2. amps-preamps
  3. 1142033337

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

Showing 4 responses by phd

phd
1,330 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.
phd
1,330 posts
 
Snipes, I did get an estimate from SF but that was last year & can't remember the exact dollar figure but it wasn't alot. In my case as well, sometimes the hiss was not there when it was initially turned on but became evident as the unit was left powered up.

With hybrids tube rolling can be fun but you always have to make sure the unit has been powered off at least two minutes before swapping tubes, (100% tube designs being the exception). My belief that this is the leading cause of failure to the solid state followers. Some mfgs will stress this in the manual. Unfortunately not all of us are aware of this problem as some units are purchased without the manual or the manual is not read thoroughly.

Jea48 mentioned another good point with cold solder joints & could be looked into as well.
phd
1,330 posts
 
Snipes, I agree with Jea48, let us know!
phd
1,330 posts
 
Snipes that's excellent! Keep playing them tunes!
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