6G2.5 Amp Build

Discussion in 'Shock Brother's DIY Amps' started by Che_Guitarra, Sep 16, 2021.

  1. Che_Guitarra

    Che_Guitarra Tele-Holic

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    Hi all.

    Thought i'd start a thread on my next build. Or more accurately, a ground up rebuild.

    I built a 6G2 many years ago - it sounded great. But over the years, it became a guinea pig to my learning curve, and suffered upgrade after upgrade, as I tried to turn it into something it was never meant to be. All these experiments conducted, I realise they were pretty close to getting it right from the factory.

    So, back to the drawing board. Time to rebuild my 6G2, but with three variations to the Fender design:
    - No tremolo
    - Long Tail phase inversion taken from a 6G3 deluxe schematic (thus, this will be a 6G2.5 :D)
    - 12 inch speaker
    And potentially, a 6G3 OT. But i'll make that assessment down the line.

    Hopefully this will result in an amp that plays friendly with single coils and humbuckers, and cover territory from rockabilly to hard rock. And at a volume that won't have my inner city neighbours in a twist.
     
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  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Does the 6G3 have an OT with 6.6K or 8K primary?
     
  3. Che_Guitarra

    Che_Guitarra Tele-Holic

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    Thanks to another thread, i'll be using Printer2's 6F10 schematic as a running guide (thanks Printer2 :) - I can pull this image down if you like), although I plan to stick with 5Y3 rectification.


    [​IMG]

    Starting point - designing the board. I think this looks OK. Been a while so my terminology has slipped, but wondering if I can jump pins 3 and 8 on V1, and share the same 1500k/25uf cathode to ground?

    [​IMG]


    Otherwise, i've got some sheet metal getting bent up right now, so the chassis build is also underway.
     
  4. Che_Guitarra

    Che_Guitarra Tele-Holic

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    No idea i'm sorry. Haven't bought this OT yet. Would have to check the Classictone website.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes you can. The standard way is to drop the 1.5K cathode resistor to an 820 Ohm resistor. Each cathode (pin3 & pin8) will then see roughly the 1.5K resistance by sharing the 820R.
     
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  6. Nickfl

    Nickfl Friend of Leo's

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    It should be 6.6K IIRC.

    It is also worth noting that the 6g3 has a split load on the second gain stage before the PI that dumps something like 87% of the signal and it is still a very hot amp. The 6g2 preamp does not attenuate the signal before the PI because the cathodyne PI has less than unity gain so it doesn't need to, but if you are going to use a LTP PI you will probably want to cut the signal to some degree like the deluxe does.
     
  7. 2L man

    2L man Tele-Holic

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    Obviously it would be better to use own cathode resistors, when series triode phases come 180 decrees like in 6F10.

    I would select OT impedance after or same time with the PT output selection and they match better!

    i have used 8,5k H1760E for 330VAC PT and 1750E for 300VAC PT and 300V did sound bit better with 8,5k but it might have been something else in amps.

    I did like best 6950 ohm H1750F OT with 260VAC PT. Again it had bit different capacitors, but there came great power stage distortion, using lowest volume. I think when its bias current came higher, the switch between A/B operation came to sweeter power range on loadline?

    Those PT voltages are their ratings and actual idle voltages were bit higher. I did use 5Y3 rectifiers.
     
    Last edited: Sep 17, 2021
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  8. Che_Guitarra

    Che_Guitarra Tele-Holic

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    Thanks all. I'm certainly open to suggestions and they will all be taken on board.

    As a reference point, this is the ballpark of sonic outcome I am chasing.

    Although I can hear the cathodyne PI of Kenny's amp being ridden a hair within its toneful limit. Thus fuels my desire for a LTPI - I don't want to overwork the PI above all other triodes - I want to get all the tubes working in their sweet spot. Which is what I hear in Marty's playing.

     
  9. Wally

    Wally Telefied Ad Free Member

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    Ime, the simpler tone circuit in the 6G2 and 6G3 differentiate those two amps from anything in the Bf/SF stable. These two 6G amps are hotter than those amps in that video….very different sonics, ime. If you want what is happening with Marty’s sound there, I say build a DR.
     
