Recently I was entrusted with a Radford STA-25 valve power amplifier and its matching SC22 preamplifier, both renowned for their sound quality and timeless design. The request for service arose after an unpleasant incident: during the connection between the power amplifier and the preamplifier, via their characteristic umbilical cable, a flash occurred accompanied by a loud bang and smoke.

Problem Diagnosis

After carefully examining the units, I carried out a thorough check of all components and precise measurements of the valves. The diagnosis revealed:

• A pair of worn EF86 valves. Although they required replacement, they were not the cause of the fault.
• The interference suppression capacitor inside the preamplifier was short-circuited. This component turned out to be the real cause of the incident.

Work Performed

1. EF86 replacement: I replaced the worn valves to ensure correct circuit operation.
2. Preamplifier repair: The faulty suppression capacitor was removed and replaced with a new one, compliant with the original specifications.
3. EL34 re-matching: Using a curve tracer, I carefully re-matched the EL34 output valves to achieve balanced performance.
4. Bias adjustment: I carried out an accurate bias setting to optimize efficiency and sonic performance.

Testing and Verification

After completing the work, the units were subjected to extended testing to verify reliability and sound quality. As is my usual practice, I performed the following measurements:

• Output power: 36 watts RMS per channel.
• Damping factor: 34.
• Total harmonic distortion (THD): 0.2%, with many small harmonics at high frequencies. This behavior is due to the heavily feedback-based design of the unit, but it is nothing unusual.

Unfortunately, I did not record the bandwidth during testing. Repeating the measurement setup would be too demanding, so I decided to omit this data.

THD

100 Hz square wave

1 kHz

10 kHz

The amplifier and preamplifier played for several hours without any issues, confirming the success of the repair and overhaul. This intervention restored the Radford STA-25 and SC22 to their original performance, preserving their historical and sonic integrity. The ability to diagnose and resolve problems like these is essential to keep valuable vintage equipment alive, offering enthusiasts a unique and authentic listening experience. If you also have an amplifier or preamplifier that requires maintenance or overhaul, feel free to contact me. I will be happy to help bring your audio system back to life.