5F6A/JTM45 Head Thread

Discussion in 'Shock Brother's DIY Amps' started by magic smoke, Jun 7, 2018.

  1. magic smoke

    magic smoke Tele-Meister

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    This thread will chronicle the construction of a custom head built around the 5F6A circuit. The amp has been successfully completed. I took quite a few pics and tried to carefully document each step of the process. Feel free to ask questions along the way.

    I decided to depart from the original in a few key areas. Some will cry blasphemy but I am not ashamed. It's nice being able to build an amp any way you want, and no others were harmed in the process.

    The list of mod were influenced by Robrob's excellent website and are as follows;
    • Power transformer, tube sockets and chassis layout to accommodate 6L6/5881/KT66/EL34 tubes
    • External bias probe points and pot accessible without removing chassis
    • 50uf fixed bias supply caps
    • 5W screen resistors
    • Backup diodes on rectifier
    • B+ fuse on center tap
    • Standby switch pop resistor
    • 1W metal film resistors throughout except power supply and plates
    • CC plate resistors in select areas
    • Separated cathodes on V1 to allow different voicings
    • Channel 1 traditional voice with a switched bright cap
    • Channel 2 RobRob lead voicing
    • Internal channel jumper on switch
    • Tone stack bypass switch *footswitch jack added at end of build
    • NFB switch for Marshall Fender and Raw
    • Multi tap OT with impedance selector on back
    • Smal box head cab
     
  2. magic smoke

    magic smoke Tele-Meister

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    After a fair bit of research it became obvious that there aren’t any kits available to meet these requirements on the cheap. The first items chosen are the transformers. Cassictone offers a PT that can deliver 5A to the heaters covering power tube rolling requirements and they also offer an OT with multiple impedances. Add on a choke for good measure and it’s a done deal. I couldn’t find any ready made chassis with a guaranteed fit for the chosen x-formers. The size of KT66’s and spacing for heat dissipation is another issue and faceplate layout was the final consideration that led me to decide on a custom drilled chassis.

    The next step in my process is to draw a schematic of the intended circuit. It helps me visualize all of the intended mods and serves as a reference for parts ordering and a guide during the build. It was a fine moment when I figured out how to draw a channel jumper switch between the inputs!

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  3. magic smoke

    magic smoke Tele-Meister

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    Parts sourcing is my least favorite part of the build.
    I tried to condense my order down to as few suppliers as possible. I order extras of most pieces and several of the cheapos especially the parts that come up in other builds. (looking at you resistors) I’m no scrooge about quality components and use the best where I feel it makes a difference i.e. transformers, tube sockets, wire, jacks, etc… I do believe every piece adds up to create the whole, but there is no way in hell I’m spending $18 on a signal cap.
     
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  4. magic smoke

    magic smoke Tele-Meister

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    The next step is creating a layout drawing/ drill template. I use a combination of Adobe Illustrator and Rhinoceros switching back and forth between the two. With Rhino it’s easier to place lines precisely and dimension, so it pulled primary duties for the chassis layout. Super77 on the paper printouts sticks them on the metal chassis and we’re ready to drill. I used a screw at each end of the face plate to secure it to the chassis and drilled pilot holes through both at the same time for all locations.

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  5. magic smoke

    magic smoke Tele-Meister

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    Might as well cut some steel for the power transformer reinforcement.

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    It's not pretty but it sure is strong ;)
     
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  6. magic smoke

    magic smoke Tele-Meister

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    This was my first foray into the world of turret boards. I didn’t know quite what to expect so I ordered a pre-made board with the intentions of modding it. After building a swaging tool, this has become a very pleasant part of the process for me. I now make my own boards from scratch.

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  7. magic smoke

    magic smoke Tele-Meister

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    It seems like I find a way to incorporate a laser into every one of my builds. This one is no exception! The beauty sleeps in the basement of a local art college. Let’s go wake her up!

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    I like to use artist’s matt board for various templates and stencils. The drill template art from earlier cuts a precise jig for wiring off-board components. IMG-0701.JPG
     
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  8. magic smoke

    magic smoke Tele-Meister

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    I haven’t been able to figure out how to run DIY Layout Creator on my ancient mac yet. Therefore I had to work mostly from my schematic. I did find the C****tone layout to be closest to my board, so I used that for reference but it did not correspond 100% with the schematic. This part of the build required much coffee and patient contemplation.

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  9. magic smoke

    magic smoke Tele-Meister

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    I chose topcoat wire for this build and really like the quality of the product, but it sure is a beast to strip. Until this point I had been using a pair of 4 1/2” angle cutters for stripping, and I was starting to develop symptoms of carpal tunnel. Buying this tool was probably the best money spent in a while. Auto strippers from this point forward!
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  10. magic smoke

    magic smoke Tele-Meister

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    IMG-0707.JPG

    1% 1R resistors to the bias probe points.

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  11. bftfender

    bftfender Poster Extraordinaire

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    great stuff !!
     
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  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Magic, your neat, elegant build process is perfectly documented by your sharp, organized photos and text. Thanks for sharing.
     
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  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Really nice work. Can I use your bias probe photo on my website?
     
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  14. magic smoke

    magic smoke Tele-Meister

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    Go for it Rob, It’d be an honor . Thank you for all of the information you have made available to us . Your website has been a huge help in the journey of many.
     
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  15. magic smoke

    magic smoke Tele-Meister

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    You’ll notice that I (eventually) run coaxial shielded wire from the inputs, skip the board and solder the grid stoppers directly to the socket pins. The un-amplified input is especially susceptible to hum and interference and these measures help reduce non-guitar noises.
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    Now is probably a good time to mention that I tried to integrate all switching options into push\pull pots for a cleaner look.

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    Chaos reigns!
     
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  16. magic smoke

    magic smoke Tele-Meister

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    Switching gears now. It’s time for the “other” side of amp building. I have been building acoustic and electric guitars for longer than I’ve been into amps, but cabinet making is a new game for me. I recently built a box joint sled for my table saw and I’m really digging the Freud box joint blade set. It was spendy, but this is one of the tools where quality counts. I wonder how often people stop to take pics of boards in the big box lumber store?
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    Last edited: Jun 7, 2018
  17. magic smoke

    magic smoke Tele-Meister

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    Up and running... I intentionally left the power side a bit loose because a few of these wires will have to be disconnected when installing the faceplates.

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  18. magic smoke

    magic smoke Tele-Meister

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    This one will have a figured maple front. This template will serve double duty for the front and back panels. To stain the wood I used an old bottle of leather dye mixed with denatured alcohol and sanded it back repeating the process a few times. A coat of tongue oil pops the figure and after that dries shellac seals it all in.

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  19. magic smoke

    magic smoke Tele-Meister

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    Skipping ahead a little, its upholstery time! I find this part to be more challenging than any other aspect of amp building. Patience and practice are key. I used 3M Super 90 as an adhesive mostly because it was too cold to open a window and I don’t want the fumes from contact cement melting my thoughts. This stuff seems to hold up pretty well so far, much stronger than the ‘77. IMG-0834.JPG IMG-0835.JPG IMG-0836.JPG
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    This last photo shows how I cut the tolex to conform around the back panel
     
  20. magic smoke

    magic smoke Tele-Meister

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    Be sure to let the solvent off gas before mating the pieces unless you want to do it twice!
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