1990 fender red knob twin crucial problems

[g1]

Yes, R215 and R216 are the only ones you need to check.
Make sure your meter reads zero when probes are shorted together. If it does not, you need to subtract that amount from your reading.

[Jquijano]

I have also noticed that some power tube sockets and preamp tube sockets are flimsy, meaning they are not gripping the tubes very tight. I have replacement for the 7 preamp tube sockets. Is it really worth investing time and effort with this or should i only retighten the pins with a jewelers screwdriver?

[Tom Phillips]

I have also noticed that some power tube sockets and preamp tube sockets are flimsy, meaning they are not gripping the tubes very tight....

Keep in mind that the individual female contacts in a tube socket are not themselves tight in the base material. They float so that the individual sockets can align with the tube pins without causing undue stress to the tube pine to glass interface. This is especially evident with the larger octal sockets. You can check it out by installing a tube in a new tube socket that is not mounted on a chassis.

When your amp is warmed up and operating, you can test the integrity of the individual tube sockets by gently wiggling the tubes one at a time. If, after you have cleaned and re-tensioned the original tube sockets, this test does not produce noises then the existing tube sockets are OK.

[Jquijano]

Thank you very much for your advice. I jist want to reduce the background noise as much as possible as my amp does generate some hum audile hum even with no guitar cable plugged in.