.3VAC secondary to output probably over 8VDC (assuming the 6.3VAC is more like 7VAC), using 7805 voltage regulator, and powering 5V relays. You can buy or build a footswitch to go with it.

    Other dudes may pop by, suggest the 7805 is unnecessary or use a different voltage/rectifier/relay configuration. They are probably all correct.

    Hoffmania has a good page on this actually:
    https://el34world.com/projects/relay_switch.htm

    You might enjoy having an extra bias trimmer, I think a matched bias in a Dumble is more important than usual.

    Finally, you did such a good job explaining the circuit but you skipped over your rationale for your mods/omissions. Interested party here! I assume it is based in the spirit of simplification.
     
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  8. FenderLover

    FenderLover Poster Extraordinaire

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    I appreciate the spirit of simplification. The trouble in mentioning the D-word brings out the naysayers with 'you can never duplicate perfection' or some nonsense, but 'D-inspired' is a perfectly functional starting point. A great sounding amp doesn't need to sound like anything else, so make it great and good luck.
     
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  9. kleydejong

    kleydejong Tele-Holic

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    I have been chewing on asking for input on how to design a faceplate that actually looks nice. All my DIY amps end up being blank or sharpied. The way my mind works, the appearance falls far below the circuit and the tone. I think I'll revisit this in a bit.

    I also think I will need a fair bit of research and input on relay switching. I've seen schematics and layouts, but it hasn't clicked yet in my head.

    Tone stack location is very interesting. The standard advice I have read and experienced personally is that the later you put it in the circuit, the more it functions like a master EQ to straightforwardly shape the final EQ of the tone. The earlier you place it the more it functions to change the nature or character of the distortion. For clean tones the differences aren't perhaps as great. But as you drive the amp, the differences become more pronounced.

    I have read that earlier Dumble amps having the tone stack placed so early with 3 stages following it means it can be hard to dial in a good clean tone and drive tone at the same time. That's why it evolved to the HRM adding a second tone stack. I don't really get why you'd put that on internal trimmers though.

    I've also seen a design that simply make the channels independent. You add an extra tube and double the first two stages, but there is no compromise. You get control to dial in the clean channel and the drive channels however you wish.

    Good amp design involves shaping EQ all throughout the amp though. Basically any place you have a capacitor you make a choice to shape the low or high frequencies. The tone stack is just an obvious and variable place to shape EQ.
     
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  10. davemontalvo

    davemontalvo TDPRI Member

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    Is it true that he poured some kind of stuff all over his board so you can’t really take it apart to see exactly what he did?
     
  11. _Steve

    _Steve TDPRI Member

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    Cool thread KDJ! Im looking forward to seeing the progress as I want to do an ODS some day.

    As someone with only a handful of builds under my belt so far I can't help wonder if EL84s are the best choice for this? I've built a couple of EL84s so far and while I personally love their sound I suspect they might be a bit 'snarly' for an ODS. I guess with that tranny though you could always try with EL84s first and if you dont like them switch to 6V6s...

    Anyway best of luck with your build!
     
  12. Teleguy61

    Teleguy61 Friend of Leo's

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    They are very challenging amps to build, and to play.
    Lead dress is very important. Very.
    ampgarage is your friend. They are very helpful, and there are some extremely knowledgeable builders there who will answer your questions.
    I've built an HRM ODS, with 78 Twin transformers, and it is a beast. I built a couple of Thiele cabs with EVM 12Ls, and that amp into those speaker cabs is phenomenal. Just huge sounding.
    I don't use it much, because I don't need that kind of power any more, but it is a great project, and good luck.

    I also have built a D-Lite, an ODS with 6V6s, and it was never the same. You may have the same experience with the EL84s--they are big amps, with a big sound, and trying to throttle them down with a smaller output section may be unsatisfying.

    I wish you the best, it is a worthy venture.
    Good luck!
     
