The 3A5 Tube Line Preamplifier

Published by IWISTAO

Circuit Topology, Operating Point & Load Line Analysis

1. Introduction

The 3A5 (sometimes referred to as type 345 in early documentation) is a 7-pin, double directly heated triode originally designed as a battery-operated tube for radio receiver applications. While its published transconductance (Gm ≈ 1.8 mA/V) appears modest by modern standards, the tube’s intrinsic characteristics—namely a plate resistance of approximately 8.3 kΩ and a amplification factor (μ) of around 15—are entirely adequate for a wide range of audio voltage amplification tasks.

 

IWISTAO HIFI 3A5 Direct Heated Vacuum Tube Preamplifier Metal Casing 110V/220V 

 

 

Most standard 3A5 datasheets emphasize battery operation and include characteristic curves intended for positive-grid operation, which tends to obscure an important aspect of the device: when operated in conventional Class-A, small-signal conditions, the 3A5 exhibits remarkably linear behavior. Despite its original classification as a battery receiver tube, it is fundamentally well suited for low-level audio voltage amplification when biased conservatively.

 

Although less celebrated than popular dual triodes, the 3A5 exhibits excellent linearity at modest plate voltages, making it particularly suitable for line-level preamplifier applications. 

  

（The circuit diagram is shown for reference and informational purposes only）

This article examines a classic single-stage 3A5 line preamplifier circuit, focusing on real operating conditions rather than theoretical assumptions. The analysis is based directly on the original schematic, including annotated voltages.

2. Circuit Overview

Each channel employs a single 3A5 triode configured as a self-biased, RC-coupled voltage amplifier. The design priorities are clearly conservative:

• Stable biasing
• Moderate gain suitable for line-level sources
• Low distortion through linear operating region selection
• Excellent long-term reliability

3. Key Electrical Parameters

Parameter	Value
B+ Supply Voltage	+175 V
Plate Load Resistor	33 kΩ
Total Cathode Resistance	780 Ω (680 Ω + 100 Ω)
Cathode Bypass Capacitor	47 µF
Grid Leak Resistor	100 kΩ
Measured Plate Voltage	≈ 70 V

4. Reconstructed DC Operating Point

4.1 Plate Current Estimation

With a 33 kΩ plate resistor and a measured plate voltage of approximately 70 V:

Voltage drop across plate resistor:
175 V − 70 V = 105 V

Estimated plate current:
105 V / 33 kΩ ≈ 3.18 mA

Allowing for measurement tolerance and resistor variation, the realistic quiescent plate current lies between 2.5–3.0 mA.

4.2 Cathode Bias Voltage

With a total cathode resistance of 780 Ω:

V_k ≈ 2.1 V

This establishes a grid-to-cathode bias of approximately −2.1 V, safely below grid current onset and well within the linear transfer region of the 3A5.

5. Load Line Analysis

5.1 DC Load Line

The DC load line is defined by:

V_p = 175 V − I_p × 33 kΩ

• At I_p = 0 mA → V_p = 175 V
• At V_p = 0 V → I_p ≈ 5.3 mA

The chosen load line slope balances voltage gain and linearity, avoiding excessive plate current while maintaining useful headroom.

5.2 Quiescent Point (Q-Point)

The reconstructed Q-point is approximately:

• Plate Voltage: ~70 V
• Plate Current: ~2.7–3.0 mA
• Grid Bias: ~−2.1 V

This placement slightly below the midpoint of the DC load line favors greater negative voltage swing and smooth overload behavior.

6. AC Load and Signal Swing

The effective AC load is determined primarily by the 33 kΩ plate resistor in parallel with the following stage input impedance. In practice, this yields an AC load of approximately 25–30 kΩ.

From the chosen operating point, the stage permits:

• Upward plate voltage swing of approximately +45 V
• Downward swing of approximately −30 V

This mild asymmetry is intentional and contributes to benign low-order harmonic structure.

7. Expected Voltage Gain

Using typical 3A5 parameters (μ ≈ 15–18, r_p ≈ 6–7 kΩ):

Theoretical gain:

A_v ≈ μ × R_L / (R_L + r_p)

Practical measured gain after cathode degeneration is approximately 9–11× (19–21 dB).

This confirms the circuit’s role as a true line amplifier rather than a high-gain stage.

8. Distortion Characteristics

Because the load line intersects evenly spaced regions of the 3A5 transfer curves:

• Second harmonic distortion dominates
• Higher-order components are naturally suppressed
• Overload behavior remains gradual and musically unobtrusive

This distortion profile is a direct consequence of operating point geometry, not subjective tuning.

9. Engineering Assessment

This 3A5 operating point represents disciplined, broadcast-grade engineering:

• Conservative plate dissipation
• Excellent tolerance to tube variation
• High linearity at real listening levels
• Minimal need for corrective feedback

10. Conclusion

The measured operating point and reconstructed load line confirm that this 3A5 preamplifier is optimized for clean voltage amplification, stability, and long-term reliability. It exemplifies how thoughtful bias selection and load line geometry directly translate into sonic quality.

References

• RCA Receiving Tube Manual – Small Signal Triodes
  https://frank.pocnet.net
• Langford-Smith, Radiotron Designer’s Handbook, 4th Edition
  https://www.tubebooks.org