Safety

A 9V pedal circuit cannot push enough current through dry skin to be dangerous — a tube amp's power supply can kill you, silently, hours after it's been unplugged, because its filter capacitors keep holding a lethal charge until something actually discharges them. This chapter covers the current thresholds that actually determine shock severity, the capacitor-discharge procedure every amp chapter on this site depends on, and the one-hand rule for working on any live circuit above 50V.

Everything else on this site assumes a reader can safely get their hands on the circuit being discussed. That assumption holds without a second thought for a 9V pedal — see Power Supply Conventions and Polarity Safety for what actually can and can’t go wrong at that voltage. It does not hold for a tube amp, and the difference isn’t a matter of degree — it’s the difference between a circuit that can startle you and one that can kill you, and knowing which one is in front of you, and what to do about it, matters more than any other chapter on this site.

Shock severity is about current, not voltage

The number that actually determines how dangerous a shock is isn’t the voltage — it’s how much current ends up flowing through the body, and specifically through the heart. Sensation starts around 0.5-1mA. Painful shock and the “can’t let go” muscle-contraction threshold sits around 5-10mA for many people. Ventricular fibrillation — a heart rhythm that stops effective pumping — becomes a real risk starting around 20-50mA, and is likely without immediate intervention above 50-100mA. A 9V pedal supply, even shorted directly against skin, cannot push current through the body’s own resistance (roughly 10-100kΩ for dry skin) anywhere near that range — it can startle, burn slightly, or damage a circuit, but it cannot deliver a lethal shock. A tube amp’s 300-500V DC supply absolutely can, if the current path happens to run through the chest — which is exactly the scenario the “one hand in your pocket” rule described below exists to prevent.

Why a tube amp is dangerous even unplugged

A tube amp’s filter capacitors store real electrical energy, and they keep storing it after the amp is switched off and even after it’s unplugged from the wall — the stored charge only goes away through a bleed resistor (if the design includes one) or through deliberate discharge, not through time alone. A 100µF capacitor charged to 400V stores about 8 joules — more than enough to cause severe muscle contraction, burns, or worse if the discharge path runs through the chest, and that capacitor can hold a dangerous charge for hours, sometimes longer, after the amp was last powered.

The discharge procedure that has to happen before touching anything

Before opening any tube amp chassis: unplug it from the wall, then wait at least five minutes for any bleed resistors to do their job. Then use a proper discharge tool — a power resistor (commonly 10kΩ, rated 5-10W) with insulated leads and clips, never a bare screwdriver blade shorted across the terminals, which creates an arc and can weld itself to the terminals. Connect the resistor across each filter capacitor’s terminals, matching polarity, and hold the connection for ten to fifteen seconds. Then verify — don’t assume — by measuring each capacitor’s terminals directly with a multimeter set to DC volts, confirming a reading below roughly 10V before touching anything else in the chassis. If every node can’t be reached and measured at once, discharge and verify them individually; a node that still reads 50V can deliver a genuinely painful shock even after the “big” nodes have been handled.

The one-hand rule, and why an ESD wrist strap is dangerous here

When probing a live circuit above roughly 50V, keep one hand in a pocket or behind your back rather than resting anywhere on the chassis. The reason is specifically about current path: if the probing hand contacts something live while the other hand rests on a grounded chassis, the current’s path runs directly across the chest, through the heart. With one hand tucked away, an accidental contact instead sends current down through the body to the feet — a meaningfully less dangerous path, even though it’s still a shock worth avoiding entirely. This is also exactly why an ESD (electrostatic discharge) wrist strap — a deliberate low-resistance connection from your body to ground, entirely appropriate protection for handling static-sensitive digital ICs — becomes actively dangerous around a high-voltage circuit: it guarantees a low-resistance path from any live node your other hand touches straight through your body to ground. Never wear a grounding wrist strap while working inside a powered or recently-powered tube amp chassis.

Fuses: a safety device, not an inconvenience to bypass

A fuse’s job is to fail before the wiring or components around it do, and replacing a blown fuse with a higher-rated one, or bypassing it with a wire, removes exactly the protection that fuse existed to provide — the underlying fault that blew it in the first place is still there, and now nothing stops it from causing more damage or starting a fire. Match fuse type to the job: slow-blow (time-delay) fuses tolerate the brief inrush current of a transformer or a charging filter capacitor without nuisance-blowing, while fast-blow fuses are for protecting sensitive semiconductors where even a brief overcurrent is a problem. Match the voltage rating to the circuit, not just the current — a fuse rated below the circuit’s actual operating voltage can arc internally when it blows, failing to fully interrupt the circuit exactly when it’s needed most.

Common mistake: treating “unplugged” as “safe”

The single most dangerous assumption in this entire chapter is believing that a tube amp with its power cord removed is automatically safe to open and touch. It isn’t, and the reason isn’t subtle or theoretical — it’s the filter capacitors, sitting there fully charged, indifferent to whether the wall outlet is connected, as covered in Rectification and Power Supply. Every amp-design chapter on this site that involves opening a chassis, including the fault-tree diagnosis in Amp Troubleshooting, links back to the discharge procedure above for exactly this reason: skipping it because the amp “is unplugged so it should be fine” is the specific mistake that turns a routine repair into a medical emergency.