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

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

  1. sds1

    sds1 Tele-Afflicted

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    Hang in there Kley! I'm sure you'll figure it out.

    I could hear the little ghost note thingie in the first video this time. Great news is that it seems super reproducible for you.

    A couple of quick thoughts as I pass thru:

    1) I don't think you need to worry about the tone stack/yellow stuff. It's all in phase but not strong signals, it's common for all the tone stack signal to intermingle without issue.

    2) @Lynxtrap makes a good point about filtering/coupling, those UCC electrolytics are ample and brand new so I wouldn't expect problems, but just wanted to verify you are following the D-schematic exactly for the power supply as well as which nodes feed which triodes? The filter caps also decouple AC, if you were to start sharing power nodes across the wrong stages you are subjected to yucky interactions.

    3) Are you reproducing this issue on the clean channel or OD ?

    4) Are you using a speaker here? If so, the speaker could be the problem, I'd try to rule it out. Had this problem recently.

    5) I'd roll preamp tubes before getting to far into rewiring. Not sure if you had done this yet but I know you mentioned it.

    6) No nukes!! You'll get it.
     
  2. kleydejong

    kleydejong Tele-Holic

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    HOT DANG!!! I was reviewing my filter supply as suggested. It appears to me that my B+2 cap may be BACKWARDS!!! I can't tell 100% from the picture, but it looks like there is no grey strip going to the ground terminal.

    [​IMG]

    If this works I will be a HAPPY CAMPER!!!

    Thanks a lot guys, I appreciate your help!
     
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  3. sds1

    sds1 Tele-Afflicted

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    Yeah that sure looks backwards from here too, Kley. Nice catch.
     
  4. kleydejong

    kleydejong Tele-Holic

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    I spoke too soon. Fooled by a flair of light on that photo. Checked them when I got home and it appears to be wired correctly...

    [​IMG]

    These are $1 27uf at 450v caps from United Chemi Con. I openly admit that they are budget oriented. I finished one amp with them successfully, but I suspect them as well. Some google searching on ghosting indicates checking these as well. I will have to take them out of circuit to test with my $15 amazon parts tester thingy.

    https://www.mouser.com/ProductDetail/661-EKXL451ELL270MK2
     
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  5. kleydejong

    kleydejong Tele-Holic

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    As I fall on my face I just as quickly get right back up! I believe I fixed it! Two issues:

    1 - My B+5 cap leg was not strongly connected to its terminal strip. I went to remove it and found that it seemed to be lightly stuck by solder to the face of the terminal strip, but it did not poke through the hole and did not seem to be securely connected. I pulled it out and measured - it checked out fine. Reattached. I believe the cause was when I reinstalled the yellow B+5 wire running clear across to V1 I pushed this lead from the other side out of the hole.

    2 - My bias cap was bad. It is coming out of its enclosure. I put it on a meter and it measured quite low - at like 16 uf where it should be 47 uf. I believe this contributed to my weird bias issues.

    I fixed these two issues and did a test and did not sense any ghosting! My kids were asleep so it was through a load box and into headphones, so it was not a very good test. I will 100% confirm tomorrow - but it felt A LOT better. Much lower noise as well.

    Thank you all very much for your help!
     
  6. kleydejong

    kleydejong Tele-Holic

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    [​IMG]

    [​IMG]

    I also took new bias measurements. I'm getting 486vdc on the plates. I set the cathode to 22.5 mv.

    My B+ step down is:

    1 - 478
    2 - 470
    3 - 438
    4 - 418
    5 - 409

    V1 sits at 262vdc and 293vdc on the plates.

    I can't wait to let her rip tomorrow through a speaker in the room!
     
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  7. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Glad it was solved!
    With such high voltages I would want minimum 500V filter caps. You are already exceeding the rating of the ones you have - at idle.
     
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  8. kleydejong

    kleydejong Tele-Holic

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    I just finished my first tone demo on a speaker in the room and it was quite glorious! A really fantastic amplifier. Very fun to play.

    Hmm, yes that is concerning. Also I noticed the weakness in my power supply design as a whole when I cranked the gain and OD drive to max. The amp really couldn't handle it. That sag you get from a 5E3 combined with a super high gain preamp just doesn't work great. I probably won't use it at that level, but I think beefing up the power supply with stiffer capacitors + a choke is something that I can now feel the need for.

