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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 3 responses by mechans

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.
mechans
3,048 posts
 
It is a moot point at this juncture. The tube is bad as demonstrated by introducing another tube and getting good results. Now we can argue about which tube brand and model he should try. NOS vs current production striaght swap or a sub etc.etc.
mechans
3,048 posts
 
I collect 5751s and have some experience subbing them in for 12AX7s. If you want a softer edged fuller sound then go against the wave of opinion and get an older grey plated example. If you crave detail space and definition then get black plated tubes. The best tube were the triple mica supported late 1950s to early 1960s, however it is not absolutley necesarry for a creamy musical tube. Probably the best depending on your budget are the Sylvania Gold Brand which were double mica in the grey plated versions. If you just want to experiment get the ubiquitous GEs of that era which are very underrated, they deliver volumes of good bass, and are cheaper that most other brands.
If you want to hear what a blackplate sounds like you can get RCA white lable triple micas or the later "Command" series.
If you really just want a sense of what a 5751 sounds like get fairly current (approx. 1980+) JAN tubes or the current Sovtek butI warn you that Sovtek just doesn't sound right to my ears. There is a lot of 5751s out there espcially GEs so finding a decent example shouldn't prove hard or very expensive. If confused write me an email and I will guide you.
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