[marvda1]

i have mono blocks with a quad each of eh 6ca7's. one of the 6ca7's dark colored top cap area turned white. i assumed it was gone but the amp still played music. my amps will burn resistors when power tubes go. got in a replacement tube and listened for while then i noticed that one of the power tubes on the other monoblock cap was about 3/4 white. are my power tubes going? do i need to get a new octet or just replace them one at a time as they go? are they bad when the top turns white?

[Ericus Rex]

Silver getters going white means you've lost the vacuum in the tube.  I've heard the EH 6CA7 has lots of problems.  Depending on the amp of course, you can still have sound with one tube gone.  You'd just have reduced volume in 1/2 of the wave cycle in one channel.

Git yerself some gooder tubes!

[Ericus Rex]

Sorry, I forgot about the second half of your questions.  It's always best to replace with a two matched quads.  I don't see any need for a matched octet.  I don't recommend it, but you can get away with replacing them one at a time if each tube is individually biasable...though I believe the other tubes can wear faster if you do this.  If they bias in pairs or are if the amp is self-biasing forget about it!  Get matched quads.