Phase Inversion

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C Moore
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Joined: Mon Oct 22, 2007 9:28 am
Location: USA, California, 94585

Phase Inversion

Post by C Moore »

OK, keeping in mind that this stuff is still pretty new to me.....
I understand how/why phase inversion occurs with a (I hope I have my terms correct) Para-phase and a Cathodyne inverter. What I do not understand is the Long Tail (again, terminology) Phase Inverter.
To me it looks like the same signal is fed to both halves of the dual triode grids and that signal leaves the plates of both triodes. So wouldn't the phase be the same at both plates.? Obviously it is not, and that is what I do not understand.
Hopefully my confusion is apparent to you guys.
Thank You


http://www.el34world.com/charts/Schemat ... _AA864.pdf
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FYL
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Joined: Fri Feb 17, 2006 1:05 am

Re: Phase Inversion

Post by FYL »

This is a differential PI, aka Schmitt or AC long tail pair. Two inputs, two outputs; the PI amplifies the difference between inputs and outputs two out of phase signals of equal amplitude.

In the Fender schematics signal is applied to input 1 thru the 500p cap, feedback to input 2, which is grounded thru a 100R. The LTP amplifies the difference between signal and feedback.

Check for instance http://www.freewebs.com/valvewizard/acltp.html for more info.
Firestorm
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Location: Connecticut

Re: Phase Inversion

Post by Firestorm »

I'll give it a shot. There are basically three ways to use a triode. You can ground the cathode (usually through a resistor, but it doesn't have to be), put the signal in through the grid, and take the amplified signal off the plate (which requires a plate resistor because the signal develops across the voltage difference between the "top" and "bottom" of the plate resistor). This is what you see in most guitar amp preamps. You can also ground the plate (it isn't really "grounded," but is connected directly to the the power supply without a plate resistor; you put the signal into the grid and take it off the cathode (again this requires a resistor so the signal has a place to develop). This is a "cathode follower" arrangment and it has, at best, unity gain (no gain). But you can also ground the grid (usually through a capacitor for AC signals), put the signal in through the cathode and take the amplified signal off the plate (again, a resistor is required).

In the long-talied pair, the first triode is the first type ("common cathode"), but the second triode is the third type ("grounded grid" or "common grid.") The signal actually comes in through the cathode (the cathodes are connected together). This type of amp does not invert the phase, so you get an out-of-phase signal on the first triode plate and an in-phase signal on the second triode plate.

I hope that didn't make it worse.
C Moore
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Joined: Mon Oct 22, 2007 9:28 am
Location: USA, California, 94585

Re: Phase Inversion

Post by C Moore »

I wanted to get back to both of you guys and say THANKS. Correct me if I am wrong, but this has got to be one of the more confusing/hard to grasp circuits in a tube guitar amp. At least it is for me...that is, from the perspective of a guy who is just learning electronics/electricity. IME so far, this has been about the least intuitive circuit I have stared at.
Thanks Again....belated
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Structo
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Location: Oregon

Re: Phase Inversion

Post by Structo »

Randall Aiken has some great technical articles on his website.

There is one on the Long Tail Pair phase inverter.

For a push/pull type audio amp the LTP is perhaps the best although some other older types can be found in some of the tweed Fender amps.

http://www.aikenamps.com/TI_Aiken_adv.htm
Tom

Don't let that smoke out!
tubeswell
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Location: Wellington. NZ

Re: Phase Inversion

Post by tubeswell »

Here's an explanation I have given elsewhere

The tail resistor and bias resistor are both un-bypassed so the signal being inputted from the '1st' stage grid (which is a typical inverting gain stage - hence it inverts the signal on that plate) causes a cathode resistor 'feedback' swing, which (since the 2 stages are connected by a shared cathode) transfers to the '2nd' stage (which - since the grid of that stage is grounded w.r.t. AC - causes the plate to swing), but this time the plate swings in synch with the cathode (i.e. it is non-inverting). So you get two opposite phases coming off each plate.

The tail resistor is much bigger than the cathode resistor, so the idea is that you get all the swing happening in the cathode resistor, and not much at all happening in the tail. In fact the tail is supposed to be a 'constant current source', but it actually does wriggle a little because it isn't bypassed.

Now the signal on the 1st side typically tends to be a bit stronger than the other side (unless you 'dilute' the plate-to-cathode voltage - on both sides - by increasing the size of the tail resistor), so you typically see 82k plate resistor on the 1st stage and 100k on the second to counteract this, in situations where you have a smaller tail resistor (like around 10k or so).

But, as I alluded to just before, if you increase the tail resistor, you lose more of that overall B+/HT voltage drop in the tail, and this reduces the plate-to-cathode voltage accordingly (remember that the B+/HT voltage is constant), and then you can get away with evening up the plate resistor on both sides, but at the expense of gain.
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