Building the "Blue Thing" a 1W Micro Bassman Amp
1. Project Summary
The Fender Bassman is an iconic tube guitar amplifier from the heyday of the Fender amp era. It also served as the foundation for the Marshall Plexi series. Featuring two gain stages, a cathode follower, and a long-tail pair phase inverter, this amplifier delivers legendary tone and versatile controls. The micro version keeps all circuit stages identical to the original 5F6A-M Bassman schematic, but the push-pull output stage is built around a 12AU7 dual-triode tube, producing about 1W of power. This is perfect for a bedroom practice amp. It can also serve as a standalone preamp to feed a modern guitar amplifier or a PA system.
Because of the circuit's complexity, the build required a structured layout. I opted for two rows of terminal lug boards. All passive components sit between the rows, while the control pots and tube sockets line the outer sides. This maintains a true point-to-point design while offering enough room to mount components in three dimensions if needed.
A Hammond 125B output transformer handles up to 5W of power, which is ideal for this design. Its multiple primary windings provide the flexibility to experiment with a variety of output tubes. To keep costs down, I reused the original EICO power transformer. This transformer originally fed two octal tubes, so it proved beefy enough to power four 9-pin miniature tubes. However, the original metal chassis had to be replaced with a custom-fabricated one to host nearly three times the original component count.
2. Tone Control Placement and the Cathode Follower
One of the challenges with my previous builds like this Princeton clone was tone control placement. A passive tone stack eats a massive amount of gain. In a Princeton or Deluxe design, the tone stack sits between the first and second gain stages. Most of the gain achieved by the first tube stage is lost in the tone stack, meaning the second stage is never pushed into overdrive. Furthermore, the cathodine phase inverter typical of those amps has a gain of one. This makes it difficult to get the output tubes to break up. While excellent for clean, warm, tubey tones, it is not ideal if you want heavy overdrive.
A quick look through popular high-gain amp schematics shows that they don't just add more gain stages, they also move the tone stack behind the second or third gain stage. This is exactly how the Fender Bassman is engineered.
On this particular build, the front panel controls and jacks run left-to-right: Input Jack, Treble, Bass, Bright Switch, and Master Volume. Right in the center sits the Gain potentiometer, paired with a vintage-style dial scale. Because original front panel limited the number of pots, I sacrificed the Middle and Presence controls, replacing them with fixed resistors. The back panel features a Speaker Jack for an external 8-Ohm speaker and a Line Out jack to connect directly to another amplifier or a PA system. I have a 4W 8-Ohm resistor across the speaker jack tip to present some load to the output, when only Line Out is connected.
Another key element of this high-gain architecture is the cathode follower. It reduces insertion loss through the tone stack by lowering the driving impedance of the previous stage. It also introduces hard, asymmetrical compression, resulting in a compressed, harmonic-rich tone. As Rob suggested, I put a small neon bulb to protect the cathode follower, it glows mysteriously when the amp warms up.
3. Output Stage Setup
I used a 5963 tube in the push-pull output circuit, which is a ruggedized 12AU7 equivalent. I considered a 12BH7, but the original power transformer could not handle its doubled heater current demand. Rob Robinette did an excellent job adjusting the input signal swing to fit the exact specifications of the 12AU7. Naturally, a triode output stage has its own distinct sonic character compared to the beam power pentodes found in a full-sized Fender amp. Rob also offers many speaker suggestions for this build. I am currently using a 12-inch Yamaha speaker. It is not highly sensitive, but it is plenty loud for home use.
4. Bench Testing and more AlNiCo Speakers
The client gave the finished amplifier a thorough test. First, he compared a 12AY7 and a few NOC 12AX7s in the first preamp position, ultimately picking the tube he liked best. Based on his feedback, I reduced the second grid stopper resistor to roughly 180K to unleash even more gain. We also experimented with some vintage AlNiCo speakers that I salvaged from old desktop radios and stereo consoles. They sound decent, but nothing special so I put them back to where they came from. Anyway, this build was aimed to be a preamp, so I don't really worry about the speakers.
5. Conclusion
Moving from a simple, single-ended Champ-style amplifier to an eight-triode Bassman circuit was a massive step forward for me. The same goes for transitioning from socket-to-socket point-to-point wiring to a structured terminal lug layout. I see incredible potential in using small, affordable preamplifier tubes as power stages paired with smaller, budget-friendly transformers. The Bassman Micro amplifier turned out incredibly well and is already being played live on stage as a pre-amp. I am planning to build a few more amps from Rob's site with different tubes and layouts to gather some more experience.