⚡
Supply
Capacitor voltage
Time to safe
53.2 s
Time constant τ
22 s
Stored energy
10.1 J
Bleeder power
920 mW
use ≥ 2 W
Bleed current
2.05 mA
R for 60 s to safe
248 kΩ
Never trust a bleeder alone. Always measure with a meter before touching; resistors fail open, and capacitors can recover charge (dielectric absorption).
Engineering Notes
How it works
Filter capacitors in a valve amp can hold a lethal charge for hours after power is removed. A bleeder resistor across the supply drains them automatically. The voltage decays exponentially:
V(t) = V₀ · e^(−t / RC) t_safe = RC · ln(V₀ / V_safe)
After one time constant τ = RC the voltage has fallen to 37%; after five, below 1%. A lower resistance discharges faster but wastes power continuously while the amp is on and runs hot:
P = V₀² / R E_stored = ½·C·V₀²
A good compromise for most amps is 100–220 kΩ at 2–3 W, giving a safe voltage within a minute or two. Stored energy above ~10 J deserves serious respect: it can deliver a fatal shock or vaporise a screwdriver tip.