This project focuses on a high-performance OTL (Output Transformerless) tube headphone amplifier engineered to handle a wide range of loads, from demanding 32 Ohm headphones to high-impedance 600 Ohm studio monitors. The goal was to build a heavy-duty, clean-sounding tube stage utilizing robust Soviet 6C19 triodes. These tubes are well-known for their remarkably low internal resistance (Ri ≈ 300 Ω) and high current capability, making them a perfect match for an OTL design.

The amplifier utilizes a White Cathode Follower (WCF) topology featuring cross-coupling feedback. Unlike classic SRPP stages, the WCF circuit acts as a true push-pull driver where the upper and lower tubes actively operate against each other. This cross-feedback significantly lowers the output impedance of the amplifier, allowing it to efficiently transfer power into lower-impedance headphones (down to 32 Ohm) without common OTL drawbacks like bloating bass or high harmonic distortion.

The driver stage is flexible and supports popular dual triodes like the ECC88, 6N23P, or direct pin-compatible equivalents.

To ensure stable thermal operation and precise operating points for the 6C19 output tubes, the circuit utilizes a mixed-bias configuration. This approach combines a fixed negative grid voltage of approximately -7V with a cathode bias resistor. The mixed bias provides the benefits of fixed bias (maximizing output voltage swing and stability) while the cathode resistor adds a layer of self-regulating protection against thermal runaway as the tubes heat up.

I originally designed two versions of the PCB:

This device was originally designed and built back in 2020. Over the years, the original schematic files for both versions were unfortunately lost.

In practice, I assembled and field-tested Version A.

To maximize the use of the PCB manufacturing area, both PCB versions A and B include two small, break-away auxiliary boards located at the bottom corners of the main panel. These sub-boards are designed to be snapped off and used as optional add-ons: Left Auxiliary Board: A small breakout PCB for mounting a power indicator LED or status lighting. Right Auxiliary Board: A dedicated utility board for implementing a custom output delay circuit.

These optional modules can be wired independently to enhance the functionality and safety of the amplifier, regardless of whether you choose to assemble Version A or Version B.

The PCB layout is optimized for compact routing and heavy current distribution. With a total power consumption of around 60W, the power supply routing requires a robust linear transformer capable of delivering high currents without voltage drops.

The required transformer outputs are:

Due to the massive 4A heater current of the output stage, the PCB features dedicated thick traces for the 6.3VAC lines to handle the thermal and current load safely.

Given the high operating temperatures of the 4x 6C19 output tubes, managing heat dissipation was critical for component longevity. To prevent baking the nearby electrolytic capacitors, I installed custom-designed sheet metal heat shields directly around the output tubes. This creates an effective thermal barrier while maintaining proper airflow.

To further ensure long-term reliability under harsh thermal conditions, all critical filtering capacitors are premium high-temperature components. Specifically, I utilized heavy-duty Epcos electrolytic capacitors rated for 125°C with an extended operational lifespan. One of these capacitors is positioned in close proximity to the output stage and its thermal sheet metal shield. Using 125°C rated components ensures that the proximity to the hot 6C19 tubes will not cause premature drying, capacity loss, or degradation of the power supply filtering over years of intensive use.

NB: This high current also places strict requirements on the power connector. To avoid voltage drops or contact overheating, using heavy-duty connectors is highly recommended. Alternatively, soldering the transformer leads directly to the PCB pads is a perfectly acceptable and reliable solution.

The combination of the low-impedance 6C19 triodes and the White Cathode Follower topology delivers a highly engaging and dynamic presentation. The sound is smooth, rich, and non-fatiguing, typical of high-quality tube stages, while the treble remains crystal clear and detailed. It is a powerful and musical amplifier suitable for long, enjoyable listening sessions.

With a 60W power draw, it doubles as a great space-heater for winter listening sessions.