The resonator guitar, or Dobro, like the electric guitar, is the result of early pioneers of the guitar looking for some way to make their instrument heard above the other instruments in a band. Using metal resonators did not catch on as amplification but the resonator guitar was embraced by slide guitar players for its expressive sound.
The basic advantages of using alternate tunings for Dobro playing are the availability of open string bass notes to accompany your melodies and the variety of combinations of chords you can make use of. Standard tuning is fine for the Dobro - you can still do alot with single note tunes - but why not look at it as a new instrument with new possibilities? You can decide which tuning to use based on the key of the song you are playing.
Bluegrass Dobro players quite often use G B D G B D - Open G tuning. There is a certain advantage for guitarists new to the Dobro in using this tuning in that the second, third and fourth strings are tuned the same as in standard tuning. This makes it a little easier to become comfortable with the new layout of the fretboard.
You cannot get minor chords using the Open G but you can find two notes of the minor triad. The second and third strings played open are two-thirds of your E minor chord - G and B. If you play the open second and third strings with the fourth string barred at the second fret, you will get the sound of the complete E minor chord.
You can also tune the Dobro to D G D G B D. This opens up the range of notes available to you in the lower range and is favored by Bluegrass guitar players. Do not forget that you can use other open tunings like Open E and Open D. If you would like to play blues music tune your Dobro to D B G G B D - gypsy tuning.
Hawaiian slack key is a little appreciated guitar style which also uses alternate and open tunings. Among the slack key tunings are D A D F# A C#, C G E G A E, Open C - C G C E G C, and Open F - C F C F A C. The key to Dobro playing is the sound and how you can bring it out in your own way by using these unusual tunings.
I should mention that you can use the capo on the Dobro, just the same as on the regular guitar. You should have no problem adjusting to the touch of the Dobro. If you are thinking of buying a resonator guitar with a pickup, just bear in mind that you may have some problems with feedback.
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