In short,
dd if=/dev/mt0 ibs=[blocksize] cbs=[conv buffer size] conv=ascii obs=[output
block size] of=[output file name]
Usually cbs and obs will be the desired record size, as the convert to ASCII
will clip trailing spaces (dd in other direction, to EBCDIC, pads out to cbs)
There's a test utility on some unix systems, testbs, that will find and
report the block size for you:
testbs /dev/mt0 r
is the syntax. Only reads up to about 32K, however.