here an interesting article in the SONIC music magazine from Dr. Peter Körner about his way to an alto trombone in D. unfortunately the article is only available in german.

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We had a post on TromboneForum from Stewbones about converting his Conn 36H to be in D (with A attachment). I believe he had a short extension made to the main tuning slide. The attachment tuning slide is long enough already

Stewbones claimed that an alto in D uses similar positions when reading alto clef to a tenor in Bb reading tenor clef. I'm not sure -- I think my head would explode. I'm better off just learning how to get the notes I want on an Eb instrument.

Absolutely. D would be 1st position. D is 3rd space in alto clef. Bb is first position on tenor trombone. Bb is 3rd space in tenor clef. You would have to mentally change the key signature though. Same trick works for reading bass clef on an Eb instrument. That's why it's a good idea for anyone trying to learn Eb alto to wean himself off of bass clef as quickly as possible, since you're not really learning the notes - you're just pretending to read tenor clef, and you will very rarely ever be reading bass clef on alto trombone in any real-life situation.