Can someone explain the inverter 12AX7 in the Fender Twin 5D8?

[bluefoxicy]

http://www.schematicheaven.com/fende..._5d8_schem.pdf

Explain this, wtf is with the 12AX7? I guess it's an inverter (I don't know how to make one), I'm trying to understand that whole area of the circuit though (right to the power tubes).

This reminds me, I still have to do a write-up about the rectification stage as someone (I believe Enzo) was kind enough to explain it a while back in another post.

[Steve Conner]

It's a see-saw PI. The top half of the 12AX7 is a regular gain stage, the bottom half is an anode follower with a gain of -1.

I used the same circuit in my Ninja Toaster and it made me happy.

http://scopeboy.com/poweramp.gif

[jrfrond]

It's referred to as a paraphase inverter. The output of one triode is gain-reduced via a voltage divider and fed to grid of the other. Both triodes operate in common-cathode configuration. The system works, but because the sections do not share a common cathode resistor (hence do not share/split cathode current), the symmetry and balance are pretty much a factor of how closely the triodes are matched.

[Steve Conner]

No, this is the floating paraphase, not the regular one. If you look, you'll see that the voltage divider has a ratio of about 2:1, and the gain of a 12AX7 stage is a good deal more than 2. Also, the divider is hooked between the two outputs of the PI, not the first output and ground.

[bluefoxicy]

sigh there's all kinds of schematics sites but is there somewhere I can go that will explain the circuit?

From what I gather, for example, the top right circuit coming off the OT secondary (27k resistor to 12AX7 cathode) is a negative feed back, to reduce THD. WTF is the presence dial there for (5k VR)?

More to my immediate interest, the signal path seems to be 12AX7(1) grid, anode follower to a pair of 100K resistors connecting ... the grids of the power tubes? Also bypassed by a 0.0001uF cap (?!), and grounded to the negative rail supply. Beyond this both ends are DC filtered with a 0.05uF cap (passes most bass freqs), then BOTH are connected again by resistors to a center point that goes to the 12AX7(2) grid. Each hits the 6L6 grid from there, while that circuit in the center goes to the ground, along with the bias circuit and power cathodes (which I assume we can ignore since the common ground is between these).

okay, but WHY? The anodes are a power supply, those have to go together that way, and that won't bleed signal from either side right? The tiny cap will bleed some extreme highs across though.... The 0.05 caps center the DC offsets (this is important), right? WTF is with those 220k and 279k resistors, and how do they bring the proper signal to the grid of the 12AX7(2) triode to produce the signal for the lower 6L6? Does any part of this circuit serve specifically to reduce THD, or is it all essential? Etc.

[Rob Mercure]

Presence in this case is upper mid frequencies - the 5K pot and the 0.1 uf cap create a frequency selective network within the negative feedback loop that cuts the upper mid "Presence" frequencies.

As far as the PI goes the way that it maintains the gain of "1" in the lower triode is by another negative feedback loop that "bucks" the voltage gain down to 1. The 0.0001uf cap just provides some high treble cut in this design - it has nothing to do with the function of the PI. The 0.05 uf cap simply blocks DC from the plates of the PI tubes to the output tubes - this is common to most PI designs (tranny/inductor excluded). The 220 and 279K resistors serve as voltage divider which functions with the little local feedback loop to keep the stage gain at 1. Nothing in the design is specific to reducing THD in the output section other than picking the operating points for the 6L6s.

Rob.

[CitizenCain]

sigh there's all kinds of schematics sites but is there somewhere I can go that will explain the circuit?

Excellent resource here, bluefoxicy, http://www.freewebs.com/valvewizard/index.html

It even explains the paraphase inverters

[BackwardsBoB]

Thanks for the great link to an excellent site, Citizen Cain!

It pays to hang around here.