Metal Guitar Sound & Recording Tips

What the amp and the speaker are actually doing

Amp (16)

Phase inverter
A long-tailed pair is the most efficient, linear and balanced-sounding splitter; older paraphase designs create smooth, early distortion with lots of compression.

an amp that stays composed right up to the power tubes, or one that softens well before them

Cathode bias
Self-regulating: rising cathode current raises cathode voltage, which increases negative grid bias and throttles the rise. Under signal it shifts progressively deeper into Class AB, which is the actual mechanism behind its compression.

an amp that squashes and gives way on its own as you push it

Fixed bias
Grid voltage held constant from an external supply regardless of signal, with no self-compensation — more headroom and more output from the same tubes.

a stiffer, louder, later-breaking amp with the attack intact

The 70 percent rule
In Class AB, peak average plate dissipation happens at mid-swing, about 30% above idle. Biasing idle to 70% of maximum rated dissipation leaves exactly enough margin. An EL34 rated 25 W at 400 V works out near 44 mA. It is a fallback for when the plate load is unknown, not a universal target.

not audible on its own — it is the difference between an amp that lasts and one that eats tubes

Class A in practice
Aiken's AC30 measurements show both output tubes shutting completely off for a considerable portion of the cycle at full output: the positive swing reaches 138 mA where Class A would require −43 mA and the circuit reaches 0 mA. Class A at idle, Class AB the moment it is pushed.

a designation that describes how hard you are playing, not what you bought

Negative feedback
A small amount of output-transformer secondary signal fed back to the phase inverter — typically 6–10 dB in guitar amps. It flattens response, reduces distortion inside the loop and raises damping factor.

a tighter, cleaner, more controlled amp with less character of its own

Presence and depth
They work by reducing that feedback at high and low frequencies. The maximum boost available equals the amount of feedback in the loop, so a 6 dB feedback amp offers at most about 6 dB of presence — and an amp with no global feedback cannot have one that behaves this way at all.

top end and low end opening up, along with a change in how tightly the amp grips the speaker

Rectifier drop
A GZ34/5AR4 drops about 10 V at 100 mA and 16 V at 200 mA; the range runs from roughly 10 V for a 5AR4 to about 60 V for a 5Y3GT; silicon drops under a volt.

stiff and immediate with solid state, soft and elastic with the saggiest tube types

Push-pull cancellation
The two output tubes' currents flow in opposite directions through a centre-tapped primary, so their signal sums while even-order harmonics cancel, leaving predominantly odd-order content.

a firmer, more austere distortion with the sweetness subtracted

Single-ended
No cancellation mechanism exists, so even-order content passes straight through, and asymmetric clipping under overload emphasises it further.

a warmer, sweeter, softer breakup at a fraction of the power

Transconductance
An EL34's gm of about 11,000 µmho is roughly double a 6L6GC's 4,700–6,000, so it saturates and compresses on a smaller input signal.

one tube type giving way earlier than the other in the same circuit — a real difference, though far smaller than the genre labels imply

Output transformer
Impedance scales with the square of the turns ratio. Real Fender primaries run 4.3–6.5 kΩ, not tens of kΩ. A measured single-ended unit held within 1 dB from 53 Hz to 25 kHz, then −2 dB at 40 Hz and −6.7 dB at 20 Hz, with 32% THD at 20 Hz — and mostly second harmonic.

the low end going soft and thick rather than harsh when the transformer runs out of core

Preamp tube mu
A 12AX7 has an amplification factor of 100, a 12AT7 about 60, a 12AU7 just 17. Swapping an AX7 for an AU7 cuts stage gain by roughly 5.9 times.

a lower-gain, higher-headroom amp from the same circuit

Ghost notes
Non-harmonic tones riding on the fundamental, produced by 120 Hz supply ripple when filtering is inadequate. Too much sag is a defect, not a feature.

a faint, unrelated pitch hanging behind low notes

Amp output impedance
Tube amps present 1–10 Ω; solid state under 0.1 Ω. Driving a speaker whose impedance rises to five times its rating at resonance from a high source impedance produces a large output rise there.

a mid-forward, resonant voicing from the tube amp that the solid-state amp flattens out

Wattage and headroom
+3 dB doubles acoustic power, +6 dB doubles pressure, and roughly +10 dB doubles perceived loudness — which needs ten times the power. Fifteen watts to a hundred and fifty buys about one perceived doubling.

not how loud it gets, but how loud it stays clean

Speaker (15)

Impedance curve
A measured 8 Ω Eminence reads exactly 8 Ω at 50, 200 and 500 Hz, peaks near 70 Ω just above 100 Hz, and climbs to about 50 Ω by 20 kHz. A V30 rated 8 Ω measures 7.3 Ω at DC.

