Introductory note: The Golden Tube Audio SE-300B II is an American-made single-ended amplifier built around the legendary 300B tubes. Released in the 1990s, it gained good commercial success thanks to its attractive design and accessible price, becoming for many enthusiasts a first step into the world of directly heated triode amplification. Over time, however, it has sparked technical debate for some unconventional design choices that favor a “romantic” sonic character over absolute neutrality. In this article, I describe the restoration of a unit that arrived in my workshop for repair and my analysis of its unusual circuit.

“For The Distortion Of The Music”

The Golden Tube Audio SE-300B II perfectly represents a period when the charm of tube sound was interpreted creatively, sometimes stretching beyond strict tonal accuracy. It’s an interesting amplifier for that very reason—an example of how some designers sought a distinctive “signature sound,” even at the cost of a few technical compromises. But let’s start from the beginning…

This amplifier was brought in because it kept blowing the fuse, and the owner requested a full circuit overhaul. Upon opening, I noticed a previous DIY “upgrade” that wasn’t particularly elegant: two large paper-in-oil capacitors roughly fixed in place with electrical tape and felt pads. I removed them and mounted them properly on the chassis, using twisted leads to the board, and took the opportunity to clean and inspect all components.

After a thorough inspection, I confirmed that the circuit had no further faults. All the tubes tested fine except for one 300B that had likely suffered from short circuits, so I replaced the pair with a new matched set.

After readjusting the bias, I noticed some odd behavior while turning the two outermost potentiometers: the oscilloscope displayed strange waveforms. To better understand, I searched for the schematic—available at this link—and identified the most interesting section:

Although the schematic is somewhat unclear, it reveals a potentiometer wired in what appears to be a variable feedback configuration. However, due to the way the dual 6SL7 triodes are arranged, this circuit behaves less like a true negative feedback network and more like a deliberate distortion modulator. It’s an unusual design choice—perhaps meant to let the user fine-tune the amplifier’s tonal character.

The driver stage uses a 6SL7 followed by a cathode follower built around an EL34 connected as a triode, driving the 300B output tube. This is a rather original approach, although the 6SL7 doesn’t provide enough voltage swing to fully drive the 300B, limiting the output to around 6 watts RMS. In a more traditional design, half of a 6SN7 per channel would have been sufficient, but the use of an EL34 as a buffer does add some stability. During testing, I also found one channel with inverted phase at the speaker output, which I later corrected.

Here are the measured performance results:

• Maximum power: 6 watts RMS
• Bandwidth at 1 watt (potentiometer open): 20 Hz – 46 kHz (-3 dB)
• Damping factor: 3.2   |   Rout: 2.5 ohm   |   THD: 0.0029%
• Bandwidth at 1 watt (potentiometer closed): 20 Hz – 85 kHz (-3 dB)
• Damping factor: 5.0   |   Rout: 1.6 ohm   |   THD: 6.3%

With the potentiometer closed, one can clearly see the effect of the feedback circuit: the bandwidth and damping factor improve, but distortion increases dramatically—a behavior opposite to what is expected in a conventional hi-fi amplifier. It’s a fascinating peculiarity, even if far from standard high-fidelity design practice.

Without the so-called “Distortion Feedback,” total harmonic distortion (THD) sits around 3%, while activating it raises THD to about 15%. Such figures would usually indicate a malfunction, but here it’s an intentional feature, shaping the amplifier’s harmonic profile. Essentially, the potentiometer allows the user to shift from “pure single-triode operation” to “triode with added harmonic coloration.” The following video shows this effect in real time, as the initially clean sine wave gradually becomes richer and more complex.

In conclusion, the Golden Tube Audio SE-300B II is an intriguing amplifier full of personality, reflecting a time when emotional musicality often took precedence over analytical precision. Despite its unconventional design choices, it remains an interesting piece to study and refine. With careful restoration and a few thoughtful upgrades, it can deliver a more balanced performance while preserving its distinct tube charm. If you own an amplifier from this brand or any other vintage model in need of restoration or optimization, feel free to contact me through the “Services” section—I’ll be glad to evaluate the best approach to bring it back to its full operational and sonic potential.