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Bad Tube? Which one?

Sorry to have to ask this question. I have been running a Rogue Cronus Magnum for two years. I've never tube rolled and still using all the original tubes. Tonight it's sounding muddy and distorted, even at lower levels. I stopped the CDP and checked the bias on the KT90's and they're all a perfect 35 mA. Does this mean that the KT90's are not the problem tubes and it's one of the 12ax7's or 12au7's?
donjr
montytx
138 posts
 
Can you isolate the problem to one side or the other? If so, move the output tubes to the other channel and see if the problem moves with the tubes. You can do the same for the small signal tubes if needed.
tom6897's avatar
tom6897
1,484 posts
 
Always a good idea to keep some spares on hand for these types of investigations.
rrog
1,958 posts
 
Tom6897 is right. When you own tube equipment you should always have a spare set of tubes on hand.
onemug
784 posts
 
I'll 3rd what Tom6897 and Rrog said.

A spare set will come in handy when:

1. Things go bump in the night. I just had a tube rectifier develop a short. Replaced the fuse and rectifier and was good to go.

2. Diagnosing. My 2a3 amp was sounding a little flat. Tubes measured ok and I could bias them properly. Put in a new set of 2a3's and all the life came back.

Even though your KT90's bias "a perfect 35 mA" they may be tired although gambling, I would bet on one or all of the 12's. Get a new set of them first, they're cheaper.
donjr OP
1,335 posts
 
I do have a spare matched quad of KT90's. I'll call Mark at Rogue and also get some spare 12's.
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