Your partner can not have 12 trick hand with fit when you hold diamond ACE. Reason is, with that hand, he would have bid differently. For instance, over 4S, he could have bid 5NT or 5S. Or, over 5D, he could ahve bid 5S or even 6C (6C must be fit for diamonds, else he would bid hearts).
A six diamond bid here is educated gamble. IF partner has six in his hand with a fit, something like...
S-A
H-AKQTxx
D-KQT98x
C=void
He would bid 5S, and with,
S-void
H-AK
D-KQTxx
C-AKQJTx
He would bid 6C.
Dont be a hero. Pass 6
♦. Besides, if he has one of those hands, you can always come post here to see how he SHOULD have bid.
Mini-plug for MishovnBg's invention:. Now if your parnter had EITHER of those hands and you were playing MisIry, it would be piece of cake..
Imagine first hand (we leave in the competition)..
Pard = 3H (preempt in spade or H/D two suiter) -
RHO = 3S (ok, less likely now, but we let him)
You = Pass
LHO = 4S
Pard = 6D = Diamond/heart two suiter, one loser, don't need any club honor
You = 7D I have the diamond ACE... cool
Imagine the second hand
Pard 3D = heart preempt, or D/S or D/C two suiter
RHO 3S = more likely now, but still not that likely
You Pass
LHO 4S = less likely as not sure who is preempting
Pard 6C = minor two suiter, one loser, no need for heart ACE
You 7D = wow, still have diamond ace and fit.
If your partner had a two loser hand, he could open the same, and then rebid at the FIVE level to show those hand. And you know to carry on to six, but not seven.
To read more about MisIry, see
http://inquiry2over1.blogspot.com/ click on chapter 3. Or search this site for MisIry or transfer preempts.