  We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals
under high He pressure. For each Ca content various samples with different
oxygen contents have been prepared to probe the influence of Ca on Tc(x),
dTc/dp(x) and Tc,max. Starting from the parabolic Tc(nh) behavior we calculated
nh values from Tc and Tc,max for each sample. It is shown that in the overdoped
region dTc/dp can be described by a pressure induced charge transfer with
dnh/dp = 3.7E-3 [1/GPa] and a dTc,max/dp value of 0.8 K/GPa, irrespective of
the Ca content. In the underdoped region additional pressure effects lead to a
peak in dTc/dp at approximately 0.11 holes/CuO2 plane. However, with increasing
Ca content this peak is strongly depressed. This is explained in terms of an
increasing disorder in the CuO chain system due to doping. Deviations in dTc/dp
at very low nh values can be assigned to the ortho II ordering in the CuO chain
system.
