Fender 5F1 based 5W Valve Amp

 

Goal : Build simple 5W Valve amplifier 

Specifications/Research :

        After delving into the history of guitar valve amplification, I came across the ubiquity of the Fender 5F1 and was enamored with both its reputation and simplicity. I found a kit of a 5F1 based design (basically a 5F1 with diode rectification as opposed to a mostly-obsolete rectifying vacuum tube) that included every part except for preamp and power tubes. Nearing my birthday, I decided to go for it. For most of this build and the ensuing troubleshooting, I referenced this annotated schematic of the originals 5F1 along with the schematic of my specific amp.

Progress log : 

• 12-05-21: I started by cutting and then bending a sheet of thin steel into a rudimentary open box approximately 10 by 16 by 2 inches. I found a satisfactory component layout and then installed the transformers first. After the tube sockets, AC input socket, and main PCB, I could finally begin wiring it up. This went relatively smooth, and I was able to install the rest of the controls and inputs as I went. 
• 12-05-21: Once my circuit was double-checked for correctness of assembly, I installed the tubes and turned on the power for the first time. After letting it warm up for a couple of minutes, I tested it through a 212 cabinet. Unfortunately, I was extremely displeased with how it sounded, especially when the high input was used. There was an excessive amount of feedback (even with the volume on one) and it was distorting more than I knew it should. After checking my circuit again and looking more closely at pictures of the actual Fender model's construction, I realized my problem: that the wire I was using was both the incorrect type and gauge. What I had was 14 AWG stranded wire and what was originally used was 18 AWG solid wire. The amp was going to need a rebuild. 
• 02-3-22: After securing the correct wire, I set out taking apart the amp. For most wires, it was easy; but in some places, I definitely got the components and PCB hotter than I would have liked. That being said, reassembly with the new wire was at least an opportunity to reroute some of wires more neatly and efficiently. Once I finished rewiring, I made sure it was correct one final time. I put in the tubes and turned it on. While the feedback was still not perfect, it was greatly reduced. In addition, whatever distortion I had been incurring initially had almost completely disappeared. 

  

  Amplifier with components installed

  
  
• 02-17-23: Since using laser cutting to make a MDF chassis for my synth project, I really started to realize all the of things I could do with it. One of the first things that came to my mind was remaking my 5F1 chassis. While the original surely got the job done, I was never 100% happy with its structural integrity or its bulky size. I started by putting representations of the crucial parts into CAD, rearranging them into as small of a box as I could. Also, since taking more courses and better understanding EMI, I purposely oriented the transforms as to not interfere with the unshielded tubes. With the constraints, I placed the components and drew a rectangle around them representing the top face of the new chassis. After fixing four more rectangles to its perimeter that would later become the side faces, I added the rest of the input and output holes to them. Lastly, I added text for the I/O's that would be engraved instead of cut. After laser cutting the design out of 1/16" steel, I used a metal bender to bend in the four faces. I had the corners tack welded together; and it now was looking and feeling much better than my first iteration. 
  
  
  
• 2-20-23: For what I hoped would be the last time, I removed the components from the old chassis and installed them into the new one along with a whole new set of wires. A final check and I installed the tubes. After letting the amp warm up, I took it off of standby and played it. Tonally, it was similar to before but the feedback had definitely been reduced even more. It was finally sounding how I thought it should, and I was certainly happy about that. 
  
  
  

Conclusion :

    Especially on the low input, I can definitely understand why this design is so tonally revered: it just gives you this blazingly loud clean tone that is both clear and rich. Even with fuzz, overdrive, or distortion, I can tell the amp just did not want to distort. Even with just 5W, the amp gets quite loud; surely loud enough to play over a drummer in the right contexts. Even with the missteps, this project was both enjoyable and rewarding; giving me a plethora of useful insights into analog signal processing and leaving me with a really cool amp to play with!