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  13. dogmeat

    dogmeat Friend of Leo's

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    silk screen looks nice for the face plate but its a lot of work. you can have a decal made. for either one you can do your own graphic. I've done a couple. I laid out my faceplate graphics in a Word document. I took the number of spaces I needed and started laying out + signs to indicate the position of the control. even in "landscape" there probably isn't enough page to do it on one line (on an 8 1/2x11) so use bigger paper or what I did is just lay it out. the 2nd line has the same spacing so cut and paste after the print. paper print it and check the spacing. adjust as required. once done, it also give marks on where to drill for the controls (too late for you). anyway, once the spacing is set, do the graphic part. it will require a font change. center each control title over an x (+). there's the graphic. from there you can make a screen or take it to a pro. I never tried to do Paint or other graphics program and I'm sure they offer more options.
    got a couple pix if you want to see
     
  14. kleydejong

    kleydejong Tele-Holic

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    Thanks for the input. I probably should add a choke. I don't have any on hand and I'm being a bit lazy about making another order. I haven't built an amp without a choke in a while and kinda want to see if it is viable for a project like this. But with the high gain channel I probably should be more concerned about keeping noise down at all levels.

    On the switching I am very much on the fence. I either need to dig in and do some research to understand out how relay switching works. I know I could just follow a layout, but that just isn't appealing to me. I'll go watch several YouTube videos and hopefully the light bulb will go off. The thing is that I'm not convinced that I want / need it for my amp. I think I would use channel switching, but that's about it for a live context. I use pedals...

    For some context I owned a Fuchs Modded Bassman some years ago. This was before I got into building amps. I remember liking it. I recall thinking that with most of the switches I would find my favorite settings and not tinker with it much past that. So that is driving some of my desire to simplify. But on the other hand I think some of the preamp switching seems too iconic for this circuit. My schematic is proposed as a starting point and I think I will drive towards adding. There are just SO MANY different circuits that it is hard to keep it all straight. Might be wise to pick one and stick to it for now. However that is very much against my nature! I love to tinker!

    As I am writing this post I think I'm convincing myself to do it right and not take any shortcuts. This isn't a 5E3. A series of compromises may result in a dud, which I think this circuit is definitely capable of.
     
  15. tubedood

    tubedood Tele-Holic

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    Yep he was a fan of "gooping" parts of his builds with an epoxy so nosey folks couldn't see what makes his amps tick.
     
  16. kleydejong

    kleydejong Tele-Holic

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    Yes, I have seen some with what appears to be epoxy smattered on the preamp. I believe the purpose would be to discourage cloning. Blue goop seen here (also spot the HRM trimmers for the drive channel).

    [​IMG]

    I get it and I don't. I don't make amps for a living and no one has used my efforts for their own profit, so I don't know what it is like to be HAD. But I love and appreciate being open to share ideas with those keen to learn. I am grateful to those who have come before me and so openly shared their expertise. I wish to pass that forward.

    I also think it is interesting to think about how a brand creates interest in their product. Mystery and intrigue can be very powerful. If you make an amp, what kind of description do you give it on the web page to sell it? My brain goes towards being technical and thorough and factual. Better marketers probably would go more flowery and try to create a feeling or an emotion. Do you love your tube amp because its 6V6's are biased at XXX plate voltage and have YYY plate current to produce XYZ watts of plate dissipation? Or do you love it because of how it feels to lay a G chord at a gig with your buddies? Or because it reminds you of a nostalgic time in your life?

    It would be a pain in the rear to service that thing. It isn't on the power supply so you may get away with it for 50+ years of life. But woof. I dunno how you'd work on that part of the circuit. Just tear it all out and replace board and all?
     
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  17. tubedood

    tubedood Tele-Holic

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    I've had great results with mojotones relay switching board that grabs power from the 6.3vac filament feed.

    AC feed! Edit
     
  18. kleydejong

    kleydejong Tele-Holic

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    A fair question. It does seem to be the trend that hotter preamps mate well with higher headroom power amps. Push Pull fixed bias EL84's aren't the highest headroom tubes. But idk, I don't need this thing to play in stadiums. I mostly play in my basement for fun. Covid killed my every other month gig schedule anyways. I also have stated that I think tubes don't have near as much character as circuits and component values. Time to put my money where my mouth is!
     
  19. tubedood

    tubedood Tele-Holic

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    KDJ I'm sure you have checked out ampgarage..they really have dug deep into Dumbles and 'wrecks.

    Looking forward to your build!
     
  20. Ess Eff

    Ess Eff Tele-Afflicted

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    Any excuse to post some pics of my VHT D-Style...
    .
    IMG_20200806_150429.jpg
    CF1oo8z.jpg
    LOkb8pJ.jpg
    Mi6ZJxD.jpg
     
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