A different type of floating paraphase

[octal]

This is an interesting variation on the floating paraphase. Sort of a reverse LTP.
Any thoughts on advantages, disadvantages?

[soundmasterg]

It looks the same as any other floating paraphase like that used in a Vox AC100 except with the addition of R2 and R3, and the bias or ground connection in between the two resistors. If there was a filter cap off of R1 then that area would be the same as the Vox. I don't see the point of connecting the bias to the junction of those two resistors as they will limit the bias beyond what it would be if it was connected in the normal manner.....unless they are trying to save a resistor or two by making them do double duty there, but if so they I would guess they are restricted in the values they can use in that spot. If that location is grounded I don't see how the circuit would work, since the first inversion signal would then go to ground instead of on to the 2nd half of the inverter. Did someone make a mistake when they designed that one? Take a look at the AC100 circuit for comparison.

Greg

[pdf64]

Greg, the way I see it is that R1,2,3 are allowing for a common mode error signal to exist at their node (with perfect balance the Vac there will be zero). That error signal is then fed to the second triode grid, thereby completing the feedback loop.
In that way it is different to the Vox self balancing paraphase, which is essentially just using an inverting NFB amp, set up for a gain of 1; although it could be argued that the linked cathodes perform a similar function to R1 in the OP. Pete.

[octal]

Pete- your analysis of how it works is essentially the same as the one in the article I cribbed the schematic from. I'm just wondering how it would compare to a regular floating paraphase... the author mentions that the drop across R1 can limit the swing of the PI (much like a tail resistor with a LTP.)

I don't recall ever seeing this arrangement in any of the bazillion guitar amp schematics I've viewed over the years...

Nathan

[pdf64]

Does the article give any guidance, with regard to component values, tube type, expected gain / output impedance etc?
Someone has to build one; Leo tried most other PI types in his amps, is there good reason this one was passed over? Pete.

[octal]

Here's the article- just scroll down to floating paraphase.

www.audioxpress.com/resource/audioclass/ga200ac.pdf

The author also gives a sample design in a later issue, but that uses nested feedback loops applied to both cathode resistors, so you couldn't easily cut and paste the circuit into a guitar amp. (presumably if you built the circuit as drawn it would be far too hi-fi to be good for guitar.)

Nathan

[ThermionicScott]

That's an interesting approach. One advantage over the "Fender" circuit is that it'll work with a fixed-bias output stage.

- Scott