    That said at more of a medium gain level the amp was a BLAST to play. The softer power supply makes it a little more forgiving.

    I will continue to tweak, but I'm thrilled to have fixed the ghosting!
     
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  9. sds1

    sds1 Tele-Afflicted

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    If you are redoing your ecaps anyhow, that 100uF-220uF at B+ feels really good, one of the most important D-features IMO. The hard part is getting a 500V rating, those are more found in the snap-in varieties. You may as well resign yourself to stacking a couple of 330uF's (or whatever) in order to get a nice ratings at B+ and the screen node. Those UCC's are totally fine, don't feel bad for using them. ;)

    Also I'm just guessing but I reckon it's those 6V6's that are crapping out on you, they get real smooshy... the bigger big bottles (6L6 EL34 etc) are the only ones that hold up in a pleasing way IMO.

    In general the best sounds out of that amp are going to be when no one stage is pushed too hard. I like to set the cleans first, say 9-2 o'clock on the preamp, to taste depending on how much grit you want on your cleans, and around noonish on the master volume so that the power tubes are just getting warm and fat but not too far. Then any heavy overdrive you need can just come from the OD side of the preamp.

    I dunno if you installed the OD trimmer or not, but you don't want to push that one too far either.

    I try to listen for fizzy artifacts and try to stay short of those, preamp tubes get fizzy when they are pushed too hard. Or sometimes swapping tubes around can help you dodge some fizz as well.

    It's fun to coax nice sounds out of these amps, congrats and enjoy!
     
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  10. kleydejong

    kleydejong Tele-Holic

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    A little preview clip from this afternoon:



    The pink tele has a grounding issue so ignore the noise it is generating.

    I haven't installed the clean channel switch yet, so it is just the drive channel hard wired. I can actually get a really nice clean tone by keeping the input gain and the drive channel 'level' control at about 9:00 and cranking the master all the way up.

    I totally agree that the sweet spot is from about 11:00 to 1:00 on both drive controls. With the drives maxed out it gets real gnarly.

    I am really excited by the rock / jazz switch. It really changes the tonal character in a good way. Jazz mode with the gain bumped up a bit is a really nice tone that I had overlooked.

    I'm very happy to be at this point. Again, you guys have been massively helpful! Thanks!
     
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  11. sds1

    sds1 Tele-Afflicted

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    Dang that sounds good.
     
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  12. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Sounds good on the clip!
    Cranking preamp gain should not load down the power section as long as you keep the master volume on reasonable levels.
    But yes, a power amp that keep things tight is a key feature of these circuits.

    Sorry if I haven't read the whole thread, but you have a 5E3 power amp with 6V6's? At 478V, you are kind of pushing your luck with the tubes and caps...
     
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  13. jmp81sc

    jmp81sc Tele-Meister

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    Amp sounds great. Worthy project
     
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  14. Telekarster

    Telekarster Tele-Afflicted

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    Oh man... this thread is getting quite tasty indeed.... ;) Congrats on the success!!! Even through my crappy laptop speakers its sounds great already! Very awesome!!!
     
  15. kleydejong

    kleydejong Tele-Holic

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    Finishing touches incoming.

    First, I have beefed up the power supply on the first B+ node. My voltage is at about 498vdc and my first filter cap is 27uf at 450v. I upped this to two 150uf at 350v caps in series with 220k resistors producing 75uf at 700v. I did a play test briefly after this and can definitely feel a difference in the response and immediacy of the attack. This was a needed and nice change.

    Second, I am yolo'ing a simple relay switch for clean / od channel switching. I believe I will quickly look back and wonder why I saw this as such a roadblock. But I believe I have a direction:

    [​IMG]

    I'm using the 5vac winding on the PT meant for a tube rectifier filament. It goes through a full wave bridge rectifier. It is filtered via a filter cap. It sends 5vdc to the relay. There is a diode to protect the relay. The relay is a DPDT switch. When the voltage circuit is completed it pulls the switch to one contact and when it is not complete it pushes to the other contact. The relay sends a ground signal out. This ground connection gets interrupted by using either the footswitch pedal and / or a front panel switch.

