Use both triode sections of the 12AX7!

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Use both triode sections of the 12AX7!

Postby michael_nipomo » Wed Jul 18, 2007 2:52 pm

This question may have been answered before, but I can not seem to find it: Why not use both triode sections of the 12AX7? I have read that there are advantages to doing that!
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Re: Use both triode sections of the 12AX7!

Postby Shannon Parks » Thu Jul 19, 2007 6:01 am

michael_nipomo wrote:This question may have been answered before, but I can not seem to find it: Why not use both triode sections of the 12AX7? I have read that there are advantages to doing that!


Hi Michael,

The positives would be:
- lower output Z for certain applications (but not really here since the phase splitter input Z is already very high)
- better PSRR which really isn't a benefit in this design - maybe in a SE
- more gain (again, not needed)

The negatives would be:
- lower input Z for the amplifier by half
- mismatched triodes less linear
- usually less linear output (according to the tube curves) in similar circuits
- the Rp and Rk values would need to be changed

Since the positives seem to not help in this design and the negatives could really hurt the linearity, the sides aren't paralleled. Indeed, the Eico HF87 used half a 12AX7 as its gain stage (using both halves in a stereo amp).

I did make an allowance for a compromise with the layout. With the diytube Poseidon board you can cut the 'X' at V1 and rotate the socket at V1 180 degrees when mounting it. Then V1 will use triode #2. With a monoblock pair you can have one amp using triode #1 and the other #2. Every six months you could rotate the 12AX7s. Too clever by - err - half?
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Re: Use both triode sections of the 12AX7!

Postby Ty_Bower » Sun Sep 30, 2007 8:29 pm

separks wrote:I did make an allowance for a compromise with the layout. With the diytube Poseidon board you can cut the 'X' at V1 and rotate the socket at V1 180 degrees when mounting it. Then V1 will use triode #2.


What are the two traces marked 'X' supposed to do at V2 (the 12AU7 socket)?
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Postby Shannon Parks » Mon Oct 01, 2007 5:43 am

You can cut the two 'X's at V2 and add a small jumper from E to pin 4. Then you can use 6.3V tubes: 6CG7/6FQ7s and 6GU7s.
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Half 2B or not 2B, is that the question...

Postby EWBrown » Mon Oct 01, 2007 7:49 am

The VTL amps use the input 12AT7 with both halves paralleled. To implement this properly, the plate and cathode resistors would each be half of the original value for a single section.

The actual gain doesn't change, since the plate resistance is effectively divided by two, and the transconductance is effectively doubled. The gain is the product of Rp and gm, so the figure theoretically stays the same.

Supposedly the increased gm allows for lower noise floor and increased dynamic range, but I'd SWAG that is more subjective than objective. Yellow_Light_Colorz_PDT_17

/ed B in NH
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Re: Use both triode sections of the 12AX7!

Postby tubularfluxus » Sat Jun 11, 2016 5:49 pm

Sorry for the stupid question...

I understand I need to rotate 180 degree the socket of the 12ax7 of one of the two monoblocks, but how about the "cut the X" part? What does it mean? Do I need to connect a pin with another pin or such?

Thanks!
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Re: Use both triode sections of the 12AX7!

Postby cdmawolf » Wed Jun 15, 2016 1:16 pm

V1 has 10 holes in the pcb. If my memory serves me correctly there is a trace between pin 9 and the unmarked pin where the "X" is.
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Re: Use both triode sections of the 12AX7!

Postby tubularfluxus » Thu Jun 16, 2016 9:13 pm

This is how the PCB looks below. Does it mean I should solder the socket rotated 180 degrees on the other board and then...cut the trace between ALT and 9, where the cross is?
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Re: Use both triode sections of the 12AX7!

Postby cdmawolf » Fri Jun 17, 2016 9:21 pm

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Re: Use both triode sections of the 12AX7!

Postby tubularfluxus » Sat Jun 18, 2016 12:51 pm

Great!!! This is very helpful! Thank you
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