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Sunn Concert Bass - EQ section help: tone stack?

Discussion in 'Amps and Cabs [BG]' started by paranoid30, Oct 11, 2018.

  1. paranoid30


    Apr 21, 2014
    Milan, Italy
    I'm currently using a Sunn Concert Bass into an Orange OBC 410 :)
    It's not the best SS head, but it's cheap, loud, aggressive and eats pedals like a champ so I'm pretty happy with it.

    I've read a couple of times that the Concert Bass EQ is a fender-style tone stack and not a parametric EQ. I never had a Fender amp so I'm a bit puzzled by this. Also the few info I found online were a bit vague.

    Anyone has some insight into how this EQ works?
    I noticed that the MID control only slightly effects the sound, while bass and treble controls have a much stronger effect. Turning up the Treble control also boosts the overall volume a lot, bringin up the low-mids too. The bass controll doesn't seem to do the same. After turning the Treble control up, the Mid control has more effect on the sound.

    Any help would be really appreciated!
  2. jnewmark

    jnewmark Just wanna play the groove. Supporting Member

    Aug 31, 2006
    Stax 1966
    Third St. Cigar Records staff musician.
    I'm not an expert but, short version goes something like : Bass and treble are boost and cut, midrange is cut only. Diming the Mid is the same as not having a mid control. There are a couple of threads here on Tb that go in to the tone stack further, like how the Treble control gets involved the more you turn it up, but, that's basically my understanding of it.
  3. Welcome to the Concert Bass club! I've had mine for 45 years - love it.

    With apologies and credit to the author, copied below is a post from the Sunn forum that explains the tone controls.

    Title: 1976 Concert Bass Mid Range Control
    Post by: JoeArthur on March 13, 2004, 05:56 :17 pm

    ... The treble frequencies are split out through a separate path and the Treble control is wired in a "cut only" type fashion - opposite of the normal Fender/Marshall tone stack. The output signal is taken off the Treble control just line the more common tone stack.

    The bass/middle frequencies go a separate path. The Middle control is first in the line and it is wired as a "treble cut" with a lower cutoff frequency than the Treble control. After that, the signal proceeds to the Bass control portion, and then to join the treble frequencies at the Treble control.

    There is a small resistor in the Bass circuit that attempts to leave a little of the signal in when the Bass control is at zero, but for most purposes, the Middle control will have very little effect at a zero bass setting because very little signal is actually going through to the Treble control. Turning the Bass control up a bit will make the Middle control have more response.

    I suggest starting out with a Bass setting of 2, a Treble setting of 10, and set the Middle control to whatever sounds good first. The Treble should be reduced to get a mellower tone if necessary. Then set the Bass control to whatever sounds good. At least this is the way I do it.

    It doesn't sound like a glitch - just the way the circuit works.

    Personally, I start with bass 3, mid 4, treble 5, and then adjust. Depends on your cabinets, the room, your bass, etc etc.
  4. P-oddz

    P-oddz Supporting Member

    Apr 7, 2009
    Milwaukee, WI
    Is your Concert Bass a Silver or Red face? I've got an early 70's Silver Face and if I set mine like this, it would be nothing but trebley "gack" tone coming out of it.
    My EQ is set with bass at like 8, Mids are like a 3 or 4, and Treble is closer to 1 or 2. Distortion channel switched on and dialed at about a 4 to introduce some grunt with a pick.

    Even with bass cranked on the amp, I still find it quite hi-mid and treble focused. Very curious.
  5. Mine is silver face. Oh yeah, it's a trebley head with a distinctive raw sound, think John Entwistle. The tone controls are unique & it can be tricky to dial it in on any given day. I'll try your settings & see what I get. What you hear definitely depends on your bass, cabinet, the room, etc.
  6. paranoid30


    Apr 21, 2014
    Milan, Italy
    Thanks guys!

    Allright, that's clearer than what I had in my mind. I didn't know the midrange is only a cut. It still puzzles me a bit, because its effect changes depending on how much treble I'm using.

    Eheh, thanks, I've actually been part of the family for a while now, this baby has been with me for a decade. I've been using it on and off and just recently moved to a new studio\rehearsal and found I had a hard time adjusting the sound to the new room.

    The explanation you quoted goes into great depth, but I'm afraid I still a few doubts: it says that the treble control is a cut only type, but I can clearly hear the bass frequencies coming up when I turn the treble clockwise. Is this because "After that, the signal proceeds to the Bass control portion, and then to join the treble frequencies at the Treble control"? Meaning that the bass frequencies are then routed into the treble control? This would make the treble controll kind of a "master" as it affects all frequencies.

