Many think this negative feedback voltage divider doesn't work because of cathode to grid current in the 30L1. Interestingly, it requires nearly 30% more power to drive the amplifier with the divider installed than with directly grounded grids. The gain of the amp is significantly reduced.
What they overlooked is, it has an RF choke, not resistor from the grid to ground. The grid and plate RF chokes are subjected to VDC or DC pulse not AC. The current in these chokes does not reverse direction so it cannot be considered Alternating current. The 6.28xFxL equation is not accurate when using a plate or grid choke who's reactance is higher than the load impedance. The wl equation requires the voltage applied to the inductor be reversing, symmetrical, sine form, and current lagging 90 degrees.
The negative voltage fed back to the grid from the plate will be approximately -22 volts when the plate swing is at maximum, the destabilizing feedback from the plate to cathode will be approximately 20 volts which is positive feedback. It is only logical is you apply a 22 volt negative feedback signal to the grid it will handily neutralize the 20 volts of positive destabilizing feedback from plate to cathode.
The same voltage divider is used to successfully neutralize every transceiver that I know of as well as grid driven amplifiers, triode or tetrode.
There is a notion that the voltage divider cannot neutralize the GG amplifier because it does not neutralize the feedback at the cathode. The fact is it equalizes the feedback voltage between the grid and cathode. NOTE: The single ended Bruene neutralizing design used in nearly every transceiver or grid driven amplifier IS NOT DIRECT NEUTRALIZATION. The input tuned circuit is equalized and the entire feed back from plate to grid is used to generate negative feed back.
There was some talk of an analysis using the Nyquist equation. Short and to the point it's simple. It includes the gain of the tube, the feedback fraction, and phase of the feedback.
What is more complex is gathering the measurements that influence those measurements i.e. the gain of the tube varies with load.
The feedback from plate to grid is 180 degrees degenerative and would have to reverse by almost 180 degrees to be regenerative. Without reactance amplification i.e. a tuned circuit on the grid of a GG amp the voltage divider can only stabilize the amplifier and improve linearity.
Last, directly grounding the grids leaves them floating! Got your attention there!
A directly grounded grid will have .080uH of inductance and 15 ohms of inductive reactance at 29MHz and there's nothing that you can do to reduce it with a direct strap to ground.
Adding a 220pf capacitor to ground changes the character of the grid connection @ 30MHz from 15 ohms inductive for directly grounded grids to a 0 ohm series resonant short to ground. The grid will be capacitive at any frequency below series resonant. The directly grounded grid will always be inductive and always more inductive than the grid connection using the 220pf capacitor. This is how the brightest engineers of that era made the 811A an audio tube function in the RF region.
I have a question for the folks who say not so. Has anyone ever measured the frequency of oscillation in a GG amp that is guessed to have been caused by the lack of neutralization? I've never seen that measurement. A GG amp will oscillate at the tuned frequency if neutralization is the cause whereas a grid driven amp will oscillate just below the tuned frequency if neutralization is the cause.
NOTE: There is a tuning combination when using chokes whose reactance is close to the load impedance. When using the lowest band, under loading the amp can result in the load impedance increasing to a value higher than the choke reactance. The effect will be the seemingly inadequate capacity of the tune capacitor. The cause is some of the current in the choke has begun to reverse enabling that part of the current to participate in the output tuned circuit.
Think for yourself --- there are many theories based on superstition out there --- Ask for explanations using words.
This is not edited if anything is unclear please send a note.
Copyright James Liles 5/2017