2. amps-preamps
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preamp tube hiss from my SF Line 1

I'm trying to diagnose a very audible hissing sound that recently showed up. The symptoms are, hissing sound coming from the right speaker. Its very noticable from my chair. It does not get louder as I turn up the volume. The sound is there regardless of which input I select. It does go away when I switch on the HT Passthrough. I thought I had a bad tube and have tried swapping out each of the three tubes ont the right channel and it makes no difference. All the tubes are NOS Amperex and I've been using them for a while now without any issue. This all started when I swapped out the bottom pair of tubes with some better ones that I had. All my tubes were recently tested so I know they still have plenty of life in them. Any ideas? I find it unlikely that I have multiple noisy tubes, but of course I could try putting the stock ones in all three right positions. Any insight would be appriciated.
snipes
brf
1,699 posts
 
I can't determine from your post if you tried swapping the left channel tubes with those of the right channel. If the hissing changes from the right speaker to the left speaker, you now know that you have a bad tube. Once you have established that you have a bad tube in one channel, switch out one tube at a time from right to left channel until you notice that the hissing has also changed from the associated channel. It is merely a process of elimination. Good luck.
jea48
2,944 posts
 
Just grabbing at straws here, but is there a chance you may have a cold solder joint on one of the tube socket terminals caused when changing out the tubes? If you put back in the three tubes you had pulled before the hiss, and you still have it, then I think that eliminates the tubes from the equations.
Jim
phd
1,008 posts
 
Snipes, I have owned a Sonic Frontiers preamp before with the same problem but my hiss was in the left channel. Called SF and they told me that the Mosfet transistor that followed the tube had become noisy & if I could tolerate the hiss it was not necessary to replace. To cure this hiss of course is to replace the Mosfet follower.

Another thing to keep in mind as when you swap out tubes it is important to wait at least two minutes after turning the unit off before doing so otherwise damaged can occur to the Mosfet follower. I am assuming the SF Line 1 is a hybrid like the SFL-1 which I had owned. Hope this helps.
snipes OP
119 posts
 
Brf: I swapped out one tube at a time using an extra tube I had. I've actually tried a few "extra" tubes, swapping out only one a time and get the same result. My next step is to put all six stock tubes in it and see what happens.

Jea48: You wrote: "If you put back in the three tubes you had pulled before the hiss, and you still have it, then I think that eliminates the tubes from the equations." I'm not following you. If I put the same tubes back in and the hiss is still there I don't see how I ruled out the tubes. If it is a bad tube, then wouldn't putting it back in cause the hiss to continue? The hissing didn't start until after I swapped out a pair of tubes. Maybe I did in fact break a solder joint. There aren't any error lights on though..

Phd: Did you happen to get an estimate on the repair work? The hiss is very audible and I won't be able to live w/it. Thanks for the info.

Thanks for all the suggestions. I'll update the thread after I swap out all six. I've had good luck with SF / Anthem tech support so I think it's time to shoot them an email as well.
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