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?
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.
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.
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.
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
"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.
Just curious as to how a bent pin would suggest an internal short?
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.
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.
Tls49, The pin may be fracturing entirely outside the glass envelope, so as not to compromise the vacuum. The argument is pointless; the OP is the only one of us who can actually examine the pin.
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.
Sounds to me that I'm risking the preamp if it is this pin issue- the term "short" is a bit disconcerting. The cartridge i'm using is a homc glider so two 12au7s match my required gain better . My spare is an rca cleartop which when paired with a cleartop in the less dominant position becomes a little bright for my system (metallic tweeters)

I listened for 3 hours yesterday on top of leaving it on for several days and no right channel cutout.

I see advertised and discussed anti oxidation fluids enabling better contact between sockets and tube pins. Is this frequently necessary?
The prudent move would seem to be to pull the tube and chase down another cv4004.
Any tube can develop an internal short, however, the external bending of a pin will not cause this. Bending of the pin could result in a glass fracture (see my previous post) or with enough bending back and forth, it could simply break off. Since everything seems to be working OK, it could have been a connection problem. It still may be wise to replace it, as the problem may develop again.

I am still a little confused when you say that your cartridge matches gain better for two 12AU7’s, and you are using one 12AU7 and one 12AX7. The CV4004 is a 12AX7. Also, you mention something about a dominant position. Is the CV4004 in the socket for the 12AX7?
My mistake Tls49, the tube in question is a cv4003 not cv4004 as I previously reported. I have it in the rear position of the unit which is designed to be a 12au7. The second position , normally a 12ax7 has an RCA cleartop(12au7)for toning down the gain. Modwright suggests the original 12au7 position is the more dominant. I haven't experimented with flip flopping the two 12au7. The cleartop is fast and revealing, the Mullard smooth and rich relatively speaking. Switching might enable a different sound. One thing I'm clear on; two cleartops is too bright in my system.
Thanks Dunenthem, that definitely clears the confusion. There are so many 12AU7 types, and the Mullard box plate CV4003 is one that seems to be quite desirable. Another is the smooth plate Telefunken ECC82.
Dunenthem, you are correctly describing an intermittent short in the tube. It is unlikely that it could damage the preamp. A replacement of the same version would probably sound better FWIW.
Before you replace the tube, try tapping it on the sides with a chopstick. Sometimes this will help a tube along if it has any residual particles accumulted that could cause intermittent shorts.
Just a comment: I think I understand you to have stated that Modwright gives permission to replace a 12AX7 with a 12AU7, to "tone down the gain". While such a substitution would surely have that effect, the characteristics of those two tube types are so different that I would wonder about a negative effect on sonics. The 12AX7 is very high mu and generally operates on less than 2mA of plate current, whereas the 12AU7 is medium to low mu but has much higher transconductance and requires much more plate current to sound best. Lots of folks use the 5751 in place of a 12AX7, when it is desireable to reduce gain. The 5751 has characteristics much more like that of a 12AX7 and usually works fine in a 12AX7 circuit. If even the 5751 has too much gain, then I would go to a 12AT7, which is kind of a 'tweener in the 12A series. And IMO the 12AT7 is a much better sounding tube than the 12AU7. I am rather surprised that Modwright would advise customers to make the 12AX vs 12AU swap, unless you are also changing a resistor here or there.

Then there is the separate but related issue that the 12AX7 in any given topology has a much different output impedance from a 12AU7. Thus when used as a gain stage in an RIAA circuit, this substitution would radically alter the RIAA. Which makes me think I've misunderstood you somewhere along the way, because Modwright would never advise such a thing.
I think the discussion in an audiocircle forum by Dan was pretty straight forward. However, as you suggest ,there may be a better way to skin the cat. How would the 12au7 negatively effect the sonics? enhancing or suppressing high frequency for instance? Do you have a recommendation on a 5751 type that is a quiet tube on the slightly organic side of the spectrum?
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.
Mechans apparently knows much more about 5751 variants than I do. I would take his advice.

Do I understand correctly now that Modwright is not the source of your idea to sub a 12AU7 for the 12AX7? It makes sense that this is so, because the idea of doing that sub makes no sense.

I would not be able to predict how a AU7 for AX7 would affect the sonics of a phono stage where the 12AX7 is used for phono gain, without seeing the schematic, but I can tell you that the RIAA equalization curve depends on very accurate filtering at very specific frequencies. The output impedance of the gain stage forms one of these filters in conjunction with a precise parallel capacitance and usually a series resistance. Since the AU7 has a VERY different output impedance from an AX7, the substitution would have a very large affect to screw up the RIAA equalization. However, nothing will blow up or anything of that sort.