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intermitant phono tube cut out?

when playing records thru mod ls100 phono circuit right channel volume drops off to near zero after 2 hours or so of playing. turning off the system and playing a day later will yield the same results meaning ok for 2 hours then notta.. digital sources thru the modwright play correctly(both channels present). Swapped left for right ics from rc and problem stays in the right channel.
swapped out both tubes in the phono circuit and problem has not reoccurred. reinstalled my sonically preferred tube back in and problem reoccurs.
My conclusion is that my preferred tube is the issue. Being new to tubes,I'm surprised this tube (mullard cv4004- 150 hrs} goes "bad" in this manner. Is this typical tube behavior near the end or am I missing something?
dunenthem
dunenthem OP
7 posts
 
A couple of clarifications - the tube in question is used for both channels so channel swapping isn't in the cards. I did notice that one of the pins easily bends upon removal and just as easily goes back into place when reinserting. So it could easily be the tube socket/pin interaction that when hot moves to a bad contact place. The spare tube I've used doesn't exhibit this behavior. I've also left the unit on for 24/7 in an effort to trigger the event but it hasn't happened yet although I've not listened for two consecutive hours yet.
I appreciate the help - thanks
lewm
12,852 posts
 
"I did notice that one of the pins easily bends upon removal and just as easily goes back into place when reinserting."

Not good. Suggests a possible internal short. I would waste nor more time on that tube.
tls49's avatar
tls49
1,351 posts
 
Just curious as to how a bent pin would suggest an internal short?
lewm
12,852 posts
 
It sounded like it was not merely bent but loose, such that maybe it is partially fractured. I did not mean to imply that a bent pin is per se a big problem.
tls49's avatar
tls49
1,351 posts
 
I do think it may possible for a bent pin to compromise the connection with the socket, however, if this pin is actually loose in the tube glass, the vacuum would be lost, the silver getter would turn white, and the tube would be totally dead.

Dunenthem, are you swapping the 12AX7 (CV4004) only, or swapping the 12AU7 as well? I believe the phono stage uses a 12AX7 (CV4004) and a 12AU7, or it can use two 12AU7’s.
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