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!2AX7, octal replacement?

76.191.236.101
Posted on November 5, 2009 at 06:36:31
tesla
Audiophile

Posts: 1727
Location: San Diego County, California
Joined: October 25, 2000
 Seems that I remember hearing somewhere that there is an octal replacement for the 12AX7. If so, I would like to consider try replacing the 8 12AX7's in my pre with the Octal replacements. Yes, I know, I will have to make 8 adapters.

If I am right though, what is the difference? Anyone done it?

What is the average cost for the replscement? I will assume that they are more expensive.

Regards,

Gene


"Son, let this be a lesson to you: never do tequila shooters within a country mile of a marriage chapel."
- Al Bundy
Ok,, posted on November 5, 2009 at 15:48:34
tesla
Audiophile

Posts: 1727
Location: San Diego County, California
Joined: October 25, 2000
 I see the errors of my ways! If I were to attempt that, I might as well just create a whole new pre!

Thanks guys!

Gene
"Son, let this be a lesson to you: never do tequila shooters within a country mile of a marriage chapel."
- Al Bundy
I have done it. I duplicated my 9 pin sockets with octal and . ., posted on November 5, 2009 at 18:22:01
John PA
Audiophile

Posts: 2858
Location: Santa Rosa, CA
Joined: August 27, 2000
 used 12SL7's and 12SN7's. Looked at the operating points etc. Good 9 pin tubes do just fine.

I have my analog section of my dac set up to do 9 pin or octal. I have some of the best 6SN7's in the world in NOS and still prefer, and this is adjusting operating points, the 6DJ8, when it is implemented correctly in this postion.

I will admit though that my favorite tube is neither 9 pin or octal but the balloon 1929 Sylvania 27 of which I also have NOS in nice old boxes.
The Elaborate DAC=Johnny's Balloon Meshplate 27 Balanced Preamp=Balanced Six Pacs=Newform Research 645
iRiver H140/120gb/iBasso D10/Monica II Dac, modified Woo 6/Ultrasone Ed. 9/JH13 Pro IEM's

see images at: www.pbase.com/jamato8
RE: !2AX7, octal replacement?, posted on November 5, 2009 at 09:36:11
Palustris
Audiophile

Posts: 251
Location: Cape Cod
Joined: September 12, 2008
 There are two octal equivalents to the 12AX7 the 6F5 and the 6SF5. The mu is the same at 100 and the Rp and transconductance are very close. The 6F5 uses a grid cap which would add to the complexity of the mod. But either should work fine in a circuit designed for a 12AX7, including the RIAA since the Rp is almost identical.

I would suggest that this would not be a practical approach for a retrofit; it would make much more sense for a new design. That said, right now Antique Electric Supply is selling the 6SF5GT for $4.32 so it would be a cheap to try a few. It would be a PIA to build 8 adapters though. If you are really curious, why not build a couple of adapters and try them out to see if you like the sound. Make sure you use the adapters in the same stage of both channels. Start with the last stage of both channels, for instance.

Experimenting with new tubes and ideas should be part of the hobby. Get yourself a good tube manual and you will come up with lots of new ideas.

reminder, posted on November 5, 2009 at 13:02:07
Jim Dowdy
Manufacturer

Posts: 1168
Location: western North Carolina
Joined: July 22, 2000
 The specs for the 6F5 and 6SF5 are indeed similar to one-half of a 12AX7...that means you would need a pair of these octals to replace a single 12AX7.

RE: reminder, posted on November 5, 2009 at 14:46:57
Palustris
Audiophile

Posts: 251
Location: Cape Cod
Joined: September 12, 2008
 Yes, I had forgotten that. These are single triodes so only relevant to new builds.

Don't do it., posted on November 5, 2009 at 07:44:24
Chris O
Audiophile

Posts: 3060
Location: Memphis, TN
Joined: March 22, 2002
 The closest octal to a 12AX7 is the 6SL7. However, the octal 6SL7 has a Mu of 70 versus the 12AX7 Mu of 100. They have different plate resistance and transconductance and need to be set at different operating points to be optimized. There is no octal tube that is 100% electrically identical to a 12AX7. If you are talking about a phono stage in there as well you will have issues with the RIAA curves.

Could you plug in the tubes and get sound out without making any changes? Yes, but it will not sound as good.

___
Long Live Dr.Gizmo
Plus..., posted on November 5, 2009 at 09:11:06
Allen Wright
Manufacturer

Posts: 4729
Location: Schaffhausen
Joined: January 31, 2002
 ...you will have all the extra connections in the adaptors - along with their associated unreliabilty issues.

Don't swollow the hype that octals are better than 9 pins, they are not! They are different, yes, but not better.

And the hassle will be flatout not worth it.

Regards, Allen

RE: I second Allen's point.........nt, posted on November 5, 2009 at 14:59:28
Neff
 .

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