    Mine is the latter, the Red Face :)
    I have no idea if the EQ section is different between them, but you can do a test: set everything to zero and turn up the treble, it should bring up a lot of bass too. I often read that this is a very bright head but I never thought so: I also play a Rickenbacker and still I have tons of bass frequencies from it. But according to what we're learning here, if you turn down the treble you also loose bass? :D
  7. jnewmark

    jnewmark Just wanna play the groove. Supporting Member

    Aug 31, 2006
    Stax 1966
    Third St. Cigar Records staff musician.
    Huh, never knew that !
  8. Yes, the treble control acts *sort of* like a master volume, but since it also affects the tone, it's a balancing act. The output always does lean to upper mids and treble. You don't lose bass by dialing up treble, you're just reducing the treble rolloff.

    I don't know if the redface schematic is different from the silverface.
  9. You can also put a conventional boost/cut preamp/EQ in front of the input to get more tone shaping.
    Last edited: Oct 11, 2018
  10. My Sunn rig :)

    That's a 215S cabinet. SO heavy; one side handle that's almost silly. It moves like one of those ridiculously large suitcases you see at the airport. Rolls great on a flat surface. Stairs, door sills, car trunks, etc. - ugh. But I can tip it into the back of my VW Golf easily.

    Last edited: Oct 11, 2018
  11. Lowbrow

    Lowbrow Supporting Member

    Apr 22, 2008
    Pittsburgh PA!
    Would it be oversimplifying to suggest that this tone stack is totally stupid?
    bumperbass likes this.
  12. paranoid30


    Apr 21, 2014
    Milan, Italy
    That looks beautiful! Mine is definitely not as pristine :D


    It was also fixed by a previous owner, the main switch is a homemade job.

    I've been reading this thread: Fender/Alembic/BBE Tone Stack Explained
    But now I'm even more confused :D

    There are really two separate sections in the circuit. The first section involves the BASS and MID pots, two caps and a fixed resistor. When the BASS control is at 0, the MID control is essentially a level control for all frequencies for this section of the circuit. When the BASS control is turned up, more low frequencies are *added* to the output of this section, increasing the overall level.

    On the Concert Bass it seems different to me: if I turn everything to zero and move the mid control there's almost no change at all. All I hear is a very slight change in tone.

    So when you turn up the TREBLE, you're actually *reducing* the level that's coming from the BASS/MID section at the same time you're *increasing* the amount of signal coming from the treble cap.

    This actually is the opposite to what I heard: turning up the treble control also brought a lot of bass frequencies back.

    I downloaded the software linked in that thread, which provides a tone stack simulator: TSC
    But the frequencies don't really move as I experienced them with the Concert Bass... so I'm thinking the Concert Bass behaves in a different way?
  13. Ha! Yes, that would be oversimplifying. It's just -- odd. You can make it work, but takes a little more effort than Ampegs or modern amps.
  14. paranoid30


    Apr 21, 2014
    Milan, Italy
    I'll get to this later, but once I understand how it works I already have the following question: what are the benefits of this kind of EQ? It seems incredibly common in the 70s: there must have been a reason, even if it's so complicated...
  15. gregmon79

    gregmon79 I did it for the muff... Supporting Member

    Dec 20, 2012
    Chicago IL

    Def sounds kinda interesting though. Unique that's for sure.
  16. Thanks. Your rig is freakin' great.

    Correct. The Sunn tone stack is not the same as a Fender.
  17. It's passive, that's why. I suppose it was easier to build & this particular stack was a variation on an earlier design. Not complicated to build. Just to use!

    I don't know if this is a Conrad Sundholm design -- he was the original Sunn guy. You can get the schematic online.
  18. paranoid30


    Apr 21, 2014
    Milan, Italy
    Well, I don't know about unique: the EQ on Fender Twin is a tone stack :D

    OK, I see. Sadly I've seen it reported as a fender-style tone stack, it's actually why I asked here for more info!
    Thanks a lot for your help, I can't wait to put my hands back on it and properly use it.

    Ha, that was my only hypothesis: it's passive, while other EQs are active!
    daveman50 likes this.
  19. gregmon79

    gregmon79 I did it for the muff... Supporting Member

    Dec 20, 2012
    Chicago IL
    Exactly. NOT the same as a Fender tone stack. So, unique ;)
  20. Yeah, it seems 3-knob passive tone stacks are referred to generically as Fender or FMV (Fender-Marshall-Vox) but that's just lazy.

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