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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

Showing 1 response by jjrenman

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.
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