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  10. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Dunno if you're planning a head or a cab, but I bought a kit for a 6G2 combo with a 12 " speaker, and there wasn't enough clearance between the speaker and the cap can. I ended up building a board with the filter caps on in the end.
    P1130456.JPG
     
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  11. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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  12. Che_Guitarra

    Che_Guitarra Tele-Holic

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    I already did that :)

    6Gx amps are the ones for me. A little nastier than BF era amps. Classic Fender cleans on one side of the dial, and a Marshall-esque bark on the top side of the dial. Perfection!
     
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  13. Wally

    Wally Telefied Ad Free Member

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    Okay, the first quote made me think that you were going for what you heard M. Stuart had going on in that video. The cleans in the 6G3 and 6G2 are quite different imho/ime from the BF thing that Stuart has going on…or even what Kenny has coming out of that smaller Princeton Reverb.
    Enjoy the search.
     
    Last edited: Sep 18, 2021
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  14. monkeybanana

    monkeybanana Tele-Holic

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    Wait is Classictone back?!

    Did you see this vid where Kenny V talks about the Letterman performance. I thought it was great. He talks about the amps used:
     
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  15. Che_Guitarra

    Che_Guitarra Tele-Holic

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    This is what remains of my DR. Planned conversion into a DR with a JCM800 pre and EQ section for a bit more flex. Project put on ice because of covid. Parts got too hard to source (in Perth, Australia), and the live scene completely died. I'm now holding off for a Bassman Tonemaster, so this project might never get used as intended.

    [​IMG]
     
  16. Che_Guitarra

    Che_Guitarra Tele-Holic

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    Circuit board design is up and running. Although forgot to add the trim resistor before the PI, as Nickfl pointed out above, sticking pretty true to the 6F10 design in the earlier post. Will add in my next tinker. Otherwise, ponder this design for a few days, stalling in case I can come up with improvements to the footprint.

    [​IMG]

    Green traces are ground.
     
    Last edited: Sep 18, 2021
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I like the placement of the 8uF filter cap on the board for the preamp node.

    For those of us that like to keep our ground nodes separate, the 1K5 PI tail resistor should not ground with the 8uF filter cap. It belongs to the next filter node.
     
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  18. Che_Guitarra

    Che_Guitarra Tele-Holic

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    Here's an alternative grounding config. Common rail for the board. All filter caps can terminate to a star ground.

    [​IMG]

    It's funny. I'm used to using modern components with very compact footprints. Metal film resistors and Wima style caps. But in this build I plan to scavenge from my existing 6G2 - the parts are all huge in size! In turn making the board very large for such a simple circuit!

    Otherwise, went timber/lumber hunting today. I was quite surprised - the quality of 'premium' ply (void free) has dropped dramatically since covid. Used to be easy to find good boards with a nice veneer, but this time only 2 boards in the pile of 50 passed a visual inspection. And who knows what I will find once I start cutting them to size.
     
    Last edited: Sep 18, 2021
  19. Che_Guitarra

    Che_Guitarra Tele-Holic

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    I've spent today expecting to get the board made for this amp with my CNC. And... kinda... sorta...

    [​IMG]


    I've built quite a few PCBs with my CNC, but never faced this issue before.

    Seems i'm having EMF interference issues with my CNC during the run. Net result is I can only get part way through carving the circuit board, before the CNC stalls and needs resetting.

    Five or six false starts, as can be seen on the top of the board.

    Thinking I had the problem solved, I turned the board around to try again. 30 minutes into a 45 minute program, it did it again. It would be less frustrating if it quit a little earlier into the carve.

    Oh well. Time for a break. See if I can think through why this would be happening all of a sudden.
     
  20. Che_Guitarra

    Che_Guitarra Tele-Holic

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    So... progress has been extremely slow due to postage delays. Thanks Covid.
    But hopefully my EMF problems are now solved with the troubleshoots below.


    New brushes for my router. Worn vs new.

    [​IMG]


    Ferrite chokes on all cables (I always wondered what these things were).

    [​IMG]


    And two chassis bent up. One for this project, and one as a spare.

    [​IMG]


    Will attempt to make the circuit board again tomorrow. Only one chance remains, or i'll need to wait until next weekend to try again (shop with copper clad board only open on weekdays).
     
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