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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Neutralizing An Amplifier

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This is the idea, it has been done several years in RF circuits, and rarely I saw it in audio circuits, but yesterday I made certain simulations in LTS, and is really better than I could imagine.

In my project, a 9.5 pF extends HL limit more than a decade.Note that no other value of component has been modified. In practice, it can easily implemented using 3-30pF, and adjusting them using 1KHz square wave, and watching the corners of the steeps.

But I didn't know if it affects sound.
 

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The circuits are too small to see, but remember that this extra capacitance has to be driven by the anode current so be careful of slew-rate limiting.

Thanks for the suggestion, but as I know this, you can see that I take the high voltage side from the WCF instead of directly from plate.

I don't know how to re-size the figures in this page, as I made it at full 1024*768 at my computer's screen.
 
My original idea was to made something similar to an operational amplifier of vacuum tubes. As you can see, there exist two inputs, an inverting and another NI, also at the outputs, having moderately high input impedance, gain and bandwidth, and relatively low output impedance, using only triodes, and single supply (in fact I have two, but none of them is negative). And finally, using as few capacitors in the signal path as possible.
 
For my audio project, link to the trafo thread:

http://www.diyaudio.com/forums/tube...-toroidal-output-transformer.html#post3252972

The idea is to apply certain amount of FB, mixed of positive current and negative voltage to obtain a near zero output impedances as is suggested in the NFB chapter of the Langford Smith book, and others I read. But for apply positive feedback, I believe that te amplifier must be stable before using it.
 

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Assuming you are using LTspice so - use a screen capture tool or print screen in windows > select paste in paint and save as a .png and post here.

If you are running LTspice in linux there are all sorts of screen capture tools you can use to capture the schematic off of the desk top.

No, Kevin, I said real world: true tubes, true voltages and currents, true caps, mica trimmers and R's. And real wires, and tubes hot. No simulations.:wave:
 
"I want to ride my bicycle / I want to ride my bike" said Freddie Mercury in a song.

I want to write about my amplifier, paraphrasing him.

The trouble:

It is well known that a triode has small pieces of metal in proximity, and in this way, they have some capacitances between them. I will no longer explain amplification theory, there are lots of text about this item. I will concentrate in the effect of this capacitances, and amplification.

When a signal is applied to the grid of a triode, it is amplified, and appears at the plate as a current variation, and in the load resistor it is converted into voltage variations again. But this is true that this voltage variations returns to the grid via plate to grid capacitances (not only internal to the tube, but also in the socket and wiring circuit capacitances.) As this coupling is capacitive, the phase of the signal returned to the grid is rotated 90 electrical degrees. This causes negative feedback, and increases the grid´s conductance, and then high frequency lost of gain and phase rotation at the plate´s output.

This feedback (Plate to grid) capacitance can be referenced to the grid to cathode circuit y means of the well known Miller Effect, and added to the grid-cathode capacitance.

The solutions:

Many solutions had been proposed:

1) The use of a screen grid, converting the triode in a tetrode, and next in pentode, but both of them becomes noisier than triodes because of the Partition Noise of electrons in the grid-screen space.
2) To tune this capacitance with an inductance, a resource used in high frequency amplifiers, but as tuning is essentially narrow band, it can’t be used in audio applications. Plus, that inductors needed becomes large and bulky, and their high internal capacitances become worse the trouble.
3) To neutralize the amplifier. This consists of inject a same voltage amplitude at the grid of the circuit that is available from the plate, but with the phase inverted.

The last of this has been extensively used in HF, VHF and UHF amplifiers, even when pentodes are used. This not only stabilizes the amplifier, it also becomes less noisy and easy to adjust in wide bandwidth circuits as in tube TV tuners.

Usually, when a tuned circuit is included in the amplifier, it is easy to obtain an out of phase signal and then return it to the grid circuit via a variable capacitor, then a bridge circuit is formed, cancelling the effect of the plate-grid capacitance. This capacitor is made “trimmable” to get the optimal value.

My circuit of a exciter for PP EL34’s UL wired, makes use of this technique. Observe screenshots of my circuits, in which can be seen its complexity. It starts with a von Skoyok phase splitter(V1 : 12AU7, V2 : 12AX7), and then “cascoded” to V3 (12AX7), finally the output is via a White Cathode Follower (V4 and V5 12AU7’s) in which the signal is bootstrapped to the plate circuit increasing output voltage and linearity, as load impedance becomes very high. (Picture #1)

But as impedance becomes higer, stray capacitances (Including Miller ones) become very important, as a high frequency pole appears at the point in which capacitive impedance (Frequency dependant) is made equal to resistance in the circuit (Above 6 mega ohms in my calculus).

Observe in pic. 1 capacitors C9 and C10, they are the neutralizing capacitances. They carries an equal amplitude signal from the opposite amplifier to act cancelling the node capacitances.

Pic. 2 illustrates what happens in my circuit when C9 and C10 are less than 9 pF (Picofarads).
Pic. 3 shows the same circuit with 10 pF. Note overshoot.

But observe when C9 and C10 are made 9.4 pF exactly(Pic. #4). The bandwidth is extended more than a decade by cancelling the effect of Miller Effect.

As I haven’t in my lab a sweep audio generator, I imagined that the improvement in bandwidth must be viewed placing a square wave at the input of the amplifier of such amplitude that not cause amplifier limiting, neither by grid current nor plate saturation, then maintaining the amp in the linear region. The square wave has lots of high frequency components in their rise and falling tracts (This is demonstrated by means of the Fourier’s Theorem). So, a lack of high frequency response can be seen as a rounded step, and an excess as a peak in the rising and falling edges.

Also, I tried it in the real world, not simulations, and results are surprising. Next week I’ll post photos about I’m talking about.

By the moment, a bit of suspense.
 

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