Neutralizing Amplifier

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Neutralization, or lack thereof, typically causes one or more of the following problems:

  • Instability near the operating frequency
  • Increased IMD
  • Decreased efficiency
  • Difficulty tuning

 

 

Grounded Grid Amplifier Neutralization

While there may be exceptions, neutralization of a high frequency PA (power amplifier) vacuum tube amplifier has little to do with VHF or UHF stability. VHF instability is almost always caused by a high impedance (or even parallel resonant) path from grid-to-ground. The high grid path impedance prevents the grid from being held at ground-potential for RF over some range of frequencies. If the high grid-path impedance occurs at or near a frequency range where the anode path to ground is parallel resonant, the tube can act like a tuned-plate tuned-grid oscillator. 

Neutralization is primarily concerned with correcting unwanted feedback that occurs from normal anode-cathode feedback in a grounded grid amplifier. Anode-cathode feedback is caused by imperfect shielding from anode to cathode inside the tube, as well as additional feedback outside the tube. In reasonable layouts the in-tube capacitance dominates, especially if multiple parallel tubes are used. 

Lack of neutralization causes the following problems:

  • Instability at or around the operating frequency
  • Increased IMD or distortion
  • Loss of efficiency on higher frequencies

 

As in other amplifier stages and systems, excessive unwanted feedback causes the plate current dip and maximum power output to be out-of-sync. This can add positive feedback, increasing intermodulation distortion. In severe cases, the amplifier becomes unstable and oscillates under some tuning conditions at or near the operating frequency. The Yaesu FL2100 series, the Dentron Clipperton L, and the Collins 30L1 amp with 811A tubes, are particularly unstable designs. Collins and Yaesu have a particularly poor feedback systems, while the Dentron has nothing at all!

In the FL2100, rumors are some tubes oscillate because they have higher mu or gain. This is actually the opposite of the truth. The FL2100 will oscillate, on standby, with any 572B tube if the bias is reduced enough to allow conduction with the antenna and exciter disconnected. The reason some tubes oscillate in the FL2100 while others do not, while on standby, is some tubes have slightly lower mu and draw current at idle while in standby. If bias is reduced on any brand tube, the FL2100 series will break into oscillation. The HF instability has little to nothing to do with gain, the instability is rooted in the lack of neutralization in early FL2100's, and a terrible neutralization system in the later FL2100Z.   

The worse thing about the incorrect mu rumors is the rumors mislead people into thinking certain tube brands cause problems from gain changes. The real problem is Yaesu used a terrible biasing system that barely cuts the tubes off on standby, they failed to swamp or load the tubes, or properly neutralize them. When Yaesu finally added a neutralization system, they used a terrible system. They added feedback from the antenna side of the tank circuit back to the filament. This creates variable phase and level feedback, with feedback depending on band, tank capacitor settings, and load impedance. This is as bad as the RF feedback in some Collins transmitters that wraps back around two tuned stages that are user adjustable.

In cases like this, where the design is flawed, blame is shifted to the tube type, even though the real problem is in the equipment design. In some Collins exciters, certain 6146's are blamed for a poor feedback system that destabilizes the stages. In the FL2100, Svetlana 572B's are blamed for Yaesu bias and feedback design problems. The result of this is certain tube brands get a bad rap, often in well-written white papers that, unfortunately, are based on speculation rather than logical verification.    

    

 Parasitic oscillation amplifier grid grounding

 

Long grid path of 572B tube and poor shielding from anode to cathode (filament) structure:

Grid path length 572B tube neutralization

 

 

parasitic oscillation circuit

The VHF parasitic circuit, unrelated to neutralization, heavily involves the grid's path to ground.

 

Neutralization can only cancel Cpk, and generally has minimal effect on VHF stability of HF power amplifiers.  Lack of neutralization, when required, causes upper HF instability. Upper HF instