High Desert Sonic Institute

Mod Blog

Follow the process of converting a 1950’s Bell and Howell Filmosound 179 tube amp into a gnarly guitar amp, with pictures and detailed descriptions.

Blues Jr Rebuild

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Fender Blues Jr Rebuild

When a Blues Jr dies, the blues can still live on.

Imagine that: another PCB-based tube amp with burned up circuit boards! Tho these amplifiers sound great, they were designed in a way that makes premature death inevitable. Amps of yesteryear were designed and built with repair in mind, so that if any one component goes you can replace it and move on. These modern amps on the other hand have everything tied together on fragile circuit boards, and so are not as repairable. Plenty of amps that need new circuit boards simply get wire jumpers made to replace burned up traces, but this fix merely delays the inevitable, and eventually replacing the board becomes an expensive repair.

But there is another way! Instead of replacing the PCB with another PCB, we can replace it with what you should find inside a tube amp: a turret board and real wire! We love taking these dead amps and replacing the insides with proper wiring, reviving them into something that’s better than when it was originally produced.

With these Blues Jr amps, there is a lot of room for creativity as well. We have 3 preamp tube sockets and 2 power tube sockets available with a transformer that provides 350VDC and a low-voltage tap perfect for a bias supply. There are several kits and designs out there that utilize these tubes and/or add additional tubes for further tonal designs. Some copy vintage Fender circuits, while other create more original circuits, and all of them sound good. For this particular build, we’ll be using a circuit of my own design that incorporates a simple but expressive preamp and cathode-bias set of EL84’s in the output.

Here’s the working schematic, along with my initial board layout idea. Many adjustments occur through the process.

Here’s the working schematic, along with my initial board layout idea. Many adjustments occur through the process.

The preamp here is influenced by classic designs like the 5F6-a Bassman, but is a modern take on it. We start with a parallel input gain stage for higher gain and a lower noise floor, which feeds our first volume or “gain” control with a push-pull bright switch. We then have a cathode follower circuit that drives the FMV tone-stack. Much has been written about how cathode followers create that creamy overdrive that has made the early Bassman and all derivative circuits (Marshall, Traynor, etc) sound so good.

Next, we have an LTP phase inverter with a post phase-inverter master volume control that feeds our cathode biased EL84’s. We’ve biased these for Class-A operation, which gives us that great overdriven output section sound typical of AC15’s. Our output is about 15 Watts total, but it’s a loud and articulate 15 Watts.

This particular rework replaces the reverb control with a switchable amount of Global Negative Feedback. The reverb in the Fender Blues Jr utilizes a real spring reverb tank, but the drive and recovery circuitry is all solid state, so to rebuild the amplifier with reverb requires adding a preamp tube as well as a transformer and a new tank. This adds about $100 to the project in parts alone and gets a bit more complicated. For players who use a reverb pedal anyways, foregoing the tube-reverb allows us to have more freedom in designing a really nice, simple circuit. The NFB switch allows us to flip between two different modes of operation: with the feedback, we get a little more headroom and a slightly tighter sound closer to a traditional Fender tone, while eliminating the feedback allows the power tubes to run wild, with more growl and bight.

Once the circuit design is settled upon, the rebuild begins by gutting the chassis. In this case, the transformers were still good, so we reused the power transformer but upgraded the output to a ClassicTone. ClassicTone transformers are made in Chicago with the same classic methods that built original transformers for Fender and other companies over the years, and their iron is arguably the best value out there.

Once the chassis is empty, new tube sockets are fitted and everything other than the turret board is mounted and wired in. I start with the power section, then move onto the filament run, which needs to be tight and low away from other wiring. I then wire up any other runs that I will want to sit below the turret board, including the output transformer and shielded runs between the controls and the tube sockets.

After that, we build and load the turret board. The layout of the board takes hours to design and makes a huge difference in how quiet your amp turns out.

Next we mount the turret board and make all our final connections. This process takes time and patience to do a nice job, and again wire placement and lead dress make a huge difference in terms of noise floor. You can have the best circuit design in the world, but it will sound like a pile of angry snakes if you don’t have good layout design and clean lead dress.

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And there it is, ready for the stage! This particular amplifier is for Tim of the Adobe Collective, a totally rad locally-based band here in Joshua Tree. Tho this process takes a lot of time and patience, the results are excellent in both sound and durability, and I strive to make it affordable to working musicians. Contact me if you have a dead Blues Jr you’d like to bring back to life!

Jacob Erwin