09/12/12 By Eddie Pletka We’ve had countless vintage Marshall's through the shop and there’s just something about the JTM45 that’s intangible. The Bright channel tends to be beamy, HIFI’esq, and hard to tame. The Normal channel is like an old pair of socks in comparison. Big, round, but still punchy and with a softness that tricks you into thinking these amps are easy to play. But within those two extremes there’s a sweet spot that’s just inspiring. It’s clean but when you dig in it gets hairy in a very expressive way. The goal with this build was to take a classic circuit and apply all of the unique build techniques I’ve seen over the years. Maintain the key aspects of the circuit that make the JTM45 what it is and have fun with the rest. An homage if you will. Specs: - Custom transformer set from O’netics. 365-0-365 PT, 4.4K OT, 10H-400mA choke. - Custom aluminum chassis, welded and milled in house. - PEC mil-spec pots. - 18AWG Teflon hookup wire throughout. - NOS Allen Bradley carbon comp resistors in pertinent locations. PRP’s just about everywhere else. - Turret strip construction, covered lead dress, cable lacing.  - Individual and independently functioning bias adjustments with test points. - Unique layout but with traditional appointments (shared cathode V1 with large value bypass cap, large value bright cap, 0.1 and 0.022 coupling        caps, low value filtering in power supply and phase inverter circuitry). - Swanson built cabs and custom hand painted control panels by the fabulous Amanda Kohn.   There are a few advantages with doing things from the ground up. While the basic “flow” of the traditional JTM45 layout was maintained, we were able to play with things like transformer orientation. So before the raw chassis was introduced to the Bridgeport Mill, we set the PT up and energized it. Then we hooked up an extremely sensitive bench top DMM to the secondary of the OT. This allowed us to see how much noise was being introduced and how the proximity and orientation of the OT relative to the PT affected it. This is a variation on the commonly known “headphone trick”. It’s eye opening to see how the slightest movement or twist of the OT’s placement changes the “coupling” effect. It’s also worth mentioning that other components and the general layout of the circuit will mess with the field as well… Another advantage with a scratch build is component layout and mounting. My chassis is slightly larger than a traditional JTM45 chassis so I have a bit of elbow room so to speak. Rather than using perf board I chose to use turret strips and modify the layout to my liking. I find this makes swapping components easier and I also find this type of layout is more intuitive to trouble shoot. Jeff Swanson is known throughout the industry for doing some of the cleanest cabinetry work. I gave Jeff the specs and a few weeks later everything came in as specified. The speaker cab is an oversized slant 2X12. It’s currently loaded with a Weber Legacy (55Hz, 65 watt) and an alnico Thames. The combo sounds great and is very unique. It sounds big and bold and does an excellent job of copping that 4X12 vibe. However, in all honesty nothing sounds like a real 4X12 except for a 4X12. These amps always tend to sound best when paired with a proper 4X12 and this build was no different. It sounds best through our straight bottom Silver Jubilee 4X12 with greenbacks. All in all I’m happy with the final result. The 4.4K OT is considerably different than the traditional JTM45 spec (which would have typically been 6.6K or 8K). This definitely gives the amp a distinct character with more growl and more “bark”. However the amp is still unmistakably JTM45 to the core. I have to thank Kyle Rhodes, Chris Merren, and Bud Purvine. A special thanks goes out to Jack Anderson, Matt Kamna and of course Bob and the whole Eurotubes Crew. Eddie

Eddie's "DeForest 45"