Dumbleator... how hard is it to build one?

Discussion in 'Amps and Cabs' started by Reyybarra, Mar 27, 2017.

  1. Reyybarra

    Reyybarra Supporting Member

    Dec 1, 2006
    I guess the title says it all...
    considering buildin rather than buying, my dads and electrical engineer so I should have some solid help,
    what y'all think?
  2. Wirezz

    Wirezz Member

    Sep 2, 2015
    Get a ceriatone kit :)


    Alternatively you can look at his schematic and try to work something out. But sourcing iron is always a pain. Finding and drilling a chassis too.
    sibyrpunk and dani_boy79 like this.
  3. candid_x

    candid_x Supporting Member

    Jan 1, 2007
    A friend has assembled one which looks clean but something's wrong. The kit source has offered to trouble shoot it. We'll see.
  4. dani_boy79

    dani_boy79 Member

    Apr 30, 2009
    I built the Ceriatone C-Lator kit in an afternoon. It works great, and it was cheap.
    Wirezz likes this.
  5. 8nthatK

    8nthatK Supporting Member

    Jan 15, 2003
    Hard to beat the Ceriatone kit per the above posts.
  6. guitarjazz

    guitarjazz Member

    Dec 29, 2009
    Cathode follower with a couple knobs?
  7. teemuk

    teemuk Member

    Apr 21, 2009
    Low output impedance 'buffer' followed by very high impedance send level control? What is the point? An external device to introduce FX loop with serious DESIGNED-IN "tone sucking" and excessive signal attenuation....? Doesn't make any sense to me.

    Today we should know better...
    Last edited: May 3, 2019
  8. El Rey

    El Rey Silver Supporting Member

    Mar 23, 2018
    Denton, TX
    Thanks for this post....you make me curious enough to examine more. I was just considering building up one to see if it could be useful to me. I put that on hold. Thank you for making me think again........I could be wasting my time.
  9. teemuk

    teemuk Member

    Apr 21, 2009
    Don’t let me get you down. I encourage everyone to build this thing for experiment if it slightly tickles interest. It should be a fairly simple project and you can always modify the thing to, say, Alembic preamp in case you actually find the thing useless for what it does.

    Just couple of pointers of what to realistically expect: It’s not a “transparent” looper device by a long shot. The recovery stage, for instance, introduces distinct low-pass filtering (“treble loss”), which explains why this thing has a reputation of “smoothing” out the tone of an amplifier. The response is actually far from “level” with bright caps disabled.

    Also, it would be somewhat sacrilegious to call this thing a “buffered” loop, because effects of that cathode follower buffer are largely negated by high impedance of the “Send” level potentiometer. (Except at settings with very little attenuation). Due to high output Z introduced by the potentiometer’s resistance the thing will highly interact with capacitance and overall impedance effecting in the loop. So, it’s not actually really “buffering” anything like REAL buffered loops and the performance will not be consistent with variable pieces of equipment, cables, etc. like with REAL buffered loops.

    For example, let’s say the Send Level attenuator introduces series resistance of 100K. That's your source impedance driving the loop. It is far from “low impedance” and imagine what happens when you plug the thing to FuzzFace with input impedance in kilo-ohm range. Next, imagine what the 100K source impedance does with varying cable capacitances. Serious interaction! These are all effects what a REAL buffered loop should NEVER introduce.

    For a valve driven FX loop this design is basically really, really mediocre. Basically a schoolbook example of what is generally wrong in poorly implemented tube-driven FX loops. All the mistakes are featured right here in this design. Could be done better. Way better. Almost any modern amp (and less modern amps as well to be exact) introduces better loops in terms of functionality for given purpose. But this thing does introduce a “tone of its own” due to designed-in treble loss, which (IMO) is a controversial feature for a FX loop. But that’s likely part of the charm of this device. Not to mention it was designed by DUMBLE!!!

    Power supply can be challenging, as it is high voltage but even that one has its strange quirks. For instance, I don’t see why a very low current draw device requires an expensive choke filter, and most likely the thing works just as good even if you don’t exactly nail in those quoted B+ voltages. The device probably retains the same functionality whether you operate it at anything between 220VDC – 350VDC B+. Heck, it’s just a cathode follower and a common cathode stage, which is not even overdriven. If you have ANY power transformer with HT winding just try that.

