Repair of a Conrad Johnson MV50: the rebirth of a Hi-Fi classic

The Conrad Johnson MV50 is a legendary tube amplifier, loved by hi-fi enthusiasts for its warm and dynamic sound. A concerned customer brought it to me after the amplifier suddenly stopped working, accompanied by a burnt smell. Upon inspection, I discovered that one of the output transformers had failed, most likely due to a faulty EL34 or an incorrect bias adjustment.

Diagnosis and disassembly

The first step was to remove the damaged output transformer. While opening the amplifier, I also noticed some burnt resistors, indicating that the fault had caused thermal stress on other parts of the circuit. The diagnosis confirmed the need to replace not only the burnt transformer but also its twin, in order to ensure perfect channel symmetry and tonal coherence.

The rebirth: new custom output transformers

I wound two brand-new output transformers, specifically designed to preserve the original sonic character of the MV50. Careful winding precision and material selection ensured optimal sonic performance and greater long-term reliability. After replacing the faulty resistors and installing the new transformers, the amplifier was ready for its first power-up.

I replaced a few worn-out resistors and installed the new transformers.

Power-up, bias adjustment and fine-tuning

The first power-up went smoothly. I proceeded with an accurate bias adjustment, a crucial step to ensure EL34 stability and minimize harmonic distortion. With a slight circuit tweak to match the new transformers, the amplifier came back to life, delivering a rich and detailed sound once again.

Instrument testing

Finally, I carried out the usual bench measurements to verify performance: