Have you ever tried plugging your acoustic guitar into a classic-style Fender amp? There are obstacles to overcome, and this month I’m going to give you some tips on how to overcome them. But first, a little background.
Amps designed for electric guitars are carefully designed and matched to the voltage and frequency profiles of signals delivered by electromagnetic pickups. An amp sounds best when it boosts or filters certain frequencies properly. Many of us have had difficulty when the input signal, say from an acoustic guitar or other instrument, is very different from what the amp expects.
A guitar signal is initially created by moving the strings. The closer the vibrating metal mass is to the pickup magnet, the greater the magnetic pull and current induced inside the coil wire in the pickups. More windings and stronger magnets induce more current, but also reduce brightness and clarity. Coil wire thickness, wire material, and coating material and thickness also play a role in signal strength and frequency response. The signal voltage produced by a sensor is small, typically between 0.1 and 1 V, and contains frequencies between 80 and 1200 Hz.
On the amp side, there are even more factors that boost or cut certain frequencies, which is called frequency filtering. Take a vintage Fender Deluxe reverb. It is designed with specific tubes, resistors and caps in the preamp stage to amplify a weak input signal and shape it through the EQ, mix in some reverb and carry the result to the amp circuitry of power, which does three things. First it divides and duplicates resulting in an inverted signal, then it amplifies the two signals as much as possible, then feeds them to either side of a power transformer which changes the resulting voltage to an appropriate level for a speaker. It is typically 30 to 50V. The enclosure and the speaker itself are the final step in providing filtered and amplified guitar sound.
For acoustic guitars, I prefer modern American-style speakers that can handle high power and both firm bass response and crisp treble.
If you plug in other instruments, like an acoustic guitar or a harmonica with a microphone, and feed their signals to an electric guitar amp, you’ll get totally different results throughout the circuit. You may not get the sound you expect or, in the worst case, you may damage the amp. But generally all passive sources with electromagnetic coil pickups are safe to use. This includes piezo pickups mounted on the bridge of an acoustic guitar and vocal microphones. Since they are not powered by an external source like a 9V battery, they are passive and create a weak signal.
You should be careful when using electrical sources like an acoustic guitar with a battery-powered preamp and EQ. In addition, electric pianos, synthesizers or Bluetooth speakers with mini-jack outputs are also dangerous, as they can easily cause the speakers to explode due to incorrect volume or equalizer settings. Electric pianos can sound great through a vintage Fender amp. I’ve seen Fender Rhodes keyboards played through Twin Reverbs, and we’ve all heard organs through Leslie/Vibratone speakers, which can be driven through Fender guitar amps.
Acoustic guitars with active pickups can be tricky. With typical default electric guitar amp settings, the tone is tight and focused around certain mid frequencies. It lacks fullness, treble clarity and overall balance. So, I have some tricks you should try if you are experimenting with this option. First, set all EQ knobs to 10. This allows the guitar signal to pass through the preamp section with minimal tonal change. Be very careful with the volume and start low – around 1.5 – and increase from there. I find the big, powerful Fender amps to be the best for this, as they have plenty of clean headroom and wide EQ possibilities with a full set of bass, mid, treble controls and bright switches. And that makes them less prone to screaming comments.
A large enclosure will enhance the bass, allowing the preamp and power amp to wind down more without maximizing clean headroom. Remember that the power and energy resides in the bass. I suggest the 40 watt silver panel Bandmaster Reverb and the 85 watt Showman Reverb as handy amp heads for acoustic purposes. I use my Bandmaster Reverb with a 1×12 extension cabinet loaded with an Eminence Maverick. For acoustic guitars, I prefer modern American-style speakers that can handle high power and both firm bass response and crisp treble. The speakers are very important for your tone. The pickups of the guitar are also important, as well as a correct setup, so the action allows for optimal proximity of the pickups.
Acoustic mics don’t have to be expensive. They just need to be balanced and clear. A good guitar amp and some fine tuning of the controls will do the rest.
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