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6SF5 preamp tubes?!?


 
10/26/1999 11:43 AM
keith
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6SF5 preamp tubes?!?
I recently purchased a Bogen J330 (1954?) PA head at auction, and I'm thinking about an amp project. It uses 6SF5 preamp tubes for each of its four channels. Is anyone out there familiar with the sonic characteristics of this tube as regards guitar? I'd listen for myself except I'm still waiting for filter caps to arrive in the mail, since it keeps blowing fuses right now. Thanks in advance!  
 
Keith
 
10/26/1999 4:00 PM
Scott
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IMO, they sound pretty similar to a 6SL7, but even cleaner. Both the 6SL7 and the 6SF5 sound cleaner than any 12AX7 to my ears, with warmer midrange and better bass. This may be good or bad depending on what kind of amp you are making. I have reworked several PA amps similar to your find and I have always stuck with the octal preamp tubes, but then I use octals for my new designs also.
 
10/26/1999 5:35 PM
Doc
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You'll find that most of the older, octal based audio tubes sound fuller and have richer bass than their miniature successors.  
 
The 6SF5 is a single high mu triode, and is exactly a 6SQ7 without the two little detector diodes. These tubes have a mu and Rp equal to one section of a 12AX7, so they are equivalent to a 12AX7 from a biasing and voltage gain standpoint. Any circuit designed for a 12AX7 can use a 6SF5 as a direct electrical replacement, although 6SF5s are a little hard to come by. When I need one for an antique piece that requires one, I have a tough time finding a good, non-microphonic one.  
 
Keith, if you decide to pull the 6SF5s and use something else for your circuit, please keep me in mind.  
 
Thanks,  
Doc
 
10/26/1999 7:31 PM
keith
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Doc, I'm still not sure what I'll end up doing with this beast, but I'll let you know if I decide not to use these. However, from yours and Scott's descriptions, I may just keep them. Thanks for the heads-up.  
Keith
 
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