Check voltage & resistors around your 6SA7. Resistors go up in value and voltage could be low. Either 6SA7 or 6SA7GT should work although they have slight different basing.
This tube is an oscillator. Oscillators stop operating at higher frequencies first. Short wave is on higher frequency than AM radio.
http://www.nostalgiaair.org/PagesByModel/999/M0007999.pdf
Norm
:Tha bands "C" and "D" do not work, I replaced the tube 6AS7 for 6AS7GT and do not work.
:It is necessary used the metal tube?
:AM is working fine, I have a good exterior antenna working fine with another radios, the band switch is OK.
:Any idea about the problem?