the load being a curve rather than a number, which is why what the amp does depends on frequency

Band-limiting
A guitar speaker is specified roughly 70–5,000 Hz. Distortion generates harmonics far above that which are simply never heard.

no fizz at all through a real cabinet, because the speaker never reproduces it

Mechanical non-linearity
Guitar drivers are deliberately non-pistonic above roughly 500 Hz, and at higher excursion produce cone cry and edge yowl.

the speaker adding its own vocal, ragged distortion on top of the amp's

Thermal power compression
Heating a copper voice coil from 20 °C to 200 °C raises its DC resistance about 72% and drops sensitivity 4.7 dB.

the last watts delivering less than the first, so the amp stops getting louder as it heats up

Sensitivity
Celestion's twelve-inch range spans 96–100 dB at 1 W/1 m, and differences under 2 dB are barely perceptible. Four decibels of sensitivity beats doubling amplifier power.

the speaker choice deciding the volume far more than the wattage on the head does

Published speaker specs
G12M Greenback 96 dB / 20 W; Vintage 30 100 dB / 60 W; G12M-65 Creamback 97 dB / 65 W; G12H-75 100 dB / 75 W; Alnico Blue 100 dB / 15 W. All five share a 75 Hz resonance and a 5,000 Hz top.

the real published differences being loudness and how hard it can be driven before it compresses — not frequency range

Open back vs closed back
Front and rear waves interfere below the dipole peak, rolling off at about 6 dB/octave. Sealing the back raises the driver's resonance by roughly two and a half semitones.

open backs airy, omnidirectional and mid-punchy; closed backs directional, tighter and deeper

Multiple drivers
Coherent sources give +3 dB from doubled radiated power alone, and up to +6 dB where spacing is small relative to wavelength.

a four-by-twelve behaving like one very large speaker, with far more weight from the midrange down

Mic placement
Moving an SM57 from dust cap to cone edge measures about 5 dB of change below 500 Hz, 8 dB at 1.6 kHz and 6 dB at 3.5 kHz.

an enormous tonal move — brighter at the centre, fuller and darker at the edge — from a few centimetres

Impulse response (IR)
A linear time-invariant capture: frequency response, phase, inter-speaker cancellation, diffraction and the whole mic and converter chain, frozen into one filter. It categorically cannot capture distortion, dynamic compression or cone breakup.

an exact snapshot of one cabinet at one volume, which never changes no matter how hard you play

Reactive vs resistive load
A reactive load presents a speaker-like varying impedance so the output stage behaves normally; a resistive load presents a flat impedance and, as attenuation rises, makes the tone more compressed and dark.

resistive loads recording far too bright and buzzy; reactive loads keeping the amp's own voice

Attenuator limits
Even a good reactive load cannot reproduce excursion-dependent breakup or thermal compression, because the cone is not actually moving that much air.

the electrical half of loud without the mechanical half

Power scaling
Reduces the plate supply voltage delivered to the power tubes before the output transformer, shrinking the whole operating envelope rather than dissipating output as heat.

a small amp's behaviour from a big amp, rather than a big amp turned down

Impedance mismatch
Mismatches within double or half the rated impedance are generally considered safe; beyond that, too high a load stresses the transformer and tubes with excess voltage and too low pulls excess current.

a slightly looser or slightly stiffer feel, until it stops being a tone question and becomes a repair

Break-in
Klippel measured about 30% total suspension stiffness loss on a complete driver, roughly 85% of it break-in, implying about a 16% drop in resonant frequency. Real and permanent — and concentrated at low frequencies, not in the 1–5 kHz breakup region that gives a guitar speaker its voice.

a modest loosening of the low end, not the transformation the folklore promises

The djent direct rig

2007 onward — The noise gate is a structural element rather than a utility — the silence between chugs is the rhythmic figure.

• Chain: seven- or eight-string → noise gate → tight boost → high-gain preamp → load box → impulse response → front of house
• Mechanisms: Two-gate detection, Pre-clipping high-pass, Impulse response, Reactive vs resistive load, Multiscale
• Gear: extended-range multiscale instruments, Fractal, Helix, Quad Cortex, Two Notes load boxes
• Appears in: pairings 07, 15, 21, 25, 36
• Correction: The word was coined by Fredrik Thordendal of Meshuggah as an onomatopoeia for the sound. Also worth knowing: an IR is a linear capture and cannot reproduce cone breakup or compression at all, so a direct rig is missing exactly the parts of a loud cabinet that change with how hard you play.