Transmutation; AO-35 to Carmen Ghia

Discussion in 'Shock Brother's DIY Amps' started by magic smoke, Nov 18, 2017.

  1. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    Greetings Shock Brothers,
    As a tireless traveler of the never-ending journey known as Tone Quest, I present another of my experiments for your amusement or horror; Transmutation of AO-35 into Carmen Ghia.

    This project has been done ad nauseam by a previous generation of builders with the result of a serious depletion of mighty Hammond Organs. Please DO NOT sacrifice a serviceable Hammond for this chassis. There are so many guitar amps in this world, and these organs, (not a renewable resource) are far more valuable in their complete state than any resulting guitar amp. To strip these from functioning Hammonds is akin to killing an endangered animal for a singular body-part. Furthermore, the transformers needed are readily available from current production, and a clean build is much easier and less cramped if you start from scratch.

    With that being said, please allow me to justify my experiment.

    1. I was given this AO-35. The organ is long gone, I did not add any Dough-Re-Mi to the cottage industry of e-bay Hammond killers.

    2. I have embarked with intent of discovering the insights of our dear DR. by following in his footsteps. By converting in a manner one might assume the young DR. took, (initial conversion first followed by methodical modding,) I hope to further my own understanding of the principles of Mystical Tone contained within this circuit.
     
    Snfoilhat likes this.
  2. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    First steps begin with research. Here are the schematics for the original AO-35 and the Phoenix interpretation of the Ghia.
    AO35.JPG 6687437273_009cc07a21_b.jpg
    Next clean the mouse debris and tone wheel oil from the chassis. This one came out quite nicely.
    IMG-0531.JPG IMG-0532.JPG


    I have learned to take detail shots of the interior before tearing anything out. Pictures have a much better memory than I.
    IMG-0533.JPG
     
  3. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    Carefully layout the additions and install some holes! I want this one to be as close to the first ghias possible, but there is scarce info to be found online. Working from a lone photo, I laid out the holes to fit around the transformers and existing voids.
    IMG-0534.JPG

    Note: Cheap stepper bits dull rapidly. ChinaFreight just had a sale and the new bit in the photo cut immensely better than it’s worn out predecessor.

    GhiaChassisFront.jpg (Is this the real deal original ghia or a clone?)
     
    FenderLover likes this.
  4. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    Install the mains section. Observe modern safety practices; Hot to fuse, fuse to switch, extra length on the safety ground and a dedicated safety ground only point. I crimp my terminal rings first and then solder the crimp joint. I prefer to use steel rivets to secure my ground connections to the chassis, and lately I’ve been putting a star washer between chassis and terminal ring. This may seem a bit like a “belt + suspenders” approach, but these ground points don’t budge.
    IMG-0537.JPG
     
    King Fan likes this.
  5. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    Wire up Leo’s HI/LO input jacks. This part always makes me scratch my head and double check the schematic against a layout drawing. It’s an elegant design conceived by someone cleverer than me. “Standing on the shoulders of giants”
    IMG-0539.JPG
    This amp is a veritable mess of ground loops and lead dress no-no’s which should be avoided in any modern design, but when working on these conversions, I prefer to leave that stuff as-is until after initial power-up. Honestly, I seldom feel the noise floor is enough to justify a re-work of the original layout.
     
  6. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    A closer look at the Hammond board.
    IMG-0540.JPG

    New electrolytics installed.
    IMG-0541.JPG

    Wired in the volume and tone circuit. I somehow overlooked these components when placing the parts order. A quick trip to my in-shop junkyard provided the resistors and ceramic cap.
    IMG-0542.JPG
     
    Piggy Stu likes this.
  7. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    Wired up the speaker out, and installed a switch for the NFB. Installed a new filter section in the power supply, with beefed up values (50, 50, 22uf!!!)

    Fired it up and it sounds like crap! Quiet, and a really sputtery bit crush type of fuzz sound. Closer inspection revealed a stray grey ground wire from the PI bias circuit floating loose. After connecting it to ground the amp came to life!


    IMG-0546.JPG
     
    FenderLover likes this.
  8. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    Sorry to leave you hanging, but it’s time to head to my shift at the brewery. Life can so hard! ;) I’ll be back in the next few days to discuss my findings and subsequent mods.
     
  9. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    *Clarification I’m not trying to dog on any of the organ amp converters of the past, just trying to dissuade any future carnage.
     
  10. kleydejong

    kleydejong Tele-Holic

    Posts:
    850
    Joined:
    Aug 30, 2010
    Location:
    Orange City, IA
    Cool project! I have two Hammond conversions that are fine amps. Gigging one this weekend in fact. Looks excellent!
     