    Hoffman sells a relay switch kit of sorts - https://hoffmanamps.com/MyStore/per... and Power supply parts.htm&ORDER_ID=!ORDERID!

    [​IMG]

    This was convenient for me. It is designed for the 6.3vac filament winding, so I am changing it a bit for my purpose. I simply use the bridge rectifier and the filter cap. I send the + of the filter cap out to the relay. I am not using the voltage regulator.

    Mounted and wired in.

    [​IMG]

    As for the circuit connections, I am taking the signal exiting the plate of the second gain stage - through a coupling cap - then into the relay switch on pin 13. The switch will either throw to pin 9 (which sends out to the OD channel) or to pin 11 (which sends out to the input of the master volume.

    The only piece of the puzzle I am not really confident in is the switching grounding. I send the ground out of the relay to the tip of a 1/4 jack and to a dpdt switch on the front panel. The panel will either connect to ground or lift. The footswitch sleeve is connected to ground as well. The footswitch will go 1/4 out to a simple pedal style 3PDT switch that again will either connect the tip to the grounded sleeve or not. Not sure if that's correct...

    [​IMG]

    I haven't tested any of this yet. We'll see tomorrow!
     
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  16. sds1

    sds1 Tele-Afflicted

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    Kley, the power supply side is floating, ideally the entire DC side is floating (you would need to isolate the footswitch jack from chassis to accomplish this):

    upload_2021-5-5_6-49-39.png

    But this would work too (but never do this if PT secondary is center-tapped):
    upload_2021-5-5_6-50-12.png
     
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  17. kleydejong

    kleydejong Tele-Holic

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    I chose option #2 above in grounding the filter cap. I tested it tonight and the front panel switch works great!

    One issue. I noticed the master and the drive channel output volume (level) are linked. This defeats the purpose of the level control to set the drive channel volume relative to the clean channel.

    Looking at the original dumble schematics it appears they use the relay to switch in the input and output of the drive channel. Some options as I see it.

    I could send the output of the drive channel level control back to the relay and then back again to the master. I'd need to use shielded cable. It would be a lot of signal running across the amp and I don't really like this.

    I am wondering if I could take the output of the level volume control. Send a line to the relay. One end is blank (leaving the drive channel on) . One end is grounded (shunting the output). Then instead of running that level control directly to the master, can I insert something to decouple them? Maybe a .02uf coupling cap?

    I could also consider trying to make some fairly radical relocation on circuitry. Like moving the relay stuff to the bottom / right area near the pilot light. Eh, I don't like this idea much.

    I could depart from dumble design and have a clean master and a drive master be separate.

    What would you do?
     
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  18. kleydejong

    kleydejong Tele-Holic

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    [​IMG]



    For visual. I currently have one long shielded cable running from the relay to the master (orange). I suppose I could try it and see if it adds noise. It kinda grates on the PTP wiring guy in me...
     
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  19. sds1

    sds1 Tele-Afflicted

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    Not sure if I quite 100% follow the entire thought process, but if this is what it takes to match the original circuit I would choose to do this, and not worry about the long signal run -- that's what the shield is for.

    If you look at the Dumble layouts you'll see you had to make these shielded runs anyhow, with an optimized layout (relays between board and chassis, closer to pots) you would maybe save a couple inches of RG-174 each? So I don't think you're in odd territory here.
     
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  20. kleydejong

    kleydejong Tele-Holic

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    I am calling it done! It is going to find a new home that will do a tolex covering for the head. But other than that she's ready to roll!

    [​IMG]

    [​IMG]

    Schematic:

    [​IMG]

    I played one gig with this amp as well and it performed great! I find the mid focused drive tones to be sublime. The Rock / Jazz switch is a really nice EQ tool. Rock mode is very classic and familiar. Jazz mode cuts the gain some, but it really adds a whole different feel. I love the capability for flatter midrange that really cuts. My gig is pretty loud, and the jazz mode with the mids boosted had NO issues cutting through.

    The clean channel is also excellent. It is a little firmer and stiffer than an AB763 Fender that I'm used to. But that's not necessarily a bad thing. It takes pedals great.

    I will be building another Dumble. They're the real deal!
     
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