JCM800 6V6 variant

Discussion in 'Shock Brother's DIY Amps' started by Tom Kamphuys, Jun 1, 2020.

  1. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    I've been thinking quite some time about building another amp. Now is the time!

    I'm planning on building @robrob 's JCM800 6V6. I did make some changes to the circuit:
    - Merlin Blencowe input circuit
    - Valvewizard bias circuit
    - diode protection for the cathode follower
    - some grid stoppers
    - snubbing network after transformer
    - Changed some filter cap values

    Standing on the shoulders of giants:

    MP2104_Master_Volume_6V6_Schematic.png

    These changes are nothing compared to what I did to the layout:
    - Grounding as tought by Blencowe
    - No can cap, but separate capacitors (result of first point)
    - Component types and (physical) sizes changed
    - JTM45 sized chassis
    - Moved on/of switch to the back to keep the 230V section cleaner
    - Pick up the pilot light supply from the humdinger pot to keep the high (filament) current more localized.

    MP2104_Master_Volume_6V6_Layout.png

    It took me ages. If only I could just copy something... But luckily my script to compare layout and schematic made sure any errors I made in the changing of the layout were catched.

    meld.png

    The script needs some tweaking for transformers (L) and tube sockets (V). Circuit equility currently only checks 'nets'. I plan to also check component values.

    I'm doing this extensive planning because I would like to build this amplifier from scratch. After my first (semi) kit build I thought it was time for the real deal.

    Any advise is welcome. If you find any mistakes, please let me know. I have the tendency to be 'penny wise' and might be 'pound foolish'; if that's the case, please let me know.

    I posted png's here, but I could also post pdf's and diy files on request.

    Edit: Changed micro to 6V6 as indicated by @Lowerleftcoast
     
    Last edited: Jun 1, 2020
  2. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

    Posts:
    1,406
    Joined:
    Dec 30, 2019
    Location:
    california
    I looked at the schematic, didn't get to the layout. I didn't find anything wrong.

    Are the 6V6 staying or will it be lower wattage tubes?

    The 0.47uF cathode cap is small. Seems like it will be a very bright amp.
     
    Last edited: Jun 1, 2020
    Tom Kamphuys likes this.
  3. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    Thanks for checking @Lowerleftcoast. I edited the title and first post to reflect that it will be the 6V6 version.

    Practical experience is lacking most; I'll buy both values.

    cathodeBypassBode.png
     
  4. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

    Posts:
    1,406
    Joined:
    Dec 30, 2019
    Location:
    california
    I have tried 0.68 and the amp ended up too brittle for me. IMO speaker choice comes into play. The old greenbacks seemed to cut highs and work well with the Marshall circuit. Chances are the Jensen I had worked against me.

    There are some guitar lead parts that really scream though. That is where bright amps seem to work well.
     
    Last edited: Jun 1, 2020
  5. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    First question:

    I've used two diodes in series to protect the amp from a shorting diode; the other one will still do it's job. This is a 'tweak' that is widely spread on the internet.
    Another option seems to be (a la Valvewizard) to fuse (2x) the AC between the transformer and the diodes.

    What are the pros and cons of each option? Which one do you prefer?
     
  6. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

    Posts:
    1,406
    Joined:
    Dec 30, 2019
    Location:
    california
    The solid state rectifier on several of the Marshall offerings use two pair of diodes in series. The Fender bassman uses a pair of three diodes in series. The two pair of diodes probably would be just fine. I would spend the few cents and upgrade unless the project was a perfect Marshall clone. (I have had a problem with diodes so that may sway my opinion.)

    Putting in fuses to protect the PT is a fine idea. I serviced an amp that zorched the PT. A board mounting screw came loose and lodged between the chassis and the 6V6 socket. Fuses could have saved that PT.

    I don't usually install them due to originality and aesthetics. It is a good idea though.
     
  7. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

    Posts:
    1,406
    Joined:
    Dec 30, 2019
    Location:
    california
    Fender installs fuses in the heater lines in the 57 Tweed and a few others.
     
  8. LetItGrowTone

    LetItGrowTone Tele-Meister

    Posts:
    222
    Joined:
    Apr 22, 2019
    Location:
    U.S.
    That .47 was one of those things I did either accidentally or because I didn't have a .68, that turned out being a *permanent* change.

    [My other 2 cents: no 470pf caps and no bright cap in the preamp, 56K and 250pf in the tone stack. Ultimately preferred 6L6s over EL34s, so I think your 6V6s might sound great. Good luck.]
     