  11. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    I’m back with some updates. When I last posted, the major parts of conversion had been completed. As stated earlier, my intention was to follow in the footsteps of Dr. Z. I got the amp to a point where it could be fired up with most of the original circuit intact and had a chance to play through it at full tilt for a few hours. The Hammond circuit in original state was a little underwhelming for guitar. It was a bit anemic and there was not very much distortion even at full volume and with the NFB removed. The amp reminded me a bit of an engine with a dirty air filter, no horsepower and struggling for O2. I have built a few 18watters, so I did have a baseline for performance from similar tube and transformers.
     
  12. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    The next steps were to go through and re-work the first two gain stages to raise voltages to V1. The Phoenix schematic proved quite useful, and it was a fun exercise comparing the two schematics and then identifying the components in real world layout. I really enjoy a good puzzle, and this was a bit like Where’s Waldo + Maze + Operation. With most of V1 reworked and a few component changes on the board, it was time to flip the switch. Raising the gain created some crazy parasitic oscillation, we’re talking dog whistle city. Replacing the long wires to the pots with shielded wire solved that issue.
    IMG-0550.JPG IMG-0551.JPG
     
  13. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    This little cap discharge tool is my new favorite! I made it from a broken test lead, a 100k resistor inside the red handle and a bit of wire with an alligator clip on the end. Now that I have it, I'm not sure how I survived so long without! IMG-0549.JPG
     
    Modman68 likes this.
  14. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    This thing finally sounds like a real guitar amp! It’s no wonder that this design has gone on to become a modern classic. I especially like the way the tone control works, it makes for a very useful tweak when switching between pickups and instruments. I tested it out at full tilt for 3 hours with a band and it was totally in the Zone. It stood up to a loud drummer, a 350w bass rig and a guitar player with a Mesa MKIII. Obviously it was not quite as loud as the Mesa, but in a club setting with a full sound system, this is plenty loud enough to piss off any engineer.

    I’d advise anyone interested in building one of these to go for it. You’ll save a lot of trouble and headache by building from scratch in a new chassis, and you’ll also have the advantage of planning a less chaotic layout and twisted pair filament wiring. Thank you to the good Dr. Z for being a bad mother f*er. Thank you to the people responsible for the Phoenix schematic, and thank you to all of the members of TDPRI who have helped me get to this point.
     
    Wally and Digital Larry like this.
  15. D'tar

    D'tar Friend of Leo's

    Posts:
    3,424
    Joined:
    Jan 11, 2013
    Location:
    WNY
    Soo.... New air filter.... Twin turbos..... And NOS injection!!!!

    Sweet! Congrats
     
    magic smoke likes this.
  16. ArcticWhite

    ArcticWhite Tele-Afflicted

    Posts:
    1,395
    Joined:
    Apr 14, 2009
    Location:
    Portland Oregon
    Using this thread, and another one started by milocj, I have nearly completed my own AO-35 conversion - which I have dubbed the Gila Monster. (Get it? Ghia > Gila.)
    One question for the board: when using shielded wire for the long runs to pots and input jacks, should I be grounding both ends or just one end of the cable sheath? And why?
     
    Last edited: Apr 6, 2018
  17. FenderLover

    FenderLover Poster Extraordinaire

    Posts:
    5,966
    Joined:
    Jun 11, 2009
    Location:
    Minnesota
    Grounding at both ends of a shield creates a ground loop and may cause hum. Check out figure 15.3.

    http://valvewizard.co.uk/Grounding.pdf
     
    ArcticWhite and magic smoke like this.
  18. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    I ended up putting insulation around my shielded cables to prevent them from shorting out other components.
     
    ArcticWhite and Snfoilhat like this.
  19. ArcticWhite

    ArcticWhite Tele-Afflicted

    Posts:
    1,395
    Joined:
    Apr 14, 2009
    Location:
    Portland Oregon
    Good idea. I wondered about that. I was lucky enough to have some insulated shielded cables from an organ pull.
    We'll see how it works!
    This thread was very helpful BTW. The pictures of the pot wiring in particular. Doing that bit from the schematic was ... um... difficult.
     
  20. magic smoke

    magic smoke Tele-Meister

    Posts:
    289
    Joined:
    May 28, 2017
    Location:
    Quantum Entanglement
    Glad to help. I’ve been gigging with this since I finished it. I’m getting ready to shoot some demo vids and I’ll post them soon. Post some pics of your build and let us know how it sounds!
     
    Wally likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.