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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
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.
dunenthem OP
7 posts
 
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.
tls49's avatar
tls49
1,351 posts
 
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?
dunenthem OP
7 posts
 
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.
tls49's avatar
tls49
1,351 posts
 
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.
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