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Rob Inspired 6G3 - Looking for Feedback:)

Discussion in 'Shock Brother's DIY Amps' started by Sam VanLaningham, Feb 9, 2018.

  1. Sam VanLaningham

    Sam VanLaningham TDPRI Member

    90
    Jan 6, 2018
    Bend Oregon
    Alright, what a revelation it was to find RobRobs website and mods. Thanks Rob if you read this!!!!

    I built a 59 Bassman long ago but have been wanting something new. I got on here, asked about AC10's etc and that led me to become friends with a forumite who steered me to Robs site. Same fella suggested I might like the Down Brownie 6G3, which indeed is a very cool amp. At the same time of investigating the 6G3 I did several of Robs mods to my bassman and its been a simply amazing experience.....in fact its done what I was looking for in a new amp! But I still got the bug and its now actually more intense!

    So now I'm building a Rob Modded 6G3 and I cannot be stopped! Its also Down Brownie inspired, which was also inspired by Rob. So Im hoping that you guys might help me not mess this up too bad since I remain entirely inexperienced having built only one amp.

    So below is a pic of the mods Im shooting for. What Im hoping to do:
    - Normal channel modded to a lead channel
    - channel jumper push pull
    - gain boost
    - 3 way NFB
    - PPIMV
    - Vox Cut

    Im confused about:

    1) How to wire the PPIMV generally but also specifically with the vox cut. Do I separate and/or remove the grid leaks etc?

    2) there are two resistors in the V1B lead channel on the down brownie but Robs bassman lead channel just adds the already existing-on-the-6G3 1.5k to the bassman. So I wonder what that 2nd resistor value should be and I also wonder why its there...not that its a guarantee I'll understand with any depth.

    3) Im seeking out less expensive decent PT's and see that the MOJO756exsp is what Mojotone puts in their 6G3 kit. This is a deluxe PT but with the voltage lowered to 330v. But I see on Hoffman amps website that the MOJO760 for the princeton runs at 330v standard. And its considerably cheaper. Can I use the MOJO760?

    4) any other thoughts on mods that would make this thing as wonderful as I think it can be?

    Thanks so much for your time!

    Sam

    [​IMG]Brownface-Deluxe-6G3-chassis-01 by Sam Van, on Flickr
     

  2. ecoast

    ecoast Tele-Meister

    473
    Jan 6, 2017
    NWNJ
    nothing to add but encouragement

    I built a 5E3+ proluxe; most of the way to 6G3, then came into an ampeg j20, which is a derivitave...


    I am VERY happy with my 5E3, but that 6G3 circuit just does it for me...
     
    Last edited: Feb 11, 2018

  3. Sam VanLaningham

    Sam VanLaningham TDPRI Member

    90
    Jan 6, 2018
    Bend Oregon
    Thanks ecoast. I checked out those 5e3 proluxes after your post and those do sound Suhweet! Thanks for the thoughts.

    Sam
     

  4. dan40

    dan40 Tele-Holic

    923
    Aug 19, 2015
    Richmond Va
    Do not use the PT that Mojo sells for the Champ. While the voltages are the same, the current rating for the Champ PT would be much less than what you will need to power the 6g3.
     
    JuneauMike likes this.

  5. dan40

    dan40 Tele-Holic

    923
    Aug 19, 2015
    Richmond Va

  6. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida

    Your gain boost switch won't work. You put a switchable .68uf bypass cap on gain stage that already has a 25uf bypass cap stock. Robs gain boost mod works by adding a bypass cap on a switch to a gain stage that is unbypassed stock. Adding it to a gain stage that is already bypassed, as you have here, is redundant.

    That being said, the last thing a 6g3 needs in more gain, so you won't miss it...

    I haven't proof read any of the other mods you put on that layout, but I'd suggest stepping back from any others that you don't fully understand.

    P.S. Actually, your cut control is wired wrong too, as you have it on that layout, turning the control would have no effect...
     
    Last edited: Feb 10, 2018

  7. Sam VanLaningham

    Sam VanLaningham TDPRI Member

    90
    Jan 6, 2018
    Bend Oregon
    Thank you dan40! I knew there had to be something amiss or else everyone would be using the less expensive PT....