    ...And one can always convert this thing to classic Fender preamp (“Alembic”) with medium effort.
    davidespinosa and Jelle like this.
  10. davidespinosa

    davidespinosa Supporting Member

    Jul 10, 2007
    Silicon Valley

    I think you're saying:
    * A tube buffered effects loop can be a good thing.
    * Dumble's specific design sucks.
    * It's not hard to make a better design.
    Do you have any references for a better design ?

    Here's one, but it costs 98 CAD, not including case and power supply:
  11. teemuk

    teemuk Member

    Apr 21, 2009
    The only virtue of driving a FX loop with a tube is the high headroom provided. You can ponder how important parameter that is when overall signal levels are few volts in maximum. Personally, I would rather use a solid-state solution and by that choice simplify the design process a lot. ...Not to mention, it would be gigantically cheaper as well.

    Couple of ideas for modifications: Locate the Send Level control before the cathode follower stage so that source impedance for loop is cathode follower’s output impedance. Do not use 12AX7 tube with high internal impedance, try something like the 12AU7. Better yet, connect two in parallel. Even better, use a MOSFET. The lower you can set the source impedance the better. You will find that this is not so trivial task with vacuum tubes as it is with e.g. a plain “opamp follower”. These modifications, for example, require pretty much a complete redesign of the front stage and probably more additional active circuitry. I’m not going to start designing this, just throwing some ideas for improvements….

    As said, the recovery stage introduces a distinct “low-pass” filter. This may be the magic of the whole design or it may not, most certainly it will color the tone of the signal. The gain of the stage, by the way, is excessive for loop recovery. Personally, I would get rid of the built-in “low-pass” voicing and increase the local NFB even further to reduce the stage gain. I would be tempted to introduce “Recovery Level” control as such that it actually controls gain of the recovery stage, not input signal level.

    Tubes are - IMO - once again unessesntial, because the stage is supposed to work “transparently”, not coloring the tone of your effects with additional effects such as clipping distortion or "mud" due to treble loss. (See that notion about excessive gain).

    Output impedance of thast design is also quite high-ish: With the potentiometer set at minimum attenuation it’s still source impedance of the common cathode amp (high) and with any attenuation introduced you also need to include series resistance of the potentiometer (high again). You would ideally want this to be in the order of few kilo-ohms for extended compatibility with various audio devices (which not all have input impedances in the megaohm range). A buffered output might be a good thing. ….Or a solid-state solution with lower source impedance overall. You see, once again we are talking about a complete redesign.

    I don't see the point of paying 100$ for a tube driven FX loop when you can get the parts for a good solid-state loop for maybe $10. Just look at any modern amp with a loop for few ideas how those circuits are typically constructed. I already listed some things not to do / avoid.
    I also mentioned it earlier but the choke power supply filter for very, very low current draw circuit seems redundant.
  12. De Batz

    De Batz Member

    Nov 13, 2015
    This is the kind of reasoned and detailed analysis that TGP often asks for, then completely ignores.
    I applaud you, sir, in your quest to reduce our ignorance.
    dsmc80 likes this.
  13. HotBluePlates

    HotBluePlates Member

    Mar 27, 2017
    I have a friend who built one of these, though not from a commercial kit. He liked the sound of it in his amp's passive effects loop, even with nothing plugged into the Dumbleator and just using the normalled connection from Send to Return. Ultimately, he knew that what he liked was the EQing of the circuit rather than its excellence at being an effects loop.

    And that's really where the users of this circuit land: it's less about having a great transparent loop for your effects, but having an additional place to insert the "Dumble sound" with the local negative feedback and its treble roll-off.