  9. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    I'll take your advice. I'll experiment with the caps.

    In the mean time I've been using LibreCAD to draw my chassis. The user interface has a steep learning curve... The snap-to options are nice, but everything else feels way to difficult. Here's my progress so far.

    JCM800_6V6.png

    Any of you guys use other software that works easier? Or do I just have to get used to this?
     
  10. sds1

    sds1 Tele-Afflicted

    Age:
    43
    Posts:
    1,430
    Joined:
    May 4, 2017
    Location:
    Orlando, FL, USA
    I do everything 2D in Inkscape. Chassis design, chassis layout, faceplate design, etc.

    I tried all the free 2D CAD programs and really wanted to like them. In the end Inkscape won it for me.
     
    Tom Kamphuys likes this.
  11. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    I was able to save to SVG in LibreCAD and import that into Inkscape. So no work lost. I like Inkscape a lot more. Way more intuitive.

    Result so far:

    chassis.png

    I can overlay the exported PNG from DIYLC in Inkscape and vice versa. The scaling is a bit off it seems. No problem though. Above you can see I used a faceplate to determine the location of the holes. I know I should get the skew (?) out of the faceplate image. Another day...
     
    sds1 likes this.
  12. andrewRneumann

    andrewRneumann Tele-Meister

    Posts:
    271
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    Here’s my take on a fused PT with standby switch a la V-dub. It’s a little messy because I didn’t trim the PT leads in case I want to make a change. There’s some extra space on the board for heater droppers if they become required—which I suspect they will with this PT.

    49542621-E1DD-4E98-AAF5-CC45E235DF3A.jpeg
     
    awasson, robrob and Tom Kamphuys like this.
  13. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    First shipment arrived:

    IMG_20200623_140931175.jpg

    Feel like a kid in a toy store.
     
  14. andrewRneumann

    andrewRneumann Tele-Meister

    Posts:
    271
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    Where did you go shopping?
     
  15. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    Tube Town, Banzai Music and das Musikding. All from Germany (I'm from the Netherlands).
     
    andrewRneumann likes this.
  16. andrewRneumann

    andrewRneumann Tele-Meister

    Posts:
    271
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    Ah yes. I got an assorted shipment of resistors from AES the other day and most were unlabeled. That was an efficient way to learn the color codes...

    Happy you got your parts!
     
  17. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    After quite some dry swimming in DIYLC and Inkscape, I made a mock up where I´ve mounted some parts to check whether there was some oversight.

    IMG_20200627_201810409.jpg


    IMG_20200627_222936694.jpg

    IMG_20200627_222825979.jpg

    The pots will be placed inside, but are here to make sure they fit.

    Learned a few things:
    - Need quite a large hole for all the wires coming from the secondary of the PT. Might need to order bigger grommets.
    - bias/rectification board mounting needs some thought (located under PT and choke).
    - Need a larger collection of screws (diameter/length/...)

    Final OT position and orientation will be determined with the 'headphone trick'.

    Any remarks are welcome!
     
    Last edited: Jun 27, 2020
    andrewRneumann likes this.
  18. andrewRneumann

    andrewRneumann Tele-Meister

    Posts:
    271
    Joined:
    Mar 22, 2020
    Location:
    Cincinnati, OH, USA
    I always forget to check for screws that will end up under a transformer and are inaccessible. It looks like you have done a good job planning! My build had a stand-off for the circuit board end up right under the OT. Doh!
     
  19. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    I was just about to brag I did this so that didn't happen. Then this made itself abundantly clear:

    IMG_20200628_122341215.jpg

    Only two of the standoff screws fit in the standoffs...

    I used two separate Inkscape files for the chassis layout and the turretboard layout. I think the software adagium DRY (Don't Repeat Yourself) should be applied here too. I will see whether I can import the turret board into the chassis image.

    Also, the OT was mounted the other way around. No wonder it didn't work.
     
  20. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    40
    Posts:
    459
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    I finally had an evening free for actual building:

    IMG_20200709_213102667.jpg IMG_20200709_213110720.jpg IMG_20200709_213128787.jpg IMG_20200709_213315690.jpg IMG_20200709_232304673.jpg IMG_20200709_232314230.jpg IMG_20200709_232333446.jpg IMG_20200709_232408706.jpg

    OT position still has to be determined using the headphone trick.

    Only 'problem' so far is the huge amount of wires of the PT being very close to the large power switch.
     
    awasson and jimytheassassin like this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.