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Hi pitched hum from TT pre-amp?

Hello,

I have a Sonic Frontiers SFP-1 pre-amp. My rig is a Scoutmaster table and a Benz Micro Glider (MC). While playing a record last night, between songs I heard a relatively hi-pitched tone; probably a bit below the 1 Kilohertz frequency (test tone). I gently tapped the top of the TT pre-amp and the tone stopped! The pre-amp contains 4 tubes; two Sovtek 6922's and two 12AT7A's. Do you think I have a tube on its way out?

Any feedback would be welcomed.
judsauce
8 responses Add your response
atmasphere's avatar
atmasphere
11,574 posts
 
Sounds like your tubes are likely shot.
judsauce OP
115 posts
 
Thanks Lewm and Atmasphere. I'm going to take a look under the "hood" this weekend. And, actually, I was playing an LP last night, and it sounded muddy and a bit compacted and boomy. So, something is going on.
atmasphere's avatar
atmasphere
11,574 posts
 
If its a loose tube you will get a crackle and possible loss of signal. OTOH a microphonic tube can 'sing' (whistle, whatever you want to call it) and often responds or stops when tapped or subjected to vibration. Such tubes won't sound their best either.
lewm
12,448 posts
 
Or a tube socket going bad or a tube that is loose in a failing tube socket. Take a look inside. Could be fixable by merely straightening up the culprit tube in its socket, but then you may want to make plans to do some freshening.
atmasphere's avatar
atmasphere
11,574 posts
 
The complaint sounds like a microphonic tube.
judsauce OP
115 posts
 
Thanks for your responses. Yes, it is an old unit. I had it checked out about 7 years ago, and tweaked to match my cartridge. I think it needs to go back for a check up.

best regards.
marakanetz's avatar
marakanetz
5,473 posts
 
i believe that this unit is over 20 years old and needs power supply caps replaced as well especially if new tubes didn't solve problem.
try to visualize dc caps with loupe for misshapes, blured painted tech info, signs of burning or high heat exposure on PCB in the connection area and also cold solder joints.
mofimadness's avatar
mofimadness
4,933 posts
 
"Do you think I have a tube on its way out?"

It could have been just a loose tube and by tapping you reseated it somewhat, but it probably is a bad tube.
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