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  • Writer's pictureRyan

Building John Mayer's Tone | Two Rock JM Signature - Phase 1

This is a big one for me. Going down the tone-hunting rabbit hole of amplifier building. I've always dreamed about building a Dumble-style amplifier, I've always loved John Mayer's Continuum and Where The Light Is tone, and I love the visual look of Dumble Steel String Singer #004. Unfortunately, no such combination is readily available to purchase so I'll have to just build it!

3D rendering of the final chassis (without the sides welded)


I had the idea kicking around to make my own chassis for a couple of months after I learned how to use Inkscape vector art application to make a replica "faceplate" of Steel String Singer #004. I have limited AutoCAD experience so this endeavor would have a large learning curve ahead of me. I would say it took roughly 120 hours to learn AutoCAD and AutoDesk Inventor to finally create a chassis that I could start submitting request for quotes (RFQ) from sheet metal manufactures in the area. Soon following was Silkscreening and learning how to perfectly line up AutoDesk files with Inkscape.


  • The chassis had to support 4x 8 pin power tubes.

  • Option for a cathode follower.

  • Exact same size and front look as SSS #004. SSS inspired rear panel with courtesy AC outlet.

  • SSS style step filters

  • HT and power fuses

Chassis Drama

5052 .125" Aluminum Chassis Delivered

Since this was a personal project and I didn't not originally have intentions on building a bunch of them to sell, my cost per-chassis was going to be a lot higher compared to established builders buying at least 10x or so from sheet metal fabricators. Taylor from Amplified Nation was wonderful to bounce questions off of and helped provide insight into things to look out for when designing a chassis. After all the CAD work was done, I first reached out to a "rapid" sheet metal fabricator and their price was astronomical for two chassis which made me question if I should go down this route to begin with. I did also submit a RFQ through Alibaba (China) to see what came up. I got some great prices from China for cutting, folding, and welding the 5052 .125" aluminum but shipping to the USA was a questionable process potentially dealing with US customs and port authorities. I decided against purchasing from China.

I eventually found CAMMetals which is within Connecticut so I would be easily able to pick the item up directly from the shop. We were able to work out a price point that was decent enough for two chassis. The quality of the chassis was outstanding upon arrival of the chassis though! Well worth it in that regard.

Chassis Powder Coating

My buddy Bryan's passion project recently is building older motorcycles so inevitably he also started to get into sandblasting and powder coating. We used plain matte black epoxy powder from Harbor Freight that, apparently, is highly regarded within the firearms community for its durability. One trouble we did have with his original Harbor Freight powder gun was that it was not strong enough to get the powder to attach to the inner corners of the chassis so we eventually called it quits until he purchased a new powder applicator. He was already going to do this so it was an easy justification to his wife that he needed to buy a new one to help me out :-)

After it came out of the oven and cooled down, we were really impressed with how well the powder was stuck on. We were also surprised that the "high temp tape" shriveled up when it got hot. Not sure what was up with that but it actually made the masking lines really clean so we will take it!


This part was actually really fun but time consuming since I've never learned vectorizing artwork and

making sure the layout was perfect to the actual model I was holding in my hand. After a lot of trial and errors, I ended up exporting a 1:1 PDF from AutoCAD and importing that vector into InkScape where I aligned the artwork. I spent a lot of time analyzing #004's font, font size, spacing, etc so I could create accurate artwork. I ended up doing a little bit of "forensic" work by using GIMP (open source, Photoshop-like desktop application) to make sure my dimensions were accurate for the faceplate and any sort of general rule that Dumble used when laying out his artwork. After the artwork was complete, I applied an exported PNG image from Inkscape to Inventor and applied it to the chassis. To my delight, the alignment holes matched perfectly to the 3D model. I was good to go!

Phase 1

Fast forward from October of last year to yesterday, my build is coming along really well and ready to share the progress so far. I do have to build the cabinets and modify the circuit to include the SSS step filters and rock/jazz switch, but here's my BOM: *Note - I do not receive any discount or kickbacks from the vendors. My wallet reminds me I paid full price for services. Boo.. Chassis

  • Designed by me using AutoDesk Inventor and AutoCAD (Thank you Taylor @ AN for critical dimensions)

  • 100W 4x 6L6GC Layout designed to accompany 6x 9 pins (one for cathode follower if necessary)

  • 5052 .125" thick Aluminum

  • Waterjet cut, folded, and welded by CAMM Metals

  • Sand blasted and powder coated using Harbor Freight epoxy powder ... 93306.html (My buddy did the work)

  • Silkscreen vector artwork designed by me using Inkscape

  • Silkscreening done by Korzon Screen Printing


  • Ceriatone JM100 Assembly Boards w/ Dale mil-spec resistors, NOS 418 Polyester Orange drop caps, NOS carbon composition resistors, other real nice components.

  • Ceriatone PT/OT/Choke/Reverb Driver

  • Alpha/Mojotone/CTS pots

  • Belton Tube Sockets

  • Switchcraft 11x and L12x connectors

  • NOS Amphenol 160-2-N outlets

  • Mogami W2330 Interconnect Wire

Next Steps

  • Build #004's head cabinet and combo cabinet.

  • Order 3/4", 1/2", and 1/4" void free baltic birch. (COMPLETE!)

  • Order tolex, suede, piping, grill cloth.

  • Modify the circuit to include:

  • Incognito 3-way rotary switch to change GNFB values using NOS Allen Bradley 2 watt carbon composition resistors (4.7k, 8.2k, 10k)

  • #004's step filters

I'm going to make a few cabinets. One is going to be a straight #004 combo cabinet with external #004 turquoise suede head cabinet and then a straight Paul Reed Smith JMOD 2x12 solid pine clone cabinet. We shall see! Keep checking back for more progress updates!

Full shared Google Photo album updating with progress. Please reach out to me if you plan on using any pictures for whatever reason. Happy to answer any questions about the build you have too! Contact me here.

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