Constant Current Sources
A resistor plate load limits your gain. A CCS removes that ceiling, letting the tube reach its theoretical maximum amplification factor μ. This is the single most impactful upgrade in tube circuit design.
Why a Current Source?
The fundamental gain limitation of resistor loads
With a resistor plate load R_L, the voltage gain of a triode is limited to:
Since R_L is always finite, the gain is always less than μ. For a 12AX7 with μ = 100 and a typical 100kΩ plate load (r_p ≈ 62.5kΩ), the gain is only about 61.5 — far from the theoretical 100.
A constant current source presents a very high dynamic impedance (100kΩ to 10MΩ+). As R_L → ∞, the gain approaches μ. In practice, a well-designed CCS gets you to 95–99% of μ.
Ref: Horowitz & Hill, "The Art of Electronics" 3rd Ed. §2.2.6 — Morgan Jones, "Valve Amplifiers" 4th Ed. Ch.2
JFET CCS Designer
Self-biased JFET current source — enter your JFET parameters and desired current
A JFET (like the J310, 2SK170, or 2N5457) makes an excellent CCS. With just one resistor R_S from source to ground, the JFET self-biases to a stable current. The square-law equation gives us:
Loadline Comparison
Resistor load (diagonal) vs CCS (horizontal)
CCS Topologies Compared
Choose the right CCS for your application
| Type | Output Z | V max | Advantages | Drawbacks | Parts |
|---|---|---|---|---|---|
| JFET | 100k–500kΩ | 25–40V | Simple, low noise, 1 resistor | Low voltage, IDSS spread | J310, 2SK170, 2N5457 |
| Depletion MOSFET | 500k–5MΩ | 300–500V | High voltage, excellent rout | Higher noise than JFET | DN2540, LND150, IXTP01N100D |
| Pentode | 40k–1MΩ | 300–500V | Native HV, period-correct | Heater needed, noise | EL84, EF86, 6SJ7 |
| Cascode (JFET) | 10MΩ+ | 300V+ | Ultimate rout, bootstrapped | Complex, 2+ devices | DN2540 + J310 |
| LED-biased BJT | 1–10MΩ | 300V+ | Very stable, cheap | More noise, Vce headroom | MPSA42 + LED |
Practical CCS Design
Key considerations for real-world implementations
1. Compliance Voltage
The CCS needs a minimum voltage across it to regulate. For JFETs: V_compliance ≥ |V_GS(off)| + 2V. For depletion MOSFETs: typically 5–10V. Budget this from your B+ when designing the stage.
2. Thermal Stability
JFETs have a negative temperature coefficient for IDSS — current decreases as temperature rises, providing natural thermal stability. MOSFETs and BJTs need more care. An LED-biased BJT CCS uses the LED's ~−2mV/°C to track the BJT's V_BE drift.
3. Startup & Transients
At power-on, the CCS may source full current before the tube warms up, leading to excessive voltage across the device. Add a startup resistor in parallel (1–2MΩ) to limit the initial voltage, or use a time-delay circuit.
4. Noise Considerations
A CCS adds its own noise. JFETs are quietest (1–5 nV/√Hz), followed by depletion MOSFETs. Pentodes and BJTs are noisier. In critical first stages, a JFET CCS is preferred. The noise contribution is proportional to gm_CCS/gm_tube.
Testez vos connaissances
What is the main advantage of a CCS plate load over a resistor?
References
- Paul Horowitz & Winfield Hill, The Art of Electronics, 3rd ed., Cambridge University Press, 2015. ISBN 978-0521809269Canonical reference for analog design — covers tubes in Ch. 2.4 & Ch. 3.
- Morgan Jones, Valve Amplifiers, 4th ed., Newnes, 2012. ISBN 978-0080966403Modern engineering treatment of tube audio design.