DHT Single-Ended Stereo Tube Amplifier Schematic Explained

An In-Depth Guide to Building a High-Performance DHT Stereo Tube Amp

Highlights

Comprehensive Block Diagram: A detailed breakdown of each amplifier stage, from input to output, is provided.
Key Design Considerations: Learn the importance of tube selection, power supply design, and transformer matching.
Safety and Testing: Emphasis on proper voltage regulation, grounding techniques, and iterative testing to ensure performance and reliability.

Introduction

Designing and building a Directly Heated Triode (DHT) single-ended stereo tube amplifier can be a rewarding challenge for the DIY audio enthusiast. This guide explores a detailed schematic and explains the fundamental concepts and design decisions behind a high-quality DHT tube amp. You will find information on tube selection, component choices, power supply and filter designs, as well as steps for building both the preamplifier and the output stages. The schematic described in this guide is meant to serve as a blueprint to help you understand the architecture of a DHT amplifier, while also providing safety insights and useful testing protocols.

Overview of the DHT Single-Ended Stereo Tube Amplifier

The DHT single-ended stereo tube amplifier is designed to deliver a warm, engaging sound characteristic of tube amplification. The key concept behind DHT designs is to use directly heated triodes as the output stage. Such tubes, like the 45 triode, 2A3, or 300B, are prized for their sonic qualities, high linearity, and minimal interstage distortion. In a single-ended design, each channel operates independently in a class-A configuration, which not only simplifies circuit design but also emphasizes high fidelity and musical accuracy. The schematic discussed here is intended for educational purposes and should be refined using simulations and prototype testing.

Block Diagram

A block diagram serves as an essential roadmap to understand the signal flow. Below is the general flow of the DHT stereo tube amplifier:

Block Diagram of One Stereo Channel

Stage	Description
Input Stage	Coupling network with RC high-pass filter to remove DC offset
Preamplifier	Voltage gain stage typically using a dual triode (e.g., 12AX7) arranged in common-cathode configuration
Optional Tone/Volume Control	RC tone networks and a potentiometer for user adjustability
Driver Stage	Tube (or portion of dual-triode) used to drive the output stage with necessary impedance matching
Output Stage (DHT Stage)	The key stage using a directly heated triode tube (e.g., 300B, 2A3, or 45) in a single-ended design; operates in class-A
Output Transformer	Matches the high impedance of the tube stage to the low impedance of the speakers (typically 8Ω or 4Ω)

Detailed Schematic Description

1. Power Supply Section

The power supply is the backbone of any tube amplifier, especially when dealing with high-voltage circuits. The schematic design for a DHT tube amplifier’s power supply includes a high-voltage section (B+) and dedicated heater supplies for the tubes.

High Voltage Supply

B+ Voltage: Approximately 300-350 VDC is required to properly bias the output tube and supply the other amplifier stages. This voltage is derived from a transformer secondary winding or a dedicated power transformer. The high voltage is then rectified using rectifier tubes (such as 5Y3) in a pi-filter configuration. The filter network typically includes:

Rectification stage using dual 5Y3 tubes for reliability and proper voltage drop handling.
A pi-filter configuration composed of capacitors and a choke to reduce ripple.
Bleeder resistors (100kΩ to 250kΩ) to safely discharge large filter capacitors when the amplifier is powered down.

Heater Supply

Filament Voltage: Directly heated tubes commonly require around 6.3V for AC or DC operation. For optimum performance, use a separate transformer winding to power the tube heaters. Direct heater supplies (AC or regulated DC) are crucial, as any noise in the filament supply can modulate the signal.

2. Input and Preamplifier Stages

The input stage of the amplifier is designed to ensure that the incoming line-level signal is properly conditioned before further amplification. The preamp provides the necessary voltage gain.

Input Coupling Network

The signal enters through a coupling capacitor (ranging between 0.1 µF to 0.22 µF) to block any harmful DC components. Subsequently, a resistor divider sets the proper DC operating point. This network is critical for protecting downstream circuits and ensuring that the tube stages operate at their intended bias levels.

Voltage Gain Stage

A commonly used tube for the preamp is a dual triode (such as the 12AX7). One half of the dual triode is configured in a common-cathode arrangement. Typical components in this stage include:

Plate Resistor: Approximately 100kΩ to load the anode properly and drop the necessary voltage.
Cathode Resistor: Typically in the range of 1.5kΩ to 2.2kΩ; may be accompanied by a bypass capacitor (around 25 µF) to maximize gain.
Grid Bias: Provided by a resistor divider from the B+ supply, ensuring the tube operates in its optimal linear region.

Tone and Volume Control

Although optional, many designs incorporate a tone control section immediately following the preamp stage. A typical approach involves RC tone circuits and a potentiometer (usually around 100kΩ) for volume adjustment. Coupling capacitors (again, around 0.1 µF to 0.22 µF) are used to isolate the bias levels between the stages.

