Amp Troubleshooting

Tube amp faults split cleanly into a handful of recognizable patterns — a blown fuse, missing B+, 120Hz hum, red-plating, motorboating — each with its own specific cause and its own specific safe way to check for it. This is the only diagnostics chapter on this site where getting the procedure wrong, not just the diagnosis, can genuinely hurt you, and it starts with the safety protocol every other step in this chapter depends on.

This is the only troubleshooting on this site where a procedural mistake, not just a wrong diagnosis, can seriously hurt you. Read Safety in full before anything in this chapter — the current thresholds, the capacitor-discharge procedure, and the one-hand rule it covers aren’t background reading here, they’re the actual gate every check below passes through first. Minimum protocol for everything that follows: unplug the amp before opening the chassis, discharge the filter capacitors with a proper resistor tool, verify near-zero voltage with a multimeter before touching anything, work with one hand while probing live circuits, never wear a grounding wrist strap near a powered chassis, and use a Variac with a current meter when first powering up a repaired amp. If any of that isn’t comfortable to execute exactly, the correct next step is a professional repair technician, not pushing ahead anyway.

The five amp faults that account for most complaints

A blown fuse means “find the short,” not “fit a bigger fuse”

A fuse that blows the instant the amp is switched on means something downstream is presenting close to a dead short, and the diagnostic sequence is entirely about finding that short safely rather than guessing: with the amp unplugged and filter capacitors verified discharged, measure resistance from the B+ rail to ground. A reading near 0Ω means a filter capacitor has failed shorted or a rectifier diode has failed shorted — desoldering the first filter capacitor and re-measuring tells you which: if the short persists with that capacitor disconnected, the rectifier diodes are the culprit and each should be checked individually in diode mode; if the short disappears, that capacitor was the fault. Never replace a blown fuse with a higher-rated one to “see if it holds.” The fault that blew it is still there, and a bigger fuse just lets more current flow before something (possibly the transformer) fails instead.

No sound with the pilot light on: trace B+ before tracing signal

Tubes glowing and a pilot light lit doesn’t mean the amp has power where it matters — B+ specifically has to be present and correct before signal tracing makes sense at all. Measure B+ after the rectifier first; if it’s low or absent, the fault is in the power supply chain covered in Rectification and Power Supply, and no amount of preamp signal-tracing will find a fault that isn’t there. With B+ confirmed present, signal injection at the input jack (through a coupling capacitor to keep DC off the injected signal) followed by checking each preamp stage’s output in sequence localizes the fault the same way it would in a pedal — a stage whose plate voltage sits at the full B+ value is a tube that’s cut off, not passing any signal at all.

120Hz hum points specifically at the filter capacitors — usually

A steady, low-pitched hum that doesn’t track volume is close to always a power-supply ripple problem: measuring ripple on the B+ rail with a scope in AC-coupled mode gives a direct answer, and a reading over roughly 5V peak-to-peak means the filter capacitors have dried out and need replacing — a genuine, common failure mode as electrolytic capacitors age. If ripple measures low but the hum is still present, the next place to look is the heater winding’s ground reference; an ungrounded (or improperly referenced) 6.3V heater winding can hum on its own even with a perfectly clean B+ rail, and the fix is grounding the winding’s center tap or, on designs without one, adding two resistors from each side of the winding to ground to create a virtual center tap.

Red-plating is an emergency, not a diagnostic curiosity

An output tube’s plate glowing visibly orange means it’s drawing dangerously excessive current and overheating in real time — the correct first action is powering the amp off immediately, not continuing to play through it while you think about why. Once safely powered down, the two most likely causes are a bias supply that’s failed and left the tube running far too hot (measuring bias voltage against the amp’s specified range identifies this directly) or the tube itself failing internally (a tube that red-plates even with correctly measured bias needs replacing, followed by a full re-bias). Red-plating that goes unaddressed risks not just that one tube but the output transformer it’s driving, since the transformer’s core can saturate under the resulting excess current.

Motorboating: a power-supply stability problem wearing an oscillation costume

A slow, rhythmic “putt-putt” sound is a power supply that can’t hold the B+ rail steady under the preamp’s own current demands, not a signal-path oscillation in the usual pedal-troubleshooting sense — the fix is almost always in the preamp filter capacitors specifically (not just the main supply filter), sometimes resolved by increasing the value of the first preamp filter stage, and it’s worth checking the output tubes’ screen grid resistors too, since an open screen resistor is a separate, real cause of the same symptom.

Common mistake: assuming the amp is safe because it’s unplugged and quiet

Every fault pattern in this chapter assumes the discharge-and-verify procedure from Safety has already happened before any measurement or component swap — an amp that’s silent, unplugged, and cold can still have filter capacitors holding a lethal charge, and skipping verification because “it’s obviously off” is the single most common way amp troubleshooting turns dangerous. Confirm the voltage is actually low before treating the chassis as safe to touch, every time, regardless of how confident that feels unnecessary in the moment.