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Chasing down the problem

Couple of nights ago, I was listening to my mono block tube amps and after about ten minutes the right channel goes silent! What the heck!! So begins the search for the culprit...is it upstream of the amps, or is it the right side mono block that has an issue...and what kind of issue.
Luckily, I own another stereo ss amp, so the first thing to do was to hook that up and see if the problem continued. This would eliminate a few pieces of gear as the culprit...basically everything in front of the tube mono blocks...except the connection from the preamp to the mono blocks...the ic’s.
The system played fine with the stereo ss amp in the chain...therefore leading me to believe that the following components were not at fault...1) the front end digital player and the front end turntable set up ( I did play both to be certain) 2) the tube preamp, 3) the connections between these components...speaker cable and ic’s ( I use different ic’s and sc’s to connect the preamp to the ss amp than with the tube amps).
The next day, I hooked up the tube mono blocks again...and this time after making sure that the connections at the amps were tight and that the tubes were all re-seated. Music played again in both channels ---for about twenty minutes! Then the same issue reared its head again, the right channel was silent. Hmm.

Therefore, unless I am wrong, the only two possibilities left are the right tube mono block...or the right ic feeding that mono block from the preamp. My next thing to do was to change the ic’s for another pair ( could have swapped left for right, but since I have other pairs of ic’s that i know are good, i figured why not try this next) Plan being to check the ic’s and then finally start to move tubes from one mono block to the other and vice versa to see if the issue changed channel.
Last night I listened to the same set up but with just the ic’s changed out between the amps and the preamp....and no problem. ( this after a two hour session).
Hopefully, I have identified the issue..would seem that the original right side ic has a iffy connection somewhere ( a lot easier to address than to have to get the mono amp worked on).
Anyone else have a problem like this that took a little ’work’ to identify?

128x128daveyf
jetter
3,035 posts
 
daveyf, I don’t have much knowledge about the inner workings of our gear like many here posses. I do know that preamp tubes last a very long time, but I would hope that you just need a new tube.

This is what I would do:

If you have not done so already, switch the interconnects from one preamp channel to the other and see if the problem follows the interconnect to the other channel.

If no luck, with the preamp off switch the tubes to the opposite channel and if the dead channel follows the tube switch you hopefully just need a new tube.

My preamp has two tubes for the line stage and two for the phono stage, and they are mounted horizontally, see this https://www.usaudiomart.com/details/649252938-copland-cta-305/images/1150672/. I have lost a channel two times over the years. Both times were because somehow a perfectly good tube or the vertical board the socket is mounted into had lost connection with the preamp. The first time I lost a channel I removed and reseated the two line stage tubes in the same socket and all was well. The second time I lost a channel, with the system off I just gently nudged the tube a little and the channel came back. Not sure why this happens but only twice in many years of use.

If that doesn’t work, while the lid is still off, as a poster mentioned above explore if by chance the RCA connecter in your preamp is damaged.

From here, if you haven’t already, I guess I would switch speaker cables left to right to make sure I don’t have a bad solder joint on a speaker connecter and if that doesn’t help with my limited knowledge of electronics its off to the shop for repair.
daveyf's avatar
daveyf OP
2,877 posts
 
@jetter   Thanks. I will swap tubes in the preamp...left to right. But I can not see how this could be the issue as the preamp utilizes separate/different tubes for the phono stage and the line stage ( left and right channels)...and in my case, both the phono and line stage exhibit the same problem. I checked the cables, all are fine. I think it likely that this is going to be a repair for the tech.
daveyf's avatar
daveyf OP
2,877 posts
 
To conclude this story...I finally had to take the preamp ( yes, not the amps at all!) to the tech. It turned out it was a switch on the preamp that was not functioning well and was intermittently cutting off the right channel!! My tech diagnosed this is minutes!
While I was playing with the amps, I inadvertently destroyed one of the Tele 12Au7 driver tubes and one of the KT150 power tubes went bad...but this didn't cause the problem in the OP! So, now that the preamp is up and running again, all is well. 
Motto of this story seems to be that for those who are less technically inclined--like myself--diagnostics isn't our strong point. 
Post removed 
gdnrbob
4,450 posts
 
@daveyf ,
I, for one, think you did a pretty thorough job of trying to trace the problem.
Unfortunately, it was something you didn't consider, nor would have I.
A dodgy switch in the preamp would be one of the last places I would think to check.
Though, if it were to happen to me, I would contact Ralph Karsten (atmasphere). Though I own his equipment, I do know he will help anyone in distress, as his presence on Audiogon shows.- A very nice and generous person.
Bob
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