Updated: Jun 3
The pedal mod that started it all for me! Back in 2009 or so, I started my guitar tone quest. I only had a Keeley Katana and I my wallet hurt from that investment at the time. I quickly realized I should get some dirt too and I knew John Mayer used a Keeley modded BD-2 on the Trio album (which I loved that tone) so I thought I should start there. I was quite confident in the instructions to mod a standard, $100, blues driver and a $20 kit from eBay so I gave it a try. I was happy with the tone but thought the bypass tone sucked a lot of my guitar so I stopped using the pedal. Fast forward almost 10 years of the pedal being in a drawer and I wanted to revive this classic pedal with true bypass.
Build Your Own Clone has the Blue Drive which is a BD-2 circuit but the board would likely be too large for the retrofit. PedalPCB has the Cobalt Drive which is a really small footprint that likely would work. I'm really happy with all of the circuit boards I've received from PedalPCB (I'm not sponsored by them or anything, just a really happy customer).
Build Notes and Approach
I studied a lot of pictures and articles about the Keeley "Phat Mod" changes and it appears that there was a few interpretations and iterations of the mod. From pictures of real Keeley mods, I devised a plan with the following:
For resistors, I went with 1/8 watt metal film 1% tolerance. I could have used 1/4 watt and stand them on end but 1/8" was what the original BD-2 uses and I was fine with that. Another gorgeous layout from PedalPCB. From what I could tell after comparing the schematics of an original BD-2 and the Cobalt Drive, the two changes on the Cobalt Drive is a 100nF C1 capacitor and a op amp change to TL071. The original BD-2 uses a 47nF cap for C1 position but the Keeley mod, funny enough, changes this to 100nf to allow more bottom end frequencies to bass into the circuit. The original BD-2 uses a JRC4558 capacitor but the PedalPCB changed this to the TL071 as mentioned. I kinda wish I could use a JRC4558 dual op amp pinout but I the TL071 is a nice upgrade too. (Although I could have used a TL072 and achieved the same thing though!! Grr...) I'll keeping looking to see if there is a single-ended opamp that's the same as a JRC4558 internals for testing.
I had to drill top mount jacks in order for the circuit board to fit and everything needed for this pedal to function. The standard side sockets just wouldn't work so I had to adapt and it came together quite nicely! All the pots are off-board wired also.
At first, I accidentally used .68uF and .33uF caps for the "Phat Mod" when it should actually be .033uf And .068uF. This was a silly oversight on my part but I corrected after I realized the pedal was a little too "phat" haha.
I was able to retrofit a mini 3PDT in the pedal too which feels like a big accomplishment. The stomp switch looks stock without a 3PDT sticking out from a weird drill hole.
I am hosting all my documents for this build on GitHub here which includes the picture above and corresponding InkScape original document.
documented my full build on YouTube for your educational and entertainment purposes. Be sure to subscribe since I typically push out a YouTube video before a blog post.
UPDATED 6/3/20: A YouTube member (Thanks, Steve!) brought to my attention that R24 should be 10k and not 2M as originally silkscreened. New Cobalt Drives have the 10k value which prevents popping when engaging the pedal for the first time. https://forum.pedalpcb.com/threads/does-your-cobalt-pop.2453/#post-18995
This is a legendary pedal for a reason. It sounds great, I'm really happy with this build. I'll put together a video soon of the demo. I'm really happy with the build and proud how it all came together.
Gallery (click for full resolution)