    And thank you nickfl. I need all the help I can get as I’m a shoot first ask questions later style person. Maybe I should not implement a bunch of rob mods meant for the 5f6a on a 6g3 until I, as you say fully understand.....maybe after a few amps. But it does seem many of the 5f6a mods apply to the 5e3 deluxe and it seems there’s only subtle differences between these amps and the 6g3???

    And yes I now see the redundancy....and I guess this is partly why the 6g3 has enough gain to start with eh?. Maybe I should put a switch there to shut off the bypass cap if anything.....but I don’t really need that.

    As for the vox cut, I’d love to have the later stage tone control..... I would not doubt for a second that I’m missing something fundamental to tube amp wiring. But as I see it in my layout, I have the vox cut wired just as it is meant for the 5f6a. This will not work?

    Again, a million thanks for helping and I hope my noobness combined with blind willingness to mod doesn’t dissuade anybody too much:).

    Sam
     

  8. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida
    You don't have anything connected to the wiper on the cut control pot, so when you turn the control, nothing will happen. You need to jumper between the wiper and one of the side lugs so that it will act as a variable resistor.

    I also notice some issues with the way you have the power tubes wired, lookup a 6v6 pinout and double check your wiring to make sure you have everything connected to the correct pins. You have a 1.5k resistor connecting your plates to your heaters, among other things.

    Also, you need to remove the jumper between the input jacks' switch lug and ground lug if you are going to use the pull pot jumper switch mod.

    Lastly, make sure you remember to delete the original NFB connection from the jack to the board if you are going to use the switchable NFB wiring.

    I realize some of these may just be things you didn't draw in to your layout, but I figured they were all worth mentioning just in case.

    Make sure you build an use a lightbulb limiter if you don't already have one. A current limiter is always a good idea when testing an amp, but especially so in a case like this where you are deviating from the original circuit... even more so since you have some things going on in your layout that could damage your amp if it were actually built to that layout and tested without a limiter.
     

  9. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida
    And... your PPIMV is not setup correctly for fixed bias. You need an extra set of caps between the PPIMV control and the grid leak resistors to keep the master volume from screwing with the bias of your power tubes. Check this out: http://el34world.com/Forum/index.php?topic=15026.0

    Can you read a schematic? If not, now might be a good time to learn, because this is the sort of thing that is best worked out in a schematic and then applied to a layout from there.
     

  10. Sam VanLaningham

    Sam VanLaningham TDPRI Member

    90
    Jan 6, 2018
    Bend Oregon
    Nice nick! Thanks so much. I’ll post more later as I dig deeper into your comments. But I will say that indeed I was being lazy in a few instances, kind of giving the maybe 5,000 ft flyover but I now see that is not ideal when asking for help like this.

    So I’ll redo what I’m think wanting to do and lay it out EXACTLY as I think it should be, except in instances where I might still have confusion.

    Oh and no I don’t really know how to read a schematic but I get a little better every time I stare at them for awhile and link them to layouts....

    More soon!

    Sam
     
    CoyotesGator and Nickfl like this.

  11. Sam VanLaningham

    Sam VanLaningham TDPRI Member

    90
    Jan 6, 2018
    Bend Oregon
    Ok here’s waht I hope is a much clearer view of what I’m hoping to do. After the thoughts guys provided so far I guess now I’m essentially building the down brownie, if you’ve seen or heard it.

    But here’s my layouts. I don’t yet have the skills to get them into schematic form (how quickly might those skills come from building and modding amps - will it come just by experience with hands in the circuits?).

    Ok thanks for any help or thoughts you have!

    Sam

    [​IMG]Brownface-Deluxe-6G3-chassis2-01 by Sam Van, on Flickr


    [​IMG]Brownface-Deluxe-6G3-chassis_blowups-01 by Sam Van, on Flickr
     

  12. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    Here I was pretty much having finally settled on a 5E3 build after several months of indecision, and you have to bring that 6G3 circuit to my wandering attention… and I’ve been keen on amps with trems lately. Now I can’t unsee it, and my 5E3 resolve is wavering... it looks pretty interesting.
     

