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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 4 responses by dunenthem

dunenthem OP
7 posts
 
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
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.
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.
dunenthem OP
7 posts
 
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?
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