    It is what it is.
    teemuk likes this.
  14. teemuk

    teemuk Member

    Apr 21, 2009
    I respect that opinion. I had a hunch it was more an 'effect unit' of its own rather than a mere looper device.
  15. Ayan

    Ayan Member

    Feb 2, 2002
    Los Angeles, CA
    I also wonder, to this day, why Dumble used a 250K send pot in the Dumbleator. As far as the effects of that choice, the worst-case scenario would be with the SEND pot dialed in at 50%. You get 125K in series and another 125K to ground, and since the output impedance of the CF itself is much, much lower than that, the two 125Ks will be essentially in parallel and the total output impedance of the “FX SEND” jack will be 62.5K. Set like this, the Dumbleator is definitely capable of interacting with the cable (to the FX input) capacitance. Assuming a cable capacitance of say 40 pF per foot, we can approximate: for 3ft, a 21 KHz 3dB frequency break point; for 6 ft, 10.6 KHz; and for an 18 ft cable (if you have the pedals on a pedalboard, for example), 3.5 KHz. The last case has a severe effect and can be easily checked. With the Dumbleator connected to the amp, but with no FX in its loop, set the amp a little loud and pretty bright, such that you have hear a high frequency buzz when your guitar is plugged it. Set the SEND pot at 125K to ground and then plug in and out an 18ft cable in and listen to how the HF buzz changes… It’s radical. Now, with a 10% taper 250K pot set at noon, the output impedance of the SEND jack lowers to under 10K, and tthe 3dB frequency breakpoints above change to 132 KHz (3 ft cable), 66K (6 ft cable), and 22K (18 ft cable). Not nearly as bad.

    As with all things Dumble, I believe he didn’t really do anything just because, so I’m inclined to believe this was part of a greater scheme. [ Also, the Dumbleator was put together in the rack era, when no one would be using 18 ft cables to connect to a rack unit sitting under the amp. ] Dumble was probably tuning something. I believe it’s no coincidence that, for example, all the pictures of Dumbleators by tone gurus like Larry Carlton and Robben Ford show the SEND control set on the low side. And those guys connected the Dumbleator to FX units that had input level controls, so they could have dialed the SEND up. I suspect Dumble probably told them exactly where to set those controls to get the sound that they wanted.

    Last, your suggestion that the SEND pot be moved to the input of the CF stage would make a lot of sense to me. Except, the Dumbleator was built to be used with Dumble's amps, where the preamp out signal is fed by the amp’s master volume. Moving that SEND control to the front of the CF would put two pots in series. Perhaps more importantly, it would also – depending upon the FX inserted in the Dumbleator’s loop – limit the level of the signal hitting the CF’s input to whatever the FX’s input could tolerate. But, hitting the CF hard can have its merit, and Dumble’s placement of the SEND pot allows to fully control the output signal level at the SEND jack irrespective of the signal level present at the CF’s input.

    BTW, has anyone tried using a lower SEND pot value in a Dumbleator? Curiously, some Dumbleator II schematics show a 10K SEND pot...



    woof* and davidespinosa like this.
  16. The Ballzz

    The Ballzz Member

    Mar 7, 2015
    WOW! @teemuk @Ayan
    This is a very cool discussion! I'm also interested in effects loop options, other than the Dumble/C/Klein/Ulator! I just want/need a clean way to insert rack mount style gear into the signal path of the amplifier! I just installed one of these into a home built 10-ish watt version of a Marshall JC800/2203:


    So far it works quite well and I'm using a Lexicon MX300 effects processor, but also tried it with my ZOOM G3 and it was also stellar, except for any of the boost and/or OD choices. One thing that complicates issues is the desire to use a similar setup in amps that may not have the "real estate" inside the chassis and possibly may not want to have the B+ power rail sucked down by the (although slight) power draw of something like the Granger , or even the Metro Zero-Loss loop!. I'd like to try to build something similar to that Granger unit, but built into it's own external box and it's own power supply. My intent would be to put a simple circuit breaking/making pair of jacks (or even a single switched TRS jack) on the amp to conserve space and possibly avoid any drilling!

    It seems that the Granger unit could certainly get mounted into a box, if I could just figure out how to power it. While I can solder well, follow instructions, schematics, etc, I have very little knowledge of how to design power supplies!

    Any thoughts comments and or suggestion would be warmly welcomed!

    Thanks Guys,

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