A balanced audio amplifier with a total of 7 tubes:
- 1 x 12AX7: This dual-triode tube is often used in the first gain stages of balanced amplifiers due to its high gain and low noise characteristics. It could be responsible for:
- Balanced Input Stage: Amplifying the differential audio signal from balanced source components.
- 2 x 12AU7 (or potentially more): These dual-triode tubes are commonly used for voltage gain and signal processing in various audio amplifier stages. Depending on the specific design, they might be used in:
- Balanced Input Stage (if not using a 12AX7): Amplifying the differential audio signal from balanced source components.
- Voltage Gain Stages: Providing further amplification to the signal in either the balanced or single-ended sections of the amplifier.
- EQ Stages (Optional): Some amplifiers might include tone controls or equalization circuits that could utilize these tubes.
- 2 x 6AQ5: These pentode tubes are often used in the output stage of amplifiers, particularly for lower power applications. In this case, they likely handle:
- Power Amplification (Single-Ended or Balanced): Depending on the design, they might amplify the processed signal in either a single-ended or balanced configuration to drive the speakers.
- 1 x 6X4: This is a full-wave rectifier tube responsible for:
- Rectification: Converting the incoming AC voltage from the mains power supply into DC voltage that the entire amplifier circuit can utilize.
Balanced vs. Single-Ended Design:
There are two main possibilities for the amplifier’s output stage:
- Balanced Design: If the amplifier is truly balanced, it likely utilizes a more complex circuit design to process and amplify the audio signal differentially throughout the stages. This can potentially offer better noise rejection and improved performance compared to single-ended designs. The 6AQ5 tubes would operate in a balanced push-pull configuration to drive the speakers differentially.
- Single-Ended Design with Balanced Input: The amplifier might have a balanced input stage to handle balanced source components, but internally convert the signal to single-ended for processing and amplification. The 6AQ5 tubes would then operate in a typical single-ended push-pull configuration to drive the speakers.
Additional Points:
- Without a specific model name or a schematic diagram, it’s difficult to determine the exact circuit design, functionalities, and whether it’s truly balanced throughout.
- 7 watts (assuming pentode mode for 6AQ5) is a moderate output power range, suitable for small to medium-sized rooms.
- Vintage tube amplifiers might require occasional maintenance and component replacements.
Here’s a breakdown of the circuit and its potential functionalities:
Stages (Possible Configuration):
- Input Stage (V1: 12AX7): The signal from your audio source (e.g., turntable, phone) enters the first triode section of the 12AX7 tube (V1a). This stage amplifies the weak input signal.
- Coupling Capacitor (C1): Blocks DC voltage from the previous stage while allowing the AC audio signal to pass to the next stage.
- Grid Resistor (R1): Sets the input impedance of the first triode section and affects gain.
- Cathode Bypass Capacitor (C2): Improves AC signal fidelity and stability in the first triode section.
- Phase Inverter Stage (V1b): The second triode section of the 12AX7 tube (V1b) inverts the phase of the amplified signal. This is a common technique in single-ended amplifier designs to drive the push-pull output stage.
- Coupling Capacitor (C3): Blocks DC voltage from the previous stage while allowing the AC audio signal to pass to the output stage.
- Output Stage (VT2a & VT2b: 6AQ5): The two 6AQ5 tubes (VT2a and VT2b) are connected in a push-pull configuration, a common design for power amplifier stages in SET amplifiers. This configuration utilizes both the inverted and non-inverted signals from the phase inverter stage to improve efficiency and reduce distortion.
- Output Transformer (Tr1): Matches the impedance of the push-pull output stage to the speakers for efficient power transfer and optimal sound reproduction.
- Power Supply (Not Shown): The schematic doesn’t depict the power supply section, but a complete amplifier would include a power transformer, rectifier tube (likely a 5Y3K), filter capacitors, and voltage regulator circuits to provide the necessary DC voltages for each stage of the amplifier.
Important Notes:
- Building a tube amplifier requires expertise and knowledge to handle high voltages safely. Always follow a proper circuit diagram and safety precautions when working with such equipment.
- This is a simplified explanation based on the visible components in the schematic. A complete understanding of the amplifier’s operation would require the entire circuit diagram, including the power supply section and component values.
- Transformer-coupled SET amplifiers are known for their simple design and warm sonic characteristics. However, they typically have lower output power compared to other amplifier designs.
