- Tell 50 Hz heater hum from 100 Hz PSU ripple
- Recognise a ground loop by ear
- Know which guide solves each cause
Hum Hunter
Hum is not a vintage tax — it is a clue. Listen, see the spectrum, narrow the cause, and walk away with a usable bench reflex.
Hum Hunter
Listen to four hum samples. Match each one to its cause. Submit to reveal the typical fix and the spectral fingerprint.
Audio is generated live with the Web Audio API — no asset downloads. The spectrum view is fed by an AnalyserNode tap on each sample. Headphones reveal the lowest peaks more clearly.
Diagnose your amp from the room
Place your phone or laptop near the speaker, click below, and we will listen for ~3 seconds. The page is the analyser — no audio leaves your device.
Coaching tool, not a measurement instrument. Confidence reflects how clearly each signature stands above the room's noise floor — verify on the bench before you re-cap anything.
Tube amps can retain lethal voltages even unplugged. Filter capacitors hold their charge for minutes — sometimes hours — after power-off. This page teaches recognition and diagnosis. Internal repair is for qualified people only: discharge B+ caps before touching anything inside the chassis.
Find your hum
Five questions narrow the field before you open anything. Answer them on the bench, with your ears.
- Question 1Does the hum change when you turn the volume knob?
- Question 2Is the hum still there when nothing is plugged into the input?
- Question 3Does shorting the input jack to ground silence it?
- Question 4What pitch does the hum sound like?
- Question 5When is the hum loudest?
Answer all five to see the most likely cause and the relevant guide.
Four causes, four cures
Each card is a bench-side recipe — symptom, ordered checks, and the safety call-out that matters.
50/60 Hz heater hum
Smooth, pure low tone at mains frequency. Independent of input.
- Twist the heater wires tightly — one twist per cm — and dress them along the chassis edge, away from V1 grids.
- Provide a center-tap reference: real CT on the heater winding, or two 100 Ω resistors to ground (artificial CT).
- For cathode followers and SRPP stages, elevate the heater reference to +50–70 V DC to keep Vhk within spec.
- If hum persists in a critical preamp, switch to a regulated DC heater supply for V1 only.
100/120 Hz PSU ripple
Buzzy hum at twice mains. Often louder when output stage draws more current.
- Increase the first reservoir capacitor — within the rectifier's stated max — and verify ripple drops on B+.
- Add a CRC stage (resistor + cap) between the reservoir and the preamp tap. 1–10 kΩ + 47 µF buys 30+ dB of ripple rejection.
- For high-current rails, swap the resistor for a choke (CLC). The choke also softens fast load transients.
- Check that the high-current return from the reservoir cap goes directly to the star ground — never through preamp ground.
Ground loop
Hum appears or worsens when other gear is connected. Stack of harmonics on top of mains fundamental.
- Run all returns to a single star point at the chassis bolt. Never use the chassis as a signal-ground bus.
- Group local returns by stage (input · driver · output · PSU heavy current) and join the groups only at the star.
- Ground signal cable shields at one end only. Lift the input jack from the chassis and ground it at the input-stage cathode return.
- For a stubborn loop with external gear, isolate with a balanced DI or a 1:1 audio transformer rather than lifting safety earth.
Tube microphony
Bell-like ring or "ping" that decays after a tap. Often appears on low notes through a combo cabinet.
- Tap each tube gently with a pencil eraser at low volume. The one that rings is microphonic — replace it.
- Reseat the tube — a slightly loose pin can amplify mechanical pickup. Clean the socket pins with contact cleaner.
- Add a damping ring or rubber socket grommet on V1. For combos, decouple the chassis from the cabinet floor with felt washers.
- Distinguish tube ringing from cabinet rattle: pull the chassis out of the cab and run it on the bench. If silent, the cabinet is the culprit.
Move V1 — watch the hum
A top-down chassis: drag V1 closer or farther from the power transformer and watch the hum meter respond. Toggles let you twist the heaters, fit a tube shield, or rotate the OT — visual proof that layout discipline is worth real dB.
Illustrative model: PT field falls as 1/r² with distance; heater pickup is dominated by twist; OT coupling depends on relative orientation. Real-world numbers depend on transformer construction and signal-stage gain — but the rank order holds.
Four real-world cases
Patterns that come back over and over on the bench — short before/after sketches.
Keep this open at the bench
| Symptom | Likely cause | Frequency clue | First move |
|---|---|---|---|
| Smooth low hum | Heater hum | 50 / 60 Hz | twist heaters · check Vhk · re-route |
| Buzzy, octave higher | PSU ripple | 100 / 120 Hz | larger reservoir cap · CRC · check returns |
| Hum with connected gear | Ground loop | 50/60 Hz + harmonics | star ground · shield one-end · isolate |
| Bell-like ring | Microphony | 1–3 kHz resonance | tap-test · swap V1 · damp socket |
Questions builders ask
What is the difference between 50 Hz and 100 Hz hum?
50 Hz (60 Hz in the US) is the mains fundamental — typical of heater hum or magnetic pickup from a transformer. 100 Hz (120 Hz US) is twice the mains frequency — the signature of full-wave rectifier ripple. The pitch tells you whether to look at filaments and layout, or at the power supply filter.
Why does rectifier ripple appear at twice the mains frequency?
A full-wave rectifier flips the negative half of every cycle, producing two positive humps per mains cycle. The reservoir cap recharges twice per cycle, so the residual ripple is at 2 × f_mains — 100 Hz in Europe, 120 Hz in North America.
Can a ground loop sound like heater hum?
They share the mains fundamental, but a ground loop almost always brings a stack of harmonics (150, 250, 350 Hz…) on top — modern mains current is heavily distorted by switching power supplies on the same circuit. If the hum is pure, suspect heaters; if it has a crunchy texture, suspect a loop.
Does tube microphony always sound like ringing?
Often yes — it shows up as a damped resonance in the 1–3 kHz range after a tap or a low note. But a mildly microphonic tube can also colour the sound subtly — extra reverb, "thickening", or feedback at high gain — without an obvious ping. The tap test is decisive.
Should I open my amp to fix hum myself?
Only if you are trained in high-voltage work. Tube-amp B+ rails routinely sit at 300–500 V and filter caps hold a lethal charge after power-off. Use this page to recognise the cause; for the actual repair, a qualified tech can fix in minutes what an unprepared hobbyist could die for.
Why does hum change when I turn the volume knob?
Because the noise is entering the signal path *before* the volume control — typically the input jack, the input cable, or the first preamp tube and its surroundings. Hum that is constant regardless of volume is downstream of the control: power supply, output stage, or layout-induced pickup at a later point.
What tools help identify hum frequency?
A tuner app or smartphone spectrum analyser is enough to tell 50/60 Hz from 100/120 Hz. For a closer look, free apps like Spectroid, REW, or Audacity show the full hum spectrum on a phone or laptop. A scope across the speaker output gives the cleanest reading — but only when used by someone who knows how to probe a live amp safely.