Endstufe Sarah MKII - 300B Single-Ended Tube Amplifier

Project Overview

This project documents the design, development, and implementation of a high-end single-ended (SE) tube amplifier using the legendary 300B triode output tubes. The "300B MKII" amplifier represents a refined evolution of audiophile tube amplification technology, combining classic vacuum tube designs with modern engineering practices.

Technical Specifications

Output Stage

• Output Tubes: 300B triodes (one per channel)
• Driver Tubes: 6SN7 dual triodes
• Topology: Single-ended Class A
• Output Power: ~6W RMS per channel into 8Ω
• Maximum Output: 20Vss (peak-to-peak) into 8Ω load

Power Supply

• Anode Voltage: 360V DC (±10V tolerance)
• Bias Voltage: -66V DC (adjustable)
• Heater Supply 1: 5.0V DC (symmetrical ±2.5V to ground) for 300B
• Heater Supply 2: 6.3V DC for 6SN7

Performance Characteristics

• Frequency Response: -1.5dB from 20Hz to 22kHz
• THD: <1.3% at 1W, <3.0% at 6W
• Bias Current: 63-65mA (adjustable)
• Power Dissipation: ~21W per 300B tube (52% of maximum rating)

Fusing

• Main Fuse: 1.25A
• Anode Supply: 630mA
• Bias Supply: 125mA
• 5V Heater: 3.15A
• 6.3V Heater: 1.25A

Project Structure

📁 Directory Organization

├── 00_Alte_Version/           # Previous version archives
│   ├── 01-Auslegung/         # Original design concepts
│   ├── 02-Module/            # Module designs
│   ├── 03-Test/              # Test results and measurements
│   └── 04-Bilder/            # Images and photos
├── 01_Auslegung/             # Current design files
│   ├── 01_HV_Netzteil_370V/  # High voltage power supply
│   ├── 02_Heizungsnetzteil/  # Heater power supplies
│   ├── 03_Vorspannungsnetzteil/ # Bias voltage supply
│   └── 04_6SN7_300B_SE Bauversion/ # Main amplifier circuit
├── 02_Module/                # Module implementations
│   ├── Groundbreaker neu/   # Ground isolation module
│   ├── Heizungsnetzteil MKII/ # Heater supply module
│   ├── HV Netzteil MKII/    # HV supply module
│   ├── Negative Vorspannung/ # Bias supply module
│   └── Steuerung/           # Control circuitry
├── 03_Integration und Test/  # Integration testing
├── 04_Dokumentation/        # Documentation and manuals
├── 05_Fräsen Endstufe/      # Mechanical drawings
└── 06_Order/                # Parts lists and orders

Key Design Features

Circuit Topology

• Driver Stage: Dual 6SN7 triodes in optimized configuration
• Output Stage: 300B single-ended triode per channel
• Coupling: Direct-coupled driver to output stage
• Bias: Fixed bias with adjustable regulation
• Output Transformer: High-quality audio transformer (282Ω DC resistance)

Power Supply Design

1. High Voltage (370V): MOSFET-based regulation for clean, stable anode supply
2. Heater Supplies: Separate DC supplies for optimal performance and low noise
3. Bias Supply: Adjustable negative voltage generation with current limiting
4. Safety Features: Multiple fusing levels and relay-controlled startup sequence

Protection Systems

• Soft Start: 60-second delay relay for speaker connection
• Multiple Relay System:
  • Main power relay
  • Speaker protection relay
• Current Limiting: Bias current limiting resistors (4.9Ω-5.2Ω range)

Development History

Major Revisions

• Original Version: Initial concept and prototype
• MKII Version: Refined design with improved power supplies and protection
• Current Status: Fully operational stereo pair with documented performance

Testing Timeline

• July 2013: Initial commissioning and basic measurements
• November 2019: Comprehensive integration testing and optimization
• October 2025: Latest revision and maintenance cycle

Performance Data

Measurements (November 2019)

• Channel Separation: Excellent stereo imaging
• Noise Floor: Very low hum and noise in music pauses
• Frequency Response: Measured -1.5dB, 20Hz-22kHz
• Listening Tests: "Brilliant" sound quality with enhanced detail retrieval

Bias Settings

• Target Current: 63-65mA per channel
• Voltage Drop: ~310mV across cathode resistor (4.9Ω)
• Working Point: Optimized per 300B datasheet specifications

Tools and Simulation

Design Software

• LTSpice: Primary circuit simulation platform
• Tube Models: Norman Koren tube library implementation
• Analysis: AC/DC analysis, THD calculations, frequency response

Measurement Equipment

• Signal generators and oscilloscopes for characterization
• Digital multimeters for bias adjustment
• Audio analyzers for THD and frequency response measurement

Safety Considerations

⚠️ HIGH VOLTAGE WARNING

• This amplifier operates with potentially lethal voltages (>350V DC)
• All work should be performed by qualified personnel
• Proper safety procedures must be followed at all times
• Discharge all capacitors before servicing

Documentation

• Service Manual: Complete technical documentation available in 04_Dokumentation/
• Test Results: Detailed measurement logs in 03_Integration und Test/
• Schematics: LTSpice simulation files in 01_Auslegung/
• Module Docs: Individual module documentation in 02_Module/

Future Enhancements

• Potential driver stage optimizations
• Alternative tube rolling experiments
• Enhanced measurement and characterization
• Mechanical refinements

License and Usage

This project represents a personal high-end audio development effort. All design files, documentation, and test results are provided for educational and reference purposes.

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Project Name: Endstufe Sarah MKII
Last Updated: November 2025
Status: Operational - Stereo Pair