# r-f tube amp neutralisation

Discussion in 'Homework Help' started by howartthou, Sep 11, 2009.

1. ### howartthou Thread Starter Active Member

Apr 18, 2009
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Hi All

Attached are 3 pics. The first is an r-f tube amp. The next is a circuit of Plate Neutralisation and the next is Grid (Rice) Neutralisation.

I am supposed to look at the r-f amp and determine if whether first tube is using Plate or Grid Neutralisation and the same for the second tube.

Simple I thought, just match the circuits, but not so easy for me.

I would really appreciate some guidence as to how to read the rf-ciruit and compare it to the Grid and Plate.

Here is my attempt:

Compare tube 1 in rf-amp with Plate
===========================
V1 grid --> Bottom of L1 with variable C3

This means its Plate Neutralisation.

Compare tube 2 in r-f amp with Grid
===========================
V2 grid --> Bottom L2

Ummm, looks like Plate again but I think it should be Grid

Analysis
======

I can't see the difference between V1 and V2, they both seem to be connected as Plate Neutralisation but must admit I really can't see whats happening here.

How do I know if V1 is Grid or Plate

How do I know if V2 is Grid or Plate

They look the same to me

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2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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1,776
Can you tell me the difference in the Cn's connections on both the plate and the grid neutralization diagrams?

3. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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427
Howdy:

The way to tell whether it's plate or grid neutralization is to look at where the 180 degree phase reversal takes place....the best hint is a center-tapped inductor. If this center tapped inductor is connected to the plate, it's plate neutralized, if it's connected to the grid, it's grid neutralized.

Interestingly enough, the function is IDENTICAL, either way....to supply an out of phase current (with respect to the interelectrode plate-grid capacitance)....it's generally only mechanical considerations that make the choice for you in the real world.

Eric

4. ### howartthou Thread Starter Active Member

Apr 18, 2009
96
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Hey Kl7aj

Thanks for making it much clearer but which ever way I look at it they both connect to the centre tap, thats why I am confused.

What exactly is "it" Do you mean the Cn? If so it connects to the Grid in both the Grid and Plate diagrams.

Can you please explain exactly how Cn connects to the Grid and not the Plate in the Grid diagram?

And then the same for the Plate diagram

It seems to me right now that "all paths lead to Rome", in my case Cn always leads to the Grid

5. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,208
427

"IT" is the center-tapped coil. In the first picture, it's connected to the plate on the first stage, and connected to the grid of the second stage.

Eric

6. ### howartthou Thread Starter Active Member

Apr 18, 2009
96
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Thanks Eric, if I follow the path from the top of the tap it does go where you say it does in all the diagrams.

So now I know how I should read it I still don't understand why its read from the top of the tap and not the bottom, I can only assume its the way the current flows. I wish they would put little current flow arrows on the paths....gues I could draw in a sim software and see how it behaves there. But thanks, I can see how to read it now.

One more question if you don't mind. I am asked what would happen if c4 was shorted?

I am not sure if it would reduce the overall rf amp output or if it would just reduce the plate current to V1?

I really don't know the purpose of C4?

7. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,208
427
C4 is the tank resonating capacitor. It forms a parallel resonant circuit with L1, applying the proper load impedance to the tube, (and in the case of a class C amplifier, supplies the missing 80% of the waveform!) The bottom of L1 is 180 degrees out of phase with the top of the coil, which is where you get the neutralizing phasing (for the most part)

eric

8. ### howartthou Thread Starter Active Member

Apr 18, 2009
96
0
Thanks Eric, I will need to research this a bit more but you have given me something to work on.