diff --git a/GPy/core/transformations.py b/GPy/core/transformations.py
index 0524dfe3..d475f29e 100644
--- a/GPy/core/transformations.py
+++ b/GPy/core/transformations.py
@@ -5,15 +5,15 @@
 import numpy as np
 from GPy.core.domains import POSITIVE, NEGATIVE, BOUNDED
 import sys 
+import weakref
 lim_val = -np.log(sys.float_info.epsilon) 
 
 class Transformation(object):
     domain = None
     _instance = None
     def __new__(cls, *args, **kwargs):
-        if not cls._instance:
-            cls._instance = super(Transformation, cls).__new__(
-                                cls, *args, **kwargs)
+        if not cls._instance or cls._instance.__class__ is not cls:
+            cls._instance = super(Transformation, cls).__new__(cls, *args, **kwargs)
         return cls._instance
     def f(self, x):
         raise NotImplementedError
@@ -62,14 +62,16 @@ class LogexpClipped(Logexp):
     log_max_bound = np.log(max_bound)
     log_min_bound = np.log(min_bound)
     domain = POSITIVE
+    _instances = []
     def __new__(cls, lower=1e-6, *args, **kwargs):
-        if not cls._instance:
-            cls._instance = super(Transformation, cls).__new__(
-                                cls, lower, *args, **kwargs)
-        elif cls._instance.lower == lower:
-            return cls._instance
-        else:
-            return super(Transformation, cls).__new__(cls, lower, *args, **kwargs)
+        if cls._instances:
+            cls._instances[:] = [instance for instance in cls._instances if instance()]
+            for instance in cls._instances:
+                if instance().lower == lower:
+                    return instance()
+        o = super(Transformation, cls).__new__(cls, lower, *args, **kwargs)
+        cls._instances.append(weakref.ref(o))
+        return cls._instances[-1]()
     def __init__(self, lower=1e-6):
         self.lower = lower
     def f(self, x):
@@ -136,19 +138,20 @@ class Square(Transformation):
 
 class Logistic(Transformation):
     domain = BOUNDED
+    _instances = []
     def __new__(cls, lower=1e-6, upper=1e-6, *args, **kwargs):
-        if not cls._instance:
-            cls._instance = super(Transformation, cls).__new__(
-                                cls, *args, **kwargs)
-        elif cls._instance.lower == lower and cls._instance.upper == upper:
-            return cls._instance
-        else:
-            return super(Transformation, cls).__new__(cls, *args, **kwargs)
+        if cls._instances:
+            cls._instances[:] = [instance for instance in cls._instances if instance()]
+            for instance in cls._instances:
+                if instance().lower == lower and instance().upper == upper:
+                    return instance()
+        o = super(Transformation, cls).__new__(cls, lower, upper, *args, **kwargs)
+        cls._instances.append(weakref.ref(o))
+        return cls._instances[-1]()
     def __init__(self, lower, upper):
         assert lower < upper
         self.lower, self.upper = float(lower), float(upper)
         self.difference = self.upper - self.lower
-        return self
     def f(self, x):
         return self.lower + self.difference / (1. + np.exp(-x))
     def finv(self, f):