Understanding and Selecting Chokes
Power supply chokes operate in high-voltage direct current (DC) filter networks (350V to over 500V DC). Filter capacitors store lethal electrical charges long after the amp is switched off. Always safely discharge power supply capacitors and verify 0V DC before touching internal chassis components.
Guitar Amplifier Chokes: Technical Guide & Calculator
Learn how power supply chokes work, how to calculate the exact current rating for your build, and how to select replacement chokes from the Evatco catalog.
Interactive Choke Requirement Calculator
Estimate the continuous current (mA) required through your choke based on output valve count and operating class.
Evatco Choke Catalog & Brand Cross-Compatibility
Chokes are often labeled by classic amp models (e.g., "Marshall 50W Choke" or "Fender Blackface Choke"). However, chokes are universal components. A choke designed for a Marshall can be used in a Fender, Vox, or custom build as long as the specs line up:
- Current Rating (mA): Must meet or exceed the total screen grid + preamp current draw.
- Inductance (H): Determines filtering stiffness (3H to 5H is traditional for Marshall/Vox; 4H to 10H for Fender).
- DCR (Resistance): Controls voltage drop. Lower DCR = higher screen voltage and tighter feel.
| Product / Part | Primary Specs | Associated Amps / Brands | Cross-Brand Applications |
|---|---|---|---|
| Hammond 158Q / 159Q | 5H, 150mA, 105Ω | Classic Fender (Bassman, Super Reverb) | Ideal replacement or upgrade for 50W 6L6/EL34 custom builds. |
| Hammond 159P / 158M | 10H, 125mA, 100Ω–125Ω | Fender Twin, Deluxe Reverb, Vibrolux | Excellent for high-headroom clean amps needing extra hum filtering. |
| Hammond 158L / 159L | 3H, 75mA, 100Ω | Marshall JTM45, 50W Plexi, Vox AC30 | Great for vintage EL34 or EL84 amps with punchy dynamic compression. |
| Classic Heavy Duty (Hammond 159M) | 15H, 100mA, 256Ω | Gibson, Traynor, High-Gain Preamp Mods | Perfect for hi-fi valve builds or high-gain preamps with severe hum sensitivity. |
How a Choke Works in Valve Amps
A choke (filter inductor) consists of a copper coil wound around an iron core. Installed in the high-voltage power supply between the rectifier and the screen grids/preamp, its primary role is to strip residual AC mains hum out of the DC supply.
AC Impedance (Hum Blocking)
Inductors offer high resistance (reactance) to high-frequency AC ripple voltage while allowing steady DC to pass through with minimal loss.
Transient Power Reserve
A choke stores magnetic energy. When the power section draws heavy transient current during loud notes, the magnetic field releases energy to stabilize screen voltage.
