A minor 7th chord is a minor chord plus the flattened 7th note of the scale.
A major 6th chord is a major chord plus the 6th note of the scale.
Look at the chart below;
|C major is;||C E G|
|C 6 is;||C E G A|
|A minor 7th is;||A C E G|
Don't forget that the Stradella system only plays three notes in a chord (more >>). When the chord calls for 4 notes, such as in a 7th chord, the major 5th of the scale is missed out.
Amazingly, C 6 uses the same notes as A minor 7. One is an inversion of the other. Now we must remember that the Stradella system relies on inversions for every chord it plays and so we can state confidently that C 6 and A minor 7 are the same as far as the accordion left side is concerned.
So, now we have to work out how to get C, E, G and A to sound. The answer is to hold down the C major button and the A minor button at the same time. Because of the way the Stradella system works this simply opens only one more valve on the left side than the C major chord alone so it does not mean that twice as much pumping is required. In addition, every note of the chord sounds (that is, including the 5th of the scale).
Based on this you can play any major 6th or minor 7th chord on the left hand.
If you are playing quickly so that the major 6th chord is just a passing sound, I use the correct fundamental bass (in this case C) and the appropriate minor chord (in this case A minor).
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