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octal preamp tubes - are metal ok to use?

1/8/2006 4:38 PM
Ian Anderson
octal preamp tubes - are metal ok to use?
ok, so the title says it all - I've got a SE EL84 OT and want to try some octal preamp tubes. Looking at some Fender schematics tells me that 6SC7s are the way to go BUT are the metal ones ok to use or should I stay with the glass variants.  
What about other octal preamp tubes?  
... Ian
1/9/2006 12:05 PM
Doug Patterson

Don't be afraid to use metal octal preamp tubes. Just check them out and make sure they are in good condition. I have used 6SL7 dual triodes, 6SJ7 pentodes and RCA "special Reds" 5693. The octals give a fatter sound and they are usually not real expensive. Down side if metal... you don't get to see the filament glow. Good luck.  
1/9/2006 3:57 PM
Ian Anderson

cheers Doug,  
I'm also thinking of using a dual-triode octal preamp tube for the clean channel on my hi-gain amp as I already have a hole in the chassis that will accomodate an octal socket.  
I'll probably just go with 6SC7s since there are published Fender circuits to copy from and get me started.  
What is the amplification factor with these type of tubes? (the more common ones anyway), by that I mean compared to common 9pins... ECC83=100, 5751=70 etc... etc...  
... Ian
1/9/2006 7:54 PM
Metal 6SC7's work fine and the mu is 70 like a 6SL7. The metal can serves as a shield and reduces hum considerably, but the shared cathode limits how much gain you can use in a cascade arrangement, a 6SL7 with seperate cathodes can give much more but many people have had hum problems using them as a preamp. I'm using a 6SC7 in a Fender 5C2 arrangement and it works well, also tried it with the triodes in parallel and that worked well too...
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