3. Driver Stage

The driver stage is responsible for amplifying the signal further to accommodate the current required by the output or power stage. Usually, another portion of a dual-triode or a dedicated driver tube (such as 6SL7 or 6N7) is used to drive the output grid. The interstage network often includes:

An interstage transformer or capacitor-coupled network that ensures proper impedance matching.
Component value tailoring to achieve the desired signal phase and amplitude.

4. Output Stage – The DHT Section

The output stage is the heart of the amplifier, often utilizing a classic design by using a directly heated triode (DHT) as the final amplification element. Popular tube choices include the 45 triode, 2A3, and 300B. Key design elements include:

Tube Selection and Configuration

Choice of Tube: Each tube offers a unique sound profile. The 45 triode delivers rich musicality, while the 2A3 offers warm tonal characteristics. The 300B, with its reputation for exceptional detail and presence, is also a popular choice among audiophiles.

Cathode Bias: The tube is typically operated with a self-biasing cathode resistor (commonly around 100 Ω to 250 Ω depending on the desired bias current). A bypass capacitor (often 50 µF or higher) is placed in parallel to further increase the effective AC gain in the output stage.

DC Operating Point and Biasing

The bias configuration is essential for setting the operating point of the output tube. This bias can be self-adjusting by using the voltage drop across the cathode resistor or fine-tuned using an adjustable bias potentiometer. Proper biasing ensures that the tube performs efficiently in class-A operation, thereby reducing distortion and maintaining linearity.

Output Transformer

The output transformer serves two main purposes: matching the high impedance of the output tube to the speaker’s low impedance and isolating the DHT circuit from potential load-induced instability.

Primary Winding: Connected between the anode of the output tube and the B+ supply (or ground, depending on the topology). Its impedance is often several hundred ohms to several kilo-ohms.
Secondary Winding: Designed to match standard speaker impedances (typically 8 Ω or 4 Ω). High-quality transformers, known for extended frequency response and minimal losses, are recommended.

Sample Schematic Diagram

Below is a simplified representation of the complete schematic for a DHT single-ended stereo tube amplifier. This diagram shows the signal path in one channel, which is replicated for the second channel.


  // DHT Single-Ended Stereo Tube Amplifier Schematic (One Channel)
  
  [Input Jack]
       │
       ▼
  [RC High-Pass Filter] ----- Coupling Capacitor (0.1µF to 0.22µF)
       │
       ▼
  [Voltage Gain Stage]
       │    (e.g., 12AX7 - Common Cathode Configuration)
       │       • Plate Resistor ~100kΩ
       │       • Cathode Resistor 1.5kΩ-2.2kΩ (with 25µF Bypass)
       ▼
  [Optional Tone/Volume Control]
       │    (RC tone networks, 100kΩ Potentiometer)
       ▼
  [Driver Stage]
       │    (Second half of dual triode, e.g., 6SL7/6N7)
       ▼
  [Interstage Coupling]
       │    (Capacitor or Transformer Coupling)
       ▼
  [Output Stage - Directly Heated Triode]
       │    • Tube: 45, 2A3 or 300B
       │    • Cathode Bias Resistor 100Ω-250Ω (with ~50µF Bypass)
       ▼
  [Output Transformer]
       │    (Primary: High Impedance winding; Secondary: 8Ω/4Ω match)
       ▼
  [Speaker Output]
  
  // Note: High Voltage (B+) ~300-350V is supplied via a filtered & regulated power supply.
  // Heaters are powered separately at 6.3V AC/DC for directly heated tubes.

Component and Design Considerations

Every element of this schematic has been selected with performance and reliability in mind. Here are some important considerations when building your amplifier:

Power Supply and Filtering

Importance: The stability of the B+ supply directly influences the overall performance of your amplifier.

Transformer Selection: Choose a power transformer that offers sufficient filtering and is rated for the current draw of your circuit. For instance, a transformer with both high voltage and low voltage windings (e.g., a Hammond style transformer) is ideal.
Capacitor Filtering: Use high-quality electrolytic or film capacitors in the filter network (100 µF to 470 µF values) to maintain a low ripple voltage and improved power supply stability.
Chokes and Bleeders: Implement choke filters (e.g., 10H, rated for 150mA) to further smooth out the DC supply and include bleeder resistors to safely discharge stored energy when the amplifier is turned off.

Tube Selection and Operating Points

Tube Suitability: The choice of tube critically shapes your amplifier’s sonic characteristics. For example:

45 Triode: Evokes a warm, musical presentation with a smooth midrange.
2A3: Offers rich, detailed performance, making it popular for high-fidelity applications.
300B: Renowned for its exceptional linearity and clarity, albeit with higher operating voltages.

