18W "D" type project

Discussion in 'Shock Brother's DIY Amps' started by dogmeat, Apr 11, 2019.

  1. dogmeat

    dogmeat Tele-Afflicted

    Age:
    68
    Posts:
    1,063
    Joined:
    Oct 12, 2017
    Location:
    Alaska
    I've been thinking about building a Dumble OD type amp for some time. I have seen schematics on a couple and there are some kits, but I don't see the ODS in the 18 watt range. anyway, what Iv'e done is cut and paste an assortment of diagrams and would like to hear some thoughts on it. the pieces are mostly taken from Hoffman's kit diagrams. I have built his AB763 and 18W Plexi (plus a couple other's designs). in fact, the power section and output posted below are straight out of the Hoffman Plexi, though I expect I will modify that to some extent (a more D-style cap arrangement). the real ODS has more "push button" tone modifiers in the first pre-amp stage. I won't be doing that other than the bright switch shown below (which I might delete anyway). you can probably recongnise that group as standard Fender setup. the pre-amp and tone stack on the OD side is the part I have really bodged together in the cut & paste. I have attempted to make it look like the real ODS as far as the layout of componants. the question is values. what do you see that needs fixing... or is this a fool's errand
     

    Attached Files:

    Last edited: Apr 12, 2019
  2. dogmeat

    dogmeat Tele-Afflicted

    Age:
    68
    Posts:
    1,063
    Joined:
    Oct 12, 2017
    Location:
    Alaska
    heres the D amp I am trying to model, made with EL34s...
     

    Attached Files:

  3. Lynxtrap

    Lynxtrap Tele-Holic

    Posts:
    733
    Joined:
    Feb 29, 2016
    Location:
    EU
    How are you planning to do the tone stack on the OD side? With trim pots like the HRM?
    The unusual thing about these is that both tone stacks are in the circuit when the OD channel is selected.
    If you are planning on making both stacks with big pots on the front panel, it might be worth considering independent tone controls for each channel.
     
    FenderLover likes this.
  4. sds1

    sds1 Tele-Afflicted

    Age:
    43
    Posts:
    1,071
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    Your schematic seems to deviate from your model's schematic quite a bit doesn't it?

    Some quick observations:

    1) Your preamp cathode bypass caps (22uF, 22uF, 22uF, 22uF) are all too high. D-style means using lower values here (10uF, 10uF, 5uF, 1uF). This is a big part of how Dumbles sparkle in clean mode and stay pristine when OD in engaged.

    2) Dumble used local feedback on V1B to drive the effects loop. I believe this in an impedance matching thing but I'm not 100% sure. You might look into it since you added the effects loop, your model doesn't include the effects loop and omits the local feedback. Here is an example:

    [​IMG]

    3) I don't see a coupling capacitor (likely you want .02uF, your model used .01uF) at the phase inverter input

    4) A pre-phase inverter trim (or make it a pre-PI master volume) seems like a good idea, you need some way to balance the PI input from both "channels" as well as the signal coming back in from the effects loop.
     
    dogmeat likes this.
  5. FenderLover

    FenderLover Friend of Leo's

    Posts:
    4,845
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    On the OD stages, you will also sometimes see (like on the Robben Ford annotated schematic) a 250pF cap between plate and cathode. That would be something to try during final tuning after it is up and running.
     
    sds1 likes this.
  6. dogmeat

    dogmeat Tele-Afflicted

    Age:
    68
    Posts:
    1,063
    Joined:
    Oct 12, 2017
    Location:
    Alaska
    thanks guys. so, as you might have guessed, it was never intended to be an exact copy, only a rendition of the concept. get a good clean sound and the ability to switch some of that signal into another preamp boost, with a second tone stack to shape that output.... sounds reasonable to me. as far as making it sound exactly like a Dumble amp... that was not nessicarily the goal. as an example, V1 preamp in my bodge is straight Blackface.

    there will be 2 tone stacks with controls on the face. the OD trim is the only pot on the board. the switch will be a relay on a foot pedal control.

