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[J.J. Blair]

I have a '53 Fender TV faced Deluxe amp.  At some point, somebody changed the 6SC7 tubes for 12AX7s.  Anybody here familiar enough with these?  I'm wondering how different the response will be if I put 6SC7s back in.

[bigaudioblowhard]

I don't have any of my tube substitution books here at the studio, (on the list) but at a glance I would guess the 6SC7 is 6 volts on the filament, as opposed to 12 volts on a 12AX7.

bab

[compasspnt]

Voltage on both is 6.3v, is it not?

[Dan Kennedy]

6sc7 is a dual triode octal socket with a common cathode.

I didn't dig real deep, but I'd suspect it's a bit higher in filament current, but pretty much the same performance as a 12AX7 and would be a direct substitute, with a socket change.

The difference in modern 12AX7's sound would probably swamp out the sound of the substitution.

[J.J. Blair]

Yeah.  Thanks, guys.  

Whoever did this swapped the sockets.  NOS Amphenol bakelite octal sockets are plenty available and cheap if I want to change back.  It appears that on one of the 12AX7s, a cathode self bias circuit was added, and there a couple resistors in line, as well as some caps added on the pins.  I didn't see which pins it was, to see if it's something tying the two sides of the dual triode together, but all these things don't exist on the original schematic, when the 6SC7 is employed.

Seeing how these amps are going for like $3,000 for beat up ones, maybe I should make it as original as possible.  I still don't get why these amps are worth so much money, and Ampegs aren't.  I don't honestly find this any more appealing than my R12 Rocket, and my Gemini kicks the crap out of just about any tweed Fender I've ever heard.  

[GaryR47]

compasspnt wrote on Wed, 15 July 2009 20:08

Plate voltage on both is 6.3v, is it not?

I believe you're thinking of filament voltage.  The plate(anode) voltage depends on supply voltage, cathode voltage, plate load resistor, etc.  But I'm sure you knew that.  No pun intended.

Gary

[Michael Brebes]

Like someone said, the 6SC7 an octal base tube which is also in a metal can.  Sounds a lot different than a  12AX7.  Has a lot less gain.  If you are not going to put it back to original, I would opt for a 12AY7, which has a gain more similar to the original tube and it was used in the later tweed Deluxe amps.

[compasspnt]

GaryR47 wrote on Thu, 16 July 2009 10:45

compasspnt wrote on Wed, 15 July 2009 20:08 Plate voltage on both is 6.3v, is it not? I believe you're thinking of filament voltage.  The plate(anode) voltage depends on supply voltage, cathode voltage, plate load resistor, etc.  But I'm sure you knew that.  No pun intended. Gary

Oops, yes, thanks!

[J.J. Blair]

mbrebes wrote on Thu, 16 July 2009 09:30

Like someone said, the 6SC7 an octal base tube which is also in a metal can.  Sounds a lot different than a  12AX7.  Has a lot less gain.  If you are not going to put it back to original, I would opt for a 12AY7, which has a gain more similar to the original tube and it was used in the later tweed Deluxe amps.

Hmmm.  Perhaps this is why they switched to the 12AX7, to get higher gain.

Does the 6SN7 have more gain than the 6SC7?

[Dan Kennedy]

Now that I'm at work and have my receiving tube manual in front of me, the 6SC7 is a "high-mu" dual triode, with an amplification factor of 70 vs the 12AX7's 100.

The 6SN7 is much lower gain at amp factor of 20.

The 12AY7 and 12AZ7 are closest in gain to the 6SC7 at 60.

The plate resistance and transconductance of each are different, so how they sound and ultimately perform is going to be all over the map as well. Not huge differences, but you'll hear it.

[MagnetoSound]

bigaudioblowhard wrote on Thu, 16 July 2009 02:32

I don't have any of my tube substitution books here at the studio, (on the list) but at a glance I would guess the 6SC7 is 6 volts on the filament, as opposed to 12 volts on a 12AX7.

Don't forget, the 12A*7 series has a 12.6 volt filament with a centre tap. It is usually configured as two 6.3v heaters in parallel.  

[Stephen Payne]

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