  13. Sam VanLaningham

    Sam VanLaningham TDPRI Member

    90
    Jan 6, 2018
    Bend Oregon
    Yeah cool RLee.....and though I don’t have a lot of personal experience with the deluxes etc, the Down Brownie suggests that with a NFB installed and switched to NO feedback, that’s a tone in the direction of the 5e3. So maybe you can have your cake and eat it too!

    Check out the down brownie on cutthroat audios website....very enticing!

    https://cutthroataudio.com/product/down-brownie/
     

  14. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    I think it's just the trem that I might pull and use in the 5e3... I would not want the fixed bias or special voicing in my case, as I already have an amp for those sounds.
     
    Sam VanLaningham likes this.

  15. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    This is a very cool project. In the 6G3 V1 A and B share a cathode with a 1.5k resistor which is the equivalent of each one having a 3k ohm resistor so your choice of a 2.7k for one channel will be close to stock but the other channel's bias with a 1.5k resistor will be much warmer than stock which is cool if you're going for the Down Brownie dual channel thing. 1.5k is a normal value for a first stage preamp but if you want to keep it close to 6G3 stock you should use a 2.7k.

    If you want to use a post phase inverter master volume with a dual gang pot you will have to separate the two 220k power tube grid leak resistors. I'm a big fan of the simple Type-3 MV/Cut control which would be super easy to add to your planned layout. Your layout is going to get really complicated with a dual gang pot + cut control + tremolo intensity.

    On your NFB switch, be sure and use the stock impedance for the NFB source (I believe it's 8 ohm) and use the impedance switch input (not output) as the NFB source so the NFB amount doesn't change when you switch speaker impedance.
    [​IMG]
     
    Last edited: Feb 17, 2018

  16. Sam VanLaningham

    Sam VanLaningham TDPRI Member

    90
    Jan 6, 2018
    Bend Oregon
    Rob, thanks so much. So helpful. I knew I read on your site that the two 220k power tube grid leaks needed to be separated but I couldn't see it on the down brownie.....and since I am just going for it here with little actual knowledge, Im shooting in the dark quite a bit!

    I'll contemplate the wiring, re-draft the layout and post.

    Meanwhile I already drilled holes and made my faceplate to have the Frondelli PPIMV and vox cut. I hope I am not going to run into problems!

    Also, I'm building two of these. One with all the fancy boutique caps etc, and one with a bunch of vintage caps I just got from a retiring ham radio repairman....here's that board in its infancy of layout (I did it it awhile ago before getting as fas as I am now with the mods). I did not test for ESR but all the capacitances are to spec......and theres my hand hammered chassis haha...so I can afford sozos and jupiters:)......

    [​IMG]Print by Sam Van, on Flickr

    [​IMG]Untitled by Sam Van, on Flickr
     
    cap47 and robrob like this.

  17. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I recommend you wire the master volume and don't wire up the VOX cut until you get the amp up and running.
     

  18. Sam VanLaningham

    Sam VanLaningham TDPRI Member

    90
    Jan 6, 2018
    Bend Oregon
    OK will do! Thanks a ton.

    Sam
     

  19. Sam VanLaningham

    Sam VanLaningham TDPRI Member

    90
    Jan 6, 2018
    Bend Oregon
    Ok heres the BROWNFACE JTM DELUXE hehe......now to actually wire it?????

    I do not see that the 220k power tube grid leaks separated on the down brownie....meaning it very much appears like they are in the stock setup (connected)??? But it is hard to see clearly in his photos....

    He is using a frondelli right?

    [​IMG]Untitled by Sam Van, on Flickr
     
    Last edited: Feb 18, 2018

  20. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I'm 98% sure the 220k's are connected using one eyelet/turret. It's easier to separate them if they're on a turret.

    I don't think the Down Brownie uses a Frondelli, but I'm not sure. You can use a Trainwreck Type-3 and a separate VOX cut control. It's best when you can add an eyelet or turret to your board before assembly for a Type-2 or Frondelli master volume.
     
    Last edited: Feb 18, 2018

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