In a PM, bassguy recommended to practice to a drone tune to improve your intonation and train your ear to hear intervals. This is just a recording of a precise frequency. There would be 1 drone recording for each tonic following the circle of 5ths. This is something I had never tried before, so I went online and found a set of recordings that you can buy and download. You can get them here:
Cello Drones cd by Marcia Sloane
I was practicing études in the key of D Maj last night and played them over top of the D drone. Incredible. Your ear can actually hear any little fluctuation in intonation, and for an unfretted instrument it is mandatory that your intonation be excellent. But you could do the same thing for a slide or valve instrument as well.
I don't know how I missed this all these years but this is really the cat's a$$. You can play scales, arpeggios or just improvise in a key right over top of the drone and it works like magic. I brought my 12 year old daughter into my practice area, played the drone and played D Maj scale notes and asked her if they were in tune or not - she nailed every one of them. Best $10 I have spent so far on this.
Thanks bassguy - I really appreciate the tip.
Alembic