Ensure the selected tube is always operated within its specified plate dissipation and maximum plate current ratings. Testing and iterative adjustments will help you fine-tune the operating conditions.

Signal Integrity and Noise Reduction

Grounding Considerations: A star-ground or single-point ground layout minimizes hum and ground loops. Separate signal and heater grounds where possible.

Decoupling: Adequately decouple the B+ lines between stereo channels to limit cross-talk and interference. Use RC snubbers or dedicated filter banks for each channel.
Quality Interconnects: Use solid-core wiring and ensure all joints are soldered reliably. Solid wiring helps maintain the high purity of the tube signal.
Chassis Design: A well-isolated chassis not only improves safety by physically securing high voltage sections but also helps mitigate microphonic effects.

Layout and Testing

Building a tube amplifier requires careful attention to layout and the physical arrangement of components. Best practices suggest:

Iterative Prototyping: Begin with a breadboard version (using low voltage simulation models where possible) and then move to a prototype PCB or turret board to test all connections and circuit functions safely before final assembly.
Simulations: Use SPICE or other simulation software to analyze circuit behavior under different loads and bias conditions. This helps predict performance and identifies potential issues such as oscillations or undue distortion.
Safety Precautions: Always incorporate fuse protection, proper insulation, and voltage bleeders in your design. Given the high voltages involved (~300V B+), take extra steps to minimize the risk of electrical shock during assembly and maintenance.

Practical Implementation Tips

Beyond the schematic, there are several practical tips and considerations to help ensure a successful build:

Transformer Matching

The output transformer is crucial in converting the tube’s high impedance to the low impedance of your speakers. Look for transformers known within the tube amplifier community for their performance. Whether choosing a modern custom-wound transformer or a vintage model, check that the primary and secondary impedance ratings align with your chosen tubes.

Component Quality

While sourcing resistors, capacitors, and other passive elements, prioritize high-quality parts. For instance:

Capacitors: Use paper-in-oil or high-grade film capacitors for coupling and tone circuits to ensure a clean midrange and low distortion.
Resistors: Select precision resistors with appropriate wattage and stability to manage high voltage conditions.
Wiring: Solid-core copper wiring with proper insulation helps in reducing stray capacitance and interference.

Wire Routing and Chassis Construction

Physical layout is as important as the electrical design. Keep high voltage and low level signal circuitry apart. Use shielded wiring when necessary and ensure the chassis is grounded properly. Many builders construct a multi-chassis design to separate power supply sections from signal handling circuits, reducing hum and noise.

Final Assembly and Calibration

Once all components and subcircuits are built, careful testing and calibration are required. Use a variable DC supply and multimeter to adjust operating points for the output stage. Verify that:

B+ voltage stays stable: Monitor and regulate the power supply’s filtering network.
Tubes are properly biased: Adjust the cathode resistors or bias pots to ensure that the tubes operate in optimal class-A conduction.
Signal integrity holds: Use an oscilloscope to check for unwanted oscillations or envelope distortion in the amplified signal.

Summary of Critical Parameters

Below is a concise table summarizing key component values and operating parameters for a typical DHT single-ended stereo tube amplifier design.

Parameter	Value/Range	Description
B+ Voltage	300-350 VDC	Main high voltage supply for plate circuits
Heater Voltage	6.3 V AC/DC	Voltage for directly heated tubes (e.g., 45, 2A3, 300B)
Preamp Cathode Resistor	1.5-2.2 kΩ	Sets bias current in the gain stage
Preamp Plate Resistor	~100 kΩ	Determines load for the voltage gain stage
Output Cathode Resistor	100-250 Ω	Establishes operating point for the output DHT
Coupling Capacitors	0.1-0.22 µF	Blocks DC between stages while allowing AC signal transfer
Bypass Capacitors	25 µF (Preamp), 50 µF+ (Output)	Boost AC gain by shunting cathode resistor at high frequency
Transformer Impedance	Primary: Several hundred Ω Secondary: 8 Ω/4 Ω	Matches tube output to speaker load

Conclusion and Final Thoughts

Building a DHT single-ended stereo tube amplifier is both an art and a science. This schematic provides a comprehensive starting point, detailing every major section of the amplifier—from the high-voltage power supply through the preamp, driver, and output stages to the speaker connection. Emphasis is placed on careful component selection, proper biasing, and meticulous layout to achieve the classic warm and engaging tube sound.

Safety is paramount when working with high voltage circuits. Always use proper insulation, grounding, and testing methods before fully powering your amplifier. Iterative prototyping, simulation of circuit behavior, and adjustments based on real-world measurements are key to a successful project. This guide consolidates many best practices and technical insights needed to design and build a tube amplifier that not only sounds great but is robust and reliable.