You may want to get a tech to check it out first before blowing coin on new tubes to fry them too. Screen grid resistors take the load when a tube fails but can also fail themselves whilst doing so. Replacing the tube will just result in another fried tube and no protection from the failed screen grid resistor. Having a failed SGR is like having no tune in the socket at all basically.
P.S. Disclaimer: Amplifiers carry lethal votages (that means they can kill), plugged in or not. If you are not proficient in electronics servicing and safety do NOT attempt to poke around inside your amp.