NEUTRALIZING CONCEPTS PART 1
Why is neutralization necessary in a vacuum tube amplifier? The capacity between the plate and control grid has two primary effects that are disruptive for linearity and stability.
It causes negative feedback at the operating frequency which has a degenerative effect on the drive to the grid. In the absence of neutralization the drive must be increased to achieved full output. This feedback does not affect efficiency; it simply requires more drive.
It is positive feedback to the parasitic Hartley circuit formed just below the operating frequency which occurs naturally in all tuned grid/tuned plate vacuum tube amplifiers. This effect is insidious and must be neutralized. Video clip # 2 is a good example of a seriously under-neutralized amplifier. You will see two things occur. The grid voltage will be aggressively muted as you tune the plate cap through resonance. Then, further tuning to a higher frequency the grid drive and plate voltage will increase with a peak that does not coincide with the plate dip. This peak is the point where the Hartley parasitic begins to form with the greatest risk to linearity and stability.
The common practice of tuning for peak output rather than plate dip places the radio at the highest risk for instability. Peak output occurs for two different reasons depending on the amount of neutralizing feedback.
If the PEAK OUTPUT occurs when the final tuning cap has to be adjusted to a higher frequency than the plate dip there is inadequate neutralizing feedback ---increase the neutralizing cap. This is caused by the Hartley effect which will be fully explained as we go.
If the PEAK OUTPUT occurs when the final tuning cap is adjusted to a lower frequency than the plate dip, there is too much neutralizing feedback. Decrease the neutralizing cap. Neutralizing feedback is regenerative and meant to counter the degenerative feedback from plate to grid at the operating frequency. Excessively high it will precipitate oscillation at the operating frequency like any other regenerative feedback.
SNIPPET #1: The reasons for each video clip will be fully explained as we move on.
Video clip #1 is a good example of a correctly neutralized amplifier. We use this to demonstrate the effect of final output on grid drive. This clip is a good example of the negative feedback that every designer seeks. The plate dip and peak output are exactly in sync along with mild negative feedback. This can only be accomplished when every band can be precisely neutralized. It won’work when 10 Metres is neutralized and the lower bands are swamped --- which is fairly common design practice.
The neutralizing cap is patently inadequate, causing the peak output to occur when you tune the final cap to a higher frequency. Just as you tune above resonance, the tank begins to look like an infinitely high inductance in parallel with the load resistance. The inductance will decrease rapidly as you tune further. At some point the inductance will intersect and resonate with the grid to plate capacity. It is at that point you see the plate voltage peak to the left of the grid signal. At this point oscillation or instability is most likely to occur. A full explanation of the effect will follow.
The neutralizing cap is excessive. This causes excessive regenerative feedback because there is more plate voltage fed to the bottom of the input tank than the top. Notice the grid voltage increases when the final tuning cap is rotated to a lower frequency. This causes the final tank to appear as a capacitor.
This setup simulates a disconnected antenna. It is extremely lightly loaded and properly neutralized. Note the natural plate to grid negative feedback is depressing the grid drive but not changing the phase. Also Note the control grid is going a bit unstable from a parasitic effect but is still safe. This an example of the parasitic suppressors working to dampen a parasitic oscillation and is a simple way to verify that the resistors in the parasitic suppressors are working.
This is a slightly over neutralized amplifier. Tuning the plate capacitor through resonance will have no effect on the grid voltage; if the tube is perfectly neutralized, the grid voltage will be slightly depressed.
This paper will focus on grid driven tuned in and out amplifiers. The intent is to fully explain the various attributes of grid driven amplifiers, then define the interactions that cause instability and damage to linearity.
The PLATE to CONTROL GRID capacity has a degenerative feedback effect, not regenerative on grid drive when both the input and output tuned circuits are resonant at the same frequency. It is common belief that an unneutralized TGTP amplifier will not oscillate unless the plate is tuned to a higher frequency than the grid. This is not correct; It is simply more likely to oscillate when tuned higher. This will be explained later.
Important concepts regarding parallel tuned circuits:
PARALLEL LC CIRCUIT:
Visualize a parallel LC circuit tuned to resonate at 14Mc with a 14MC signal applied to it with no resistance in the circuit. There is infinite resistance and no notable reactance.
If you tune the capacitor to a slightly higher frequency (Less capacity higher reactance) the circuit begins to exhibit immense inductive reactance, infinitely greater than the coil.
If you tune the capacitor to a slightly lower frequency (More capacity less reactance) the circuit begins to exhibit immense capacitive reactance again infinitely greater than capacitor.
TEST SETUP FOR EXPLANATIONS: