2. analog
  3. 1372534543

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 tls49

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.
tls49's avatar
tls49
1,351 posts
 
Just curious as to how a bent pin would suggest an internal short?
tls49's avatar
tls49
1,351 posts
 
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'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?
More to discover