Safety: amplifiers
PedalScope measures pedals only. It does not measure amplifiers, and there is no amplifier mode in this version. This page exists because the most dangerous mistakes are the ones made early, with pedal-measuring habits and an amp within reach.
If you take nothing else from this page, take the two rules that hold for anyone with an amp near this rig:
Never run a tube amp without a load connected. An unloaded tube amp under signal can arc across and destroy its own output transformer — often silently, sometimes not immediately, always expensively.
Never connect a speaker output directly to your interface. Speaker outputs deliver tens of volts into inputs built for about four. One patch cable from speaker-out to line-in can end an interface.
Why speaker outputs are different
Everything in the pedal loop lives around a volt. An amplifier’s speaker output is a power circuit:
A 15 W amp pushed into overdrive puts roughly 11 V RMS across an 8 Ω speaker; a 50 W amp roughly 20 V RMS, with peaks higher still. Line inputs are typically built for 2–4 V.
The danger runs both directions: the interface’s line input is destroyed by the amp’s voltage, and a tube amp that loses its load (a speaker cable used as the “measurement” path, a dummy jack, a broken cable) can destroy itself.
There is no “just for a second, at low volume.” Master volumes drift, channel switches boost, and a sweep is by design a signal that explores the amp’s range.
What a proper amp rig looks like
Two jobs, and they are not the same job — one device may do only the first:
The load protects the amp. A tube amp driven with no load can arc across and destroy its own output transformer. The load must match the amp’s impedance tap and carry its full rated power.
The line-level tap protects the interface. The load’s terminals carry the amp’s whole output voltage — the 11 V, 20 V and higher quoted above, into inputs built for 2–4 V. Absorbing that power is not the same as producing a signal anything can measure.
So a proper amp rig puts a declared load device between the amp and the interface — one of:
Reactive load box with a line out (best: presents a speaker-like impedance, so the amp behaves like itself).
Resistive dummy load with a line out (safe, though the amp’s feel and measured behavior shift slightly — a resistive-load fingerprint is a fingerprint of a slightly different amp).
Attenuator or load box in front of a real speaker, using its line/DI tap.
Take the signal from the line out, and from nowhere else. Never connect an interface input across the load’s terminals or to the amp’s speaker output. Those nodes carry speaker level whatever is absorbing the power — the load is there to swallow it, not to tame it — and one patch cable from either ends an interface. The line out is a separate, attenuated tap inside the load device, and that attenuation is the whole reason the interface survives.
A load device with no line out cannot be measured through. Dummy loads sold for silent practice often have no tap at all. Such a load protects the amp perfectly and still gives PedalScope nothing to read, and there is no safe way to improvise a tap across it — the same rule as a speaker jack, for the same reason.
The line-out pad value matters too: know what attenuation it gives, and confirm it at low power before anything loud. PedalScope never emits into an amp path — it has no amp path.
The gentler hazards
Not destructive, but worth knowing with an amp near the bench:
Effects-loop sends (“preamp only” measurements) can run several volts hotter than line level on some amps. Start input gains low.
Ground loops: an amp and an interface on different outlets invite hum into every measurement. Same outlet or power strip; use a transformer- isolated DI on the line path if hum persists; never lift the amp’s safety ground.
Heat and duty cycle: high-drive measurements dissipate the amp’s full power into the load as heat. Load boxes and output tubes both appreciate breaks between long runs.
What PedalScope measures
Measure pedals and other line-friendly boxes (preamp pedals, DI-out devices at line level). If a device’s output is a speaker jack, PedalScope does not measure it — there is no safe improvisation around that rule.