2. amps-preamps
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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
5 responses Add your response
jmowbray
42 posts
 
Neddy - according to the manufacturer (and as reflected in my SA 3.1 owners manual):

Technical Summary:
Pure tube signal path. The unit has two 6DJ8 tubes in the phono stage and a single 6DJ8 in the line stage. As with all Counterpoint preamps, there are two gain stages in the phono section with passive riaa equalization. The line stage is a single-stage.

http://www.altavistaaudio.com/Product_Guide.html#SA3
dover
2,247 posts
 
Mate, pulling tubes out and turning preamp on is likely to damage your preamp.
The power supply is designed to run with tubes in, the removal of any will mean too much current to the heaters and other issues. You will be very lucky if you haven't damaged it already. Just find a local audio store and ask them if they have a recommended technician for tube repairs and get it sorted professionally..
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.
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.
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.
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