thermostat valve on dash

1997 Buick Skylark V6 3.1Ltr

so a couple years ago the heater core went and it was bypassed at the firewall.
over the weekend i had it fixed and i bought the heater core.

now the thermostat control on the dash has a lot of resistance and will rotate back when let go. plus there is no heat.

i took it back this morning before work and the guy says it may be the heater valve, but i do not think there is a heater valve in my car.

the heater core seems easy to install(well, once you take the dash apart).
it is only a mini radiator with two small pipes extending. it does not look like it is necessary to mess with any cables or flap/diverter doors.

so
1) there is no heat - is there a heater valve?
2) the knob that rotates from cold to hot used to turn easily(well as far as i remember) but now there is tension and it will move backwards a little bit.

i just want to get some feedback because i am supposed to go back in saturday morning - maybe i shouldn't.

oops, i meant the thermostat knob, on the dash