Discussing tube amplifiers often triggers immediate concern among designers due to the challenge of finding a suitable output transformer. This component is often viewed as esoteric, bulky, and expensive, mainly because it needs to be meticulously designed and manufactured to match specific tube configurations and output power levels. Countless books have been written on tube output transformers, and experts deliver lectures on their intricacies while some enthusiasts even wind them by hand. However, a circuit design called SRPP (series regulated push-pull) offers an alternative for constructing a low-power tube amplifier without the need for the notorious output transformer. Although this approach may require some compromises in terms of distortion, it proves to be a cost-effective solution, leaving your wallet significantly heavier.
Using EL84 (6BQ5) Power Pentodes in Triode SRPP Configuration
Normally reserved for preamplifier stages, the SRPP configuration utilizes two triodes in a cascading setup. In this unconventional approach, we advocate employing two EL84 (6BQ5) power pentodes in a triode SRPP configuration. The rationale behind this choice includes the EL84’s affordability, wide availability, and ability to handle occasional overload conditions. By employing two of these tubes in an SRPP configuration, we create an amplifier that faithfully captures the sought-after ‘warm thermionic sound’ highly coveted in today’s audio landscape.
Safety Measures and Construction Considerations
Before delving into the circuit operation, it’s crucial to emphasize that constructing this circuit should only be undertaken by individuals experienced in working with tubes at high voltages or with guidance from an experienced practitioner. As a safety precaution, two anti-series connected zener diodes are incorporated at the amplifier output. These components safeguard the output (headphones and ears) against potentially hazardous voltages during power-on sequences or in the event of output capacitor C3 failure. The power supply unit (PSU) is designed for two channels, making it suitable for a stereo version of the amplifier.
Critical Details and Power Supply Design
The values provided in brackets are tailored for Elektor readers on 120 V AC networks, indicating doubled values for fuses F1 and F3 in the AC primary circuits. The PSU adheres to conventional design principles, although it deviates in raising the 6.3 V heater voltage to around +80 V through the voltage divider R7-R8. This adjustment ensures that the maximum cathode-heater voltage specified for the EL84 (6CA5) is not exceeded.
R6, serving as a bleeder resistor, rapidly and steadily discharges the reservoir capacitors C8 and C9 when the amplifier is powered down. Rectifier diodes D3-D6 are each accompanied by an anti-rattle capacitor. In the amplifier circuit, assuming the tubes utilized exhibit similar emission characteristics, a half-voltage level of approximately +145 V exists at the junction of the anode of V1 and the control grid of V2. The SRPP configuration underscores the importance of employing high-quality, preferably new capacitors. This choice not only influences sound fidelity and reproduction accuracy but also plays a significant role in ensuring the safety of the circuit.