Is it bad to put different tubes into a tube amp than what it originally came with?

i'm getting a randall rm100 soon. i've read that it comes with jj 6L6 gc tubes. i've seen that ruby and electro-harmonix tubes are really good for heavy metal. but i notice that the krank heads have 6550 tubes. so i'm wondering if i can change out all of jj tubes for ruby or electro harmonix, they would still be 6L6 tubes just a different brand of them. or if i could change them out for 6550 tubes, or if it's even possible to do a mix of 6L6 and 6550 tubes. would interchanging the tubes hurt the head in any way, if so how? and if i can't interchange 6L6 with 6550 tubes or if i can't even put 6550 tubes into the rm100 that comes 6L6 tubes. could i put in ruby or some other kind than jj tubes and just biase it to make it work?Is it bad to put different tubes into a tube amp than what it originally came with?
Sometimes OK...be careful. Search for ';swap 6L6 with 6550'; and take everything with a grain of salt. Also KT-66. ';The 6550 filament draw is twice that of the 6L6.';



There are circuits where the tubes can be swapped for alternatives without any other changes, and others that require re-biasing. There are some that are technically different in name only. And some swaps can't be done, so you need to know what you are doing. Sometimes circuits put way too much demand on a different tube. Generally, the preamp section is going to be less risky than the power section.