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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
jjrenman
279 posts
 
I suggest that you swap your preferred tube to the other channel just to confirm that issue follows that specific tube.

Could be a heat related issue. If your top cover gets very warm, as an experiment, you could try playing unit without it to see if tube still cuts out at 2 hours.
lewm
12,998 posts
 
My guess: Bad tube socket. Jiggle the offending tube, when it cuts out, or if swapping tubes between channels leaves the problem in the same channel, that is consistent with intermittent contact in that tube socket. But this is a guessing game. The problem would probably be obvious to any competent technician, on his workbench.
mechans
3,048 posts
 
It is atypical, usually an early tube failure is sudden infant death where the tube immediately blows.
Are you certain the tube was new and unused before you got it. An NOS emissions test value doesn't mean the tube is NOS and has a full lifetime in front of it. There are "life" tests that can be used to see howe viable a tube truly is.
The suggestions re tube socket and swapping the tubes from one channel for another seem logical.
I understand your taste preference for the real old stock Mullard which is not present in most current production. I understand that the Gold Lion "re-issue" has some of the characteristics you seem to want.
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