Sometimes an antenna mismatch can be fixed without an antenna tuner. A tube-type transmitter usually has a pi network in the final output. This network can usually get the rig tuned up on any decent antenna, but sometimes it can't.
I ran into this problem with a new linear amplifier. It would not load up on a coax-fed dipole that was cut for the band I was using.
My solution was to splice an extra length of coaxial cable into the feedline. It worked.
The resonance of an antenna system, including the feedline, depends on the length which the transmitter sees when in puts the signal into the line. Generally speaking, lengths which are odd multiples of a quarter wave are easy to tune, but lengths which are even multiples of a quarter wave present a very high impedance and are hard to tune.
Splicing in a piece of cable which is an electrical quarter of a wavelength or less will adjust the impedance presented by the antenna system. An electrical quarter wave is about two-thirds to three-quarters of an actual quarter wave in most coax because the signal does not move as fast through the cable as it moves through the atmosphere or empty space.
I have never tried this with a solid state rig, but I suspect it might work. I have used a coax-fed half-wave dipole cut for 20 meters without an antenna tuner without any problem. Most likely I was just lucky in getting the right feedline length.