Asymmetrical Phase Inverter?

[jbefumo]

In preparation to building a new amp, I'm working my way through the schematics of several well-known circuits, trying to understand what's going on. Thus far, I've made it through to the phase inverter. My latest confusion regards the values of the two (cathode load?) resistors. In the majority of circuits I've perused, the values are the same, which makes sense to me. However, when I came to the Fender Super Reverb, I found that they were asymmetrical (see attached image), with the top one being 82K, and the bottom, 100K. Could someone explain, please? Thanks.

Joe

[FunkyE9th]

Actually for a long tail pair PI, the 82k is supposed to make the output more symmetrical. If you were to use a 100k in place of the 82k, the inverted output of the PI will be greater than the non-inverted output. So to compensate, they use 82k to lower the gain of the inverted out.

[jbefumo]

Thank you, Funky!

I'll have to play with that until it sinks in. You guys are the best!

Joe

[jaysg]

If you see a schematic where both plate resistors are 100k, then there is probably an input at the junction where the 100 ohm resistor is place -- some voxes, marshall 18W. Put another way, when both sides of the PI are used for inputs, then the resistors are usually set to match.

[Rattlecat]

This article helped me out.


http://www.300guitars.com/articles/arti ... -inverter/

[dehughes]

Another helpful article:

http://www.aikenamps.com/LongTailPair.htm

I'm understanding then that a larger "tail" resistor will help compensate for the differences on each side, thus somewhat eliminating the need to drop one plate resistor relative to the other.

What I don't get is how this tail resistor adjusts balance and headroom, as Aiken states...

[Andy Le Blanc]

Its a form of current feedback. Audio frequency potential is developed across
the un-bypassed bias resistor and tail resistor.

[gearhead]

What I'm trying to understand is the signal polarity as it goes from the cathode (of the inverting triode) to the non-inverting triode cathode thru grid to plate.

Cathode IT is set up as a common cathode for it's output, which inverts the circuits ouput (because plate resistor drops higher voltage as the signal rises, thereby reducing the circuits output).

Cathode NIT gets it's input from the Cathode IT, as a sort of cathode follower arrangement.

The signal polarity at the IT and NIT Cathodes should be the same?

if so, for this to work, the signal at the NIT plate has to be inverse from the NIT cathode (for the circuit output to be non-inverted)

Does this mean that the grounded grid for the NIT inverts the signal? If so, how?

[Wayne]

Do what I had to do - grab an envelope, scribble down a sketch, and put a positive half cycle in at the IT's grid.

IT plate goes negative.

IT cathode goes positive. (like a cathode follower)

NIT cathode also goes positive. This increases the voltage between the cathode and the grid.

More voltage btwn cathode & grid = less current through NIT.

NIT plate voltage rises.

When I was taught this, it was with a transistor in a grounded base configuration. I scratched my head then, and still do until I follow it through like that.

W

[Kregg]

Some of the material is like having the rosetta stone, but not knowing how to use it. There has got to be a simpler way to communicate this concept. The problem with most technical minds is that outside of math they are dreadful at communicating ideas. Perhaps that's why they use press agents and are usually considered quirky (like Dumble).

But hey, as long as it equals great tone, I don't need to know exactly why it works as it does.

[gearhead]

I can see that working, but only if there is some voltage on the "grounded" grid. Aikens seems to indicate that is the case, provided by a voltage across one of the longtail resistors.

Otherwise when the NIT cathode signal goes negative, the NIT cathode would be -lower- than the grounded grid. I think you'd lose a lot of signal to ground.

BTW, a plate doesn't invert by itself in a common cathode circuit. That one took me a while to get to the "aha" moment, with a bit of help.

https://tubeamparchive.com/viewtopic.ph ... highlight=

What causes the inversion is the plate resistor. And to be perfectly clear, what we are seeing is -voltage- inversion, but not current inversion.

[FunkyE9th]

There is voltage on the grounded grid with respect to the tail resistor. As AC current flows thru the tail resistor, the voltage at the grid (NIT triode grid) with respect to the tail will vary, which in turn causes the plate voltage to vary.

You can look at a long tail pair as a common-cathode amplifier (the inverting triode) driving a common-grid amplifier (the non-inverting triode). Common-grid amps do not invert. I guess another way of looking at it is a cathode follower driving the common-grid.

[gearhead]

There is voltage on the grounded grid with respect to the tail resistor. As AC current flows thru the tail resistor, the voltage at the grid (NIT triode grid) with respect to the tail will vary, which in turn causes the plate voltage to vary.

You can look at a long tail pair as a common-cathode amplifier (the inverting triode) driving a common-grid amplifier (the non-inverting triode). Common-grid amps do not invert. I guess another way of looking at it is a cathode follower driving the common-grid.

Since am focusing on the NIT, have looked at it wrt the later. Guess what it comes down to is -what- mechanism(s) transpire to make a common-grid amp not invert? If I'm reading it correctly, the grounded grid must -always- be negative wrt to the NIT cathode voltage.

Referring to Aikens http://www.aikenamps.com/LongTailPairDesign.htm

Looks like the only source of voltage (differential) is from RG2. Looks like the bottom of the common cathode resistor Rk and bottom of RG2 are at the same voltage (as the top of the Long Tail resistor).

This one has driven me nuts, intermittently, for a year now, LOL.

[FunkyE9th]

Guess what it comes down to is -what- mechanism(s) transpire to make a common-grid amp not invert?

Referring to Aikens http://www.aikenamps.com/LongTailPairDesign.htm

What may be confusing you is that the input to common-grid is not at the grid. It's at the cathode. Under DC conditions, the point where Rg1, Rk, Rtail and Rg2 connects is setting the bias. Cin1 and Cin2 are open under DC. Then under AC conditions you can assume Cin1 and Cin2 to be shorted. So the grid of the right triode is grounded and the preamp signal drives the grid of the left triode (via Cin1). So as the voltage rises on the grid of the left triode, the voltage at cathode also rises (call this node Vk). It's in phase like the output of a cathode follower. The resulting AC signal on node Vk is the input to the common-grid. But now the grid of the common-grid (call this Vg) is grounded. It's 0. So Vg-Vk is more negative if the input is going positive. This makes the plate of the common-grid swing positive (in phase). More negative means less current through the tube, so the plate voltage swings positive. The opposite happens when the input swings negative. Vg-Vk becomes more positive, so more current through the tube, so the plate voltage drops.

It's very confusing. It took a while for it to sink in for me and I had the luxury of having a book to read several times over. This book is really good... http://pentodepress.com/power-amp/contents.html He breaks it down really well and he goes thru the process of deriving all the equations.

[FunkyE9th]

Edited the above. I hope I explained it correctly now.