Pt2399 delay oct up / down puzzle

[michael page]

 Ive played around automating the rate of a pt2399 delay but it always sounds too dischordant for me, jumping or sliding between rates without a neat harmonic relationship. I had the idea earlier that it would be nice to be able ping between normal delay rate and double delay rate at the flick of a switch.

Ive been doing something similar doubling the pitch of a 40106 oscillator by using dual pots and switching the two gangs between parralel, single and series. It works well for that because the pitch has a linear relationship to the resistance used.

So i looked up the pt2399 datasheet and at first it seemed possible because the relationship between delay length and resistance of rate control seemed linear.  2 x rate = 2 x resistance + c .  With a constant c, of about 2.3k.  After doing some sums though this constant isnt constant, it slides up. 

The only way i can figure to do something like this is to make some stupid custom 3gang pot with (perhaps 2 x 250k and 1 x 10k) using the low value to top up the sliding constant.   Urgh, its probably all too much of a faff to bother with.

Any thoughts?