Boost Pedal in Front of a Tube Amp: What It Does and How to Set It
A boost pedal in front of a tube amp raises the level of your guitar signal before it reaches the first preamp tube. On a clean amp that mostly makes you louder; on an amp that is already breaking up, it pushes the preamp into more saturation, sustain and harmonics. It does this without adding much character of its own, so you still hear your amp.
What a boost pedal actually does to a tube amp
Your guitar puts out a fairly small signal. The first gain stage in the amp, usually one half of a 12AX7, amplifies that signal and passes it on to the next stage. Every stage has a point where it can't swing any further and starts to round off the peaks. That rounding off is the warm, compressed overdrive people love in tube amps.
A boost simply makes the signal bigger before it gets there. The tubes now reach their limit sooner and harder, so you get more drive from the same amp settings. A clean boost raises the whole frequency range. A treble boost or mid boost lifts part of the range, which can tighten the low end or help a solo cut through. If you want to dig deeper into where tube distortion comes from, read preamp vs power amp distortion and the article on what a 12AX7 does in a guitar amp.
Three ways to use a boost in front of the amp
On a clean amp: more volume
If your amp has lots of headroom, a boost mostly acts as a volume jump. That is handy for a solo when you play clean, or to bring a quiet guitar up to the level of a hotter one.
On the edge of breakup: more gain
This is the classic use. Set the amp so it just starts to crunch when you dig in. Kick on the boost and the amp moves into a thicker overdrive, with more sustain and a smoother attack. Turn your guitar's volume knob down and it cleans up again.
Into an overdriven channel: tighter and more focused
On a channel that already has a fair amount of gain, a boost with the level low and the boost's own gain set to taste adds focus more than raw volume. Many players use it this way to tighten the low end.
Where it goes in your signal chain
A boost is usually placed after tuners, wah and compressor, and before or after your drive pedals, depending on what you want:
- Boost after a drive pedal makes the drive louder and pushes the amp harder.
- Boost before a drive pedal makes the drive pedal itself clip more, so you get more gain from the pedal.
- Boost in a series effects loop works differently: it sits after the preamp, so it raises the volume without adding gain. That is a clean solo boost, not a drive boost. The article on the series effects loop explains why.
Delay and reverb normally come after the boost, either at the end of your board or in the loop if your amp has one.
How to set a boost pedal
- Dial in the amp first. Get the sound you want for most of the song with the boost off.
- Start with the boost's gain or level low and raise it slowly while you play. Small moves make a big difference.
- Decide what the boost is for: volume, gain or both. If you only need more gain, keep the level close to unity and let the amp do the work.
- Check at band volume. A boost that sounds right at home can be too much on stage, and the other way around.
The Kool Ace Boost Pedal
The Ace Boost Pedal MKIII is an all-tube boost with a 12AX7 inside. It is built to drive the first gain stages of your tube amp into saturation. At low gain settings it adds harmonics and warmth to a clean sound, medium settings bring crunch, and higher settings give full lead tones. You can also use it to boost the volume of a clean amp or as a cable buffer.
It uses a true high-voltage tube circuit, so the tube runs as it would in an amp rather than on a low voltage. That is also why it needs a 12 V AC, 1 A supply instead of a typical pedal power supply. An adapter is included. The pedal has true bypass and remote control capability, and it is handmade in a powder-coated steel housing. Kool lists the MKIII as a 2016 model with total production limited to 100 units, so check availability with Kool.
Which Kool amps work well with a boost
The Ace is designed to take pedals, and boosts and fuzz pedals work well connected to its input. Its clean voice moves into breakup and subtle crunch as you raise the gain, which is exactly the zone where a boost does its best work.
On the Marvell, Kool suggests turning the gain on the Plexi Clean channel all the way up and adding a clean boost pedal for classic 70s rock sounds. If you prefer to get most of your sound from the amp and pedals together, the guide to a tube amp head as a pedal platform is a good next read.
Next step
Want to hear what an all-tube boost does in front of your amp? Look at the Ace Boost Pedal, or contact Kool to ask which amp and boost combination suits the way you play.