    the clean side has Vol/Master, the OD side has drive and volume pots (mounted on the face). that should be enough to balance the two "channels" right?

    not 100% sure on the FX loop. copied from somewhere. I read that effects in front of the amp do not fare well in the OD side, and it is better to take the OD signal to FX, then to the inverter

    bypass caps... value change. good.

    coupling cap... oooops.... forgot to put one in the OD side. best to put it right on V3 right?

    the 250pFs on plate/cathode... they were part of the plan, I just didn't draw them in. I would also use sheilded wire & follow routing practices, etc as per known examples
     
    sds1 likes this.
  7. sds1

    sds1 Tele-Afflicted

    Age:
    43
    Posts:
    1,071
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    There is no one Dumble anyhow, so many variants. But it's worth pondering the possible importance of some of the things you choose to include/omit.

    Yeah I withdraw my comment with regards to pre-PI trimmer. I guess you rely on the effects to have an output level adjustment (if effects loop is in use). A post-PI master volume is worth considering I reckon. Otherwise I think you have your bases covered -- a gain adjustment prior to every gain stage.

    I think you put it in the correct spot, it just might be worth looking into that local feedback that Dumble used and see if you wanted to include it, or not. It may be there to more closely mimic the output impedance of a guitar, which guitar effects pedals are designed for. I could be totally wrong.

    Yes it should be directly on the V3A grid.
     
    dogmeat likes this.
  8. dogmeat

    dogmeat Tele-Afflicted

    Age:
    68
    Posts:
    1,063
    Joined:
    Oct 12, 2017
    Location:
    Alaska
    really the only effect I am sure I want to use is reverb. maybe echo, slapback, etc..... things that are time/color related. no boost pedals or distortion... shouldn't need that.

    heres where I got the FX loop from... another version of the ODS with no tone stack in the OD section. note that he put the master volume on the input side of the PI, and there is no MV on the clean channel. that seems like it could be limiting to me, but I could be wrong. I have done a few amps with post PI master & would probably go that way if I wanted a true Master, but that may just be one more knob to complicate the thing. any Master downstream of the channel select would probably make the MV I have at V1 kinda pointless ODS 183 Schematic.jpg
     
  9. Lynxtrap

    Lynxtrap Tele-Holic

    Posts:
    733
    Joined:
    Feb 29, 2016
    Location:
    EU
    I guess I was trying, in a polite way, to ask if you are aware that both tonstacks are in the circuit in OD mode.
    The idea behind it was presumably that you would "set and forget" the OD tonestack, just like the OD trim.

    But if you have both on the front, there is the possibility to make each channel have its own independent tonestack.
    Not that this wouldn't work, but independent EQ on both channels might be even better.

    In the #183 schematic in the post above, the master volume works on both channels, as you'll see if you take a closer look at the switch.

    The OD channel has its own gain and volume control, often called "level" and "ratio". You set the amount of overdrive with level and balance the OD channel's volume to that of the clean channel.

    IMO and ME you don't really need a post PI master in these.
     
  10. dogmeat

    dogmeat Tele-Afflicted

    Age:
    68
    Posts:
    1,063
    Joined:
    Oct 12, 2017
    Location:
    Alaska
    yep, I was aware that the tone stacks are series in OD. there is no tone stack in 183s OD, so I chose the other setup. also aware of having the master in 183 work both channels at the same time, at least to some effect.... would it be balanced? don't know, thats why I was thinking the independant Vol and Master on each side would be more versitile for setting levels. maybe more fiddly but you could dial it right in. obviousely it would have to change for every room you play in

    edit: there other versions, but they mostly seem to look like the 183

    still pondering....
     
    Last edited: Apr 13, 2019
  11. FenderLover

    FenderLover Friend of Leo's

    Posts:
    4,845
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    I certainly wouldn't want to change your mind because holding true to the D-type structure looks pretty intentional in your plan. Just to add to the discussion, I lean towards what Lynxtrap suggests about separating Clean and OD with separate EQ. What I have been considering is to drive the clean and OD sides in parallel after a shared first gain stage. The relay then selects which output is active and there is little compromise to how the D-type circuits operate, just configured a little differently. Not that the controls on an ODS are too hard to figure out, but less interaction and dependency seem more user friendly.

    Just food for thought, there are a million ways to draw these things.
     
    dogmeat and sds1 like this.
  12. FenderLover

    FenderLover Friend of Leo's

    Posts:
    4,845
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    OK, so my coffee is still warm....just thought of another thing the ODS could improve on....

    The input of the OD section has the OD Trim, sometimes internal, sometimes mounted on the back panel. Look at the annotated schematic of the Robben Ford ODS and realize how low it is set. Not only that, but how meaningless the setting is if the Clean channel volume is turned up. The first OD stage would be swamped in that case, and the only control over OD is the input to the second stage. Compromised functionality.

    By splitting the two channels and feeding the OD directly from the first gain stage (Pre-volume) the OD Trim level is far easier to negotiate and is more functional, especially in a set-and-forget kind of way.

    When people add a Dwell control to their reverb, it is usually not well received because a high dwell setting after a low volume setting is not functional. A reverb dwell is best taken from the input side of the volume control so that the dwell control is completely functional at any volume setting. The same principle applies to the OD Trim control.
     
    Last edited: Apr 13, 2019
    sds1 likes this.
  13. sds1

    sds1 Tele-Afflicted

    Age:
    43
    Posts:
    1,071
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    I think the OD's tonestack is advantageous, a lot of harmonics are being added to the clean signal input. Of course you could always leave it set to 10-10-10 if you found it useless, but I reckon everyone would use it if they had it.

    If you're looking to clean up the front panel, I'd keep it over the Presence control, that's for sure.

    Well, the position of the OD Trim is just a user thing right? We can adjust the voltage divider going into the first OD stage if you think it's more intuitive to see OD Trim set at 50% for the same output voltage.

    But the design intent is to set up the clean channel quite clean, and then feed OD just a very small signal. The design allows the Drive adjustment to get you from a tiny bit of OD to a lot, with plenty signal to drive the PI properly and so on. So I'm not sure I see the compromised functionality here. Obviously, in any amp, when a previous stage gain is turned up this will have a cascading effect. But there is enough play in that OD Trim to feed a slightly-overdriven clean channel into the OD with all the versatility you need, that is my opinion.
     
    dogmeat and FenderLover like this.
  14. FenderLover

    FenderLover Friend of Leo's

    Posts:
    4,845
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    The local NFB decreases the output impedance, and input impedance, but in this case not too much. Merlin covers the math for this circuit real well in his book, and if you plug in the numbers the feedback is on the order of only a couple of dB.

    A more advantageous function of feedback is that the input headroom is increased by the same amount of feedback. Once the headroom is 'used up" the gain of the stage reverts to the open loop gain, as if the feedback weren't there. In this case of adding only a couple of dB to the headroom the effect is small and not even noticed by some people. A couple of dB in dynamic range with a stringed instrument will not swing the response much either way. After all, the stage would overload with open loop gain anyway, but just a few dB earlier. But there is a difference in how sharply it breaks over to its open loop gain. To hear the effect and tune to you liking, change the 40M substantially, to 10M or even 4.7M and it's function becomes more obvious.

    I completely respect that, but my example in the last paragraph shows better why the trim can be set and forget instead of needing to be functionally adjusted because the input volume changes. But yeah, I get it. The ODS is a cool amp for sure, but it's not really the Telecaster of the amp world IMO. There are many things that could be optimized, but one may not find the changes any more useful.
     
  15. dogmeat

    dogmeat Tele-Afflicted

    Age:
    68
    Posts:
    1,063
    Joined:
    Oct 12, 2017
    Location:
    Alaska
    "By splitting the two channels and feeding the OD directly from the first gain stage (Pre-volume) the OD Trim level is far easier to negotiate and is more functional, especially in a set-and-forget kind of way."

    I could swear there is a version like that but I can't find it. I posted something about that idea in my post #10 but edited it out after not being able to locate a schematic. I think its a good plan. the intent from the beginning was to have the 2 tone stacks, with controls on the face of the amp

    I recently finished a Hoffman Plexi. I chose that project because it seemed pretty close to that idea already. I had intended to build it along that line, but I wasn't that sure about certain changes and ended up just making it pretty much to the plan. looking back, I should have probably taken the leap, but then the Plexi is a great sounding little amp, so no regrets. I was going to post a report on that build but somehow most of the pix I took dissapeared. I had shots of making the chassis, doing the silk screen graphics on the face, building and covering the cabnet, etc... gone.. dunno. the speaker box I built a couple years ago to use with a Vox Night Train. here a pic..

    P1100985.JPG
    View attachment 600650
    P1100963.JPG
     
    Last edited: Apr 13, 2019
    sds1 and FenderLover like this.
  16. FenderLover

    FenderLover Friend of Leo's

    Posts:
    4,845
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    The Fender Prosonic does something similar regarding the split of channels after a common first gain stage, then relay selection and non-ODS features after that.
     
    dogmeat likes this.
  17. Lynxtrap

    Lynxtrap Tele-Holic

    Posts:
    733
    Joined:
    Feb 29, 2016
    Location:
    EU
    While you certainly can have a master volume for each channel, it is not necessary. If you look at the #183 schematic, the "OD Volume" pot is really the master for the OD channel. There is nothing between that and the master, so it's strictly there to balance the OD volume against the clean channel volume. With the channel switch placed where it is, this is as close as you can get to master volumes on each channel.

    The disadvantage of the whole Dumble setup is that the channels will never be independent of each other, as the OD channel uses the whole of the clean channel. It's part of the design, though, and not many people seem to have a problem with it. The OD tone stack no doubt offers more tweakability than the 183 setup.

    You might want to look at a confirmed schematic of a HRM model. The one in your #2 post is the one on schematicheaven, and it is supposedly kind of generic as opposed to based on a specific amp.
     
    Last edited: Apr 14, 2019
    sds1 likes this.
  18. sds1

    sds1 Tele-Afflicted

    Age:
    43
    Posts:
    1,071
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    I like the idea of setting up my clean sound, then having an overdrive that goes on top of that (as opposed to overdrive pedal driving the front of the amp). But I'm just looking for a single preamp setup, and then an overdriven version of that. I guess if you're looking for the versatility of 2 totally different sounds out of each channel, then the cascading overdrive of a D-style wouldn't be your bag. I see the tone stack in the overdrive as a fix-it type control only, not to achieve a completely different EQ from the clean channel.

    It's really not even fair to use the term "channel" here IMO. The OD is either engaged, or not.
     
    dogmeat likes this.
  19. FenderLover

    FenderLover Friend of Leo's

    Posts:
    4,845
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    That "channel" became identified in 1970 by Mesa Boogie as Dual Mode Preamp Architecture in U.S. Patent 4,211,893. I don't know who is the chicken or who is the egg, but Mesa won the foot race to the patent office.
     
    sds1 likes this.
  20. L.A. Mike

    L.A. Mike Tele-Afflicted

    Posts:
    1,030
    Joined:
    Mar 1, 2014
    Location:
    somewhere over a rainbow
    I'm not a fan of Ceriatone amps, personal reasons better not discussed here.
    But, the owner Nik has no problem copying and selling other peoples amp designs (even amps currently in production).
    He makes what he claims in a ODS clone.
    I don't know how he came up with this design for a 6V6 ODS 20 watt amp. Or even IF he was the one who came up with it.
    Maybe you might find them interesting.
    I don't build amps so I don't know how accurate or well designed they are.

    https://www.google.com/search?tbm=i...XGzlkKHaLZA1YQ4lYINCgN&biw=1600&bih=760&dpr=1
    https://www.google.com/search?tbm=i...XGzlkKHaLZA1YQ4lYINCgN&biw=1600&bih=760&dpr=1
     
    Last edited: Apr 14, 2019
    dogmeat likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.