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?

daveyf

Showing 19 responses by daveyf

@mijostyn  If you read the thread, you would see that the Nordost cable was not at fault! Good thing I did not get my soldering iron out......
@gdnrbob, Thanks, who knew when we got into this hobby that one had to learn to be a detective as well as everything else, LOL.

@millercarbon  The plug in question is a WBT locking RCA. I suspect that there is indeed a bad solder joint. The cables in question are Nordost Tyr's...which have been very reliable up to now. I am probably going to have to send it back to Nordost for servicing. 
@lwin  Your problem was actually at the RCA amp connection..is that correct?
I think mine is the RCA ic's...
@lwin Thanks. That's interesting, although I have never had a hum....just channel drop off. 
Well, the problem is back...and this with different ic's. So, I am guessing that at this point that the ic is not the issue. Next step is to swap the left side driver tube on the left side mono block to the right side mono block and vice versa to see if the issue goes to the other channel. Stay tuned.
BTW, could a driver tube failure cause a cessation of sound? Seems a little odd, but onwards I go.
@paulcreed Thanks. Yes, I tried that and the problem still persists.
I swapped one of the driver tubes to the other mono block and vice versa...still problem persists. Tomorrow, after the amp has cooled down again, I will swap the other driver tube and try again. ( The amps employ one 12au7 and one 12ax7 per channel as driver tubes. The 12ax7 swap is next.) If that still doesn't expose the issue, then it will be replacing the power tubes....in my case four KT 150's...and if that still isn't the issue...well then she's off to the repair shop..:0(
@jetter  Thanks for the suggestions. I did play the preamp with the ss amp for a few hours.Nothing showed up, so I am assuming that the preamp is ok. As a last resort, I will hook up the ss amp again and give it a very long test to see if the problem rears up again. As to replacing all of the tubes at the same time in the amps...I thought about this, but it wouldn't tell me which tube is the culprit if I did that...just that it is a tube problem in the amp. So, I am doing one set at a time, from left to right and vice versa. This is more time consuming, but should zero in on the particular tube if it is the issue. Lastly, i was thinking to swap my speakers over...left to right, as it is maybe possible that a x-over part is overheating, although I doubt this as the issue was not present when I used the ss amp. Nonetheless, it is easy to check...and it will be the last thing on my list before sending the amp for repair. Onwards--
So tonight I swapped out the 12ax7 driver tube...and after a few minutes of idle power...a loud and nasty hum and buzz through my right speaker! ( the same channel that was acting up before!) I quickly powered down...and decided to change out all of the power tubes. ( I figured this might be a classic power tube failure). Upon replacement of all KT150's, I powered up again. This time, no loud hum and buzzz...no apparent issue at idle. I switched on the high voltage...and sure enough..same channel is still down!..:0(  
I'm pretty flummoxed at this point, so I think next is off to the dealer.

@gdnrbob  I could do that, but i am hesitant to do so as the amps are "theoretically' under warranty. I say "theoretically" as you never know with this stuff until you request service!
Here's why tubes can be a major PITA! 

Tonight, I figured one of my tube mono blocks was down...so I swapped in my ss Rowland stereo amp ( in place of the tube mono blocks) for a night of music and lo and behold...the same darn issue persists!! No sound out of the right channel! 
This is so very odd as my first thought, see my OP, was to do this test and the sound was fine before when the ss amp was in the chain. This would seemingly now mean one of two things..a) the preamp has a problem or b) the right speaker is blown. 
I swapped over speakers...put the left speaker on the right side and vice versa ( easy to do with stand mounts, luckily) Nope--the right channel is still silent. Hmmm. Seems to be a preamp issue ( which could still mean a tube has gone away, as I use a CAT tube preamp...) 
So, onwards we go...
@jetter looks like you you’re right in regards to the preamp... although I am so surprised that all was fine when I hooked up the ss amp and played it for a few hours previously. Any idea how a preamp tube can cause one to lose sound in one channel?
@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.
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. 
@gdnrbob. Thank you for the vote of confidence. Luckily, the issue has seemingly been resolved with the preamp. I recently rolled in NOS Telefunken 12AX7 smooth plates and Amperex 6922 PQ's into the phono stage, a very nice addition. The CAT really responds well to NOS tubes..
@millidas   As I stated above, I did indeed go to my local tech, who solved the issue. He wasn't located in a shopping center however...:0)
@mijostyn  If i do that on my Nordost cable, i lose the warranty. No thanks.
@mijostyn  I was wondering about that...how long the repair will last. So far, so good. These toggle switches are still available and I have bought several replacements if needed. It is no easy task to replace them and my tech feels it would be a pain to do. So here's hoping the fix continues to last.
Thanks again, Ralph. This is pretty much what my tech did when he received the preamp, except he did remove the top and sprayed from behind onto the offending switch. Seems to have solved the issue. When i told Ken that is what was the issue, he was not happy that Deoxit was spayed onto his gear, feels it is like salad dressing, LOL. 
@atmasphere Thanks Ralph. I did exactly as you recommended and followed this protocol. This is what ultimately lead me to the preamp, but in my case, since the issue was intermittent, and for some reason did not happen to show up when I changed amps, the true culprit was further hidden. To add to that, i changed the left and right interconnects at the amp and the issue followed the left to right change! This lead me ( falsely) to suspect the cables. I now believe that the way Ken Stevens has these toggle switches connected can lead to a intermittent issue with either channel, as the signal passes through all of his switches before going to the input or the output connectors. Not sure if this is typical in tube preamps, but there we have it.