[Simh] changing from vaxserver to microvax

[Peter.Jakobs at selex-comms.com]

Hi

i am trying to change my vax from a vaxserver to a microvax. The manual 
says:
The system type is controlled by a "magic byte" in the CPU's boot ROM. By 
default, the system type is a VAXserver 3900. To change the type to a 
MicroVAX 3900, patch the boot ROM as follows: 
sim> set ptr ena 
sim> att ptr ka655x.bin 
sim> ie ptr 4 
4: 2 1 
sim> det ptr 
and reboot the simulated VAX. 

my system does:
sim> sho version
VAX simulator V3.7-0 [64b data, 64b addresses, Ethernet support]
sim> set ptr ena
Non-existent device
sim> 

any ideas?

Peter