Quick answer: The cable from a guitar amp head to a cabinet is a speaker cable, not an instrument cable. It carries the amplifier output current into a four, eight or sixteen ohm load through a mono TS connection, with the same conductor on tip and sleeve at both ends. Never unplug it while the amp is running, because an open circuit gives the output stage no load, and some designs do not tolerate that.
The head is on the cabinet, the lead is somewhere in the van, and the nearest spare is an instrument cable that looks identical. This is the single most common wrong cable in a guitar rig, and the failure is not subtle: the amp sees a load it was never designed to drive, or it sees nothing at all. Both are avoidable with one rule about what belongs in that jack.
What leaves the amplifier
Inside the head, an output stage drives a transformer or a direct output into a loudspeaker load. The numbers are not small. A hundred watt head at full output into four ohms pushes about five amps through the cable, and the voltage across the load is in the tens of volts rather than the millivolts of a pickup signal.
Mono TS wiring at both ends
A guitar speaker connection is unbalanced and uses two conductors, one on the tip and one on the sleeve. Both ends are wired the same way, so the tip at the head reaches the tip at the cabinet. There is no shield doing signal duty here, and no ring contact involved unless the jack is a switching type that disconnects an internal speaker.
Tip and sleeve, not tip and ring
A stereo or balanced cable has a ring contact that may not connect to anything at the cabinet end. Using one leaves the return path uncertain and can short the output if the plug is inserted halfway. Check that the plug is mono before it goes into an amplifier output.
Open circuit danger
Running a tube amplifier with no load connected is the classic way to damage an output transformer. The output stage expects a load to absorb the energy it produces, and with the circuit open the voltage across the transformer can rise well beyond normal operating conditions. Solid state designs vary in how they behave, so the safe rule is the same for both.
Why a tube amp dislikes no load
Switch off or standby before changing cables, and never pull a speaker lead to silence the amp. If the cabinet has a jack that disconnects the internal speaker when an external one is inserted, understand which jack does what before the first gig rather than during it.
| Amp rating | Cabinet load | Current at full power | Cable duty |
|---|
| 15 watts | 8 ohm | about 1.4 amps | eighteen gauge is ample |
| 50 watts | 8 ohm | about 2.5 amps | sixteen gauge, short run |
| 100 watts | 4 ohm | about 5 amps | fourteen gauge, sealed ends |
| 300 watts | 4 ohm | about 8.7 amps | twelve gauge, locking plug |
Gauge and length for a head to cabinet run
Most head to cabinet runs are under two metres, so sixteen or fourteen gauge is generous and twelve gauge is overkill for the length. The bigger issue is mechanical: the plug must survive being stepped on, and the jacket must not stiffen in a cold van. Buy for the abuse as much as for the resistance.
Reading the jack plate before connecting
Cabinet jack plates are labelled with impedance and sometimes with two jacks wired in parallel. Two eight ohm cabinets on parallel jacks present four ohms, not eight. Read the plate, set the head to match, and write the pairing on a label so the next person does not guess.
Practical scenarios
- Combo with extension cab: the internal speaker may stay connected through a switching jack. Confirm the total load first.
- Head on a four by twelve: a one metre run needs a locking plug and a floor proof jacket more than gauge.
- Attenuator between head and cab: two speaker cables, both at speaker level. Instrument cable anywhere in that chain is a fault waiting.
- Borrowed cabinet at a gig: check the impedance label first. A mismatched load is worse than a longer cable.
FAQ
Can I use an instrument cable from head to cabinet?
Not for anything beyond a quiet test. The thin centre conductor and the shield are not sized for speaker current, and the shield becomes a resistive path that heats. Use a cable with two equal conductors.
What happens if the speaker cable comes out while playing?
The amplifier briefly drives an open circuit. On a tube amp that can stress the output transformer, and on some solid state designs it can trigger protection. Switch off before touching the cable.
Does a longer speaker cable change the tone?
Longer cable adds resistance in series with the load, which slightly reduces the level the speaker receives, most of all into low impedance loads. For a normal head to cabinet length the added resistance is small; for a ten metre run, choose a heavier gauge.
Next step
Keep one labelled speaker lead with the head and the problem mostly disappears. Our 1/4 inch instrument cable page explains the shielded lead you should not use here, the quarter inch guitar cable notes cover plug and strain relief choices, and the guitar amp speaker cable page covers the load side in more detail.
See the matching cables and adapters
