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Tube Interaction in Counterpoint 3.1

I have a heavily messed-with Counterpoint 3.1 pre that I've repaired and mostly put back to original spec. It uses a pair of 6DJ8s for the line preamp and one 6DJ8 as the phono pre. I've been doing some tube rolling and discovered that when I put good Amperex tubes in the line level sockets AND the same in the phono socket, the phono tube crackles and hisses badly. When I replace the line tubes with new Russian tubes the old Amperex in the phono spot is fine! I have 6 or 7 good European Amperex tubes and they all act the same way if they're in all three sockets. They work fine in the two line level sockets no matter what's in the phono socket, but I have to use new tubes for line if I want to use an old tube for phono. What the hell! I've never seen anything like this. Any ideas?
neddyboy
mechans
3,048 posts
 
Is the very high gain phono circuit fed into the line stage then out? Many older tubes can have inherently higher noise levels. I guess it is possible that the old tubes aren't really quiet and when amplifying each other (as I have supposed is possible) the noise rises to audible levels. Just a guess.
jmowbray
42 posts
 
The 2 side positions are for the phono stage and the center tube is for the line stage. You are probably experiencing one or more bad tubes in the phono position.
neddyboy OP
2 posts
 
Thanks for the responses. Mechans, I don't know the answer to your question, but I'm going to hit up the designer at Altavista Audio if he'll answer. Jmowbray, I am SO confused. When I pull tubes, removing outside tubes cuts off right and left for line input, and pulling the center tube cuts of phono. Maybe the cascading thing Mechans mentioned is at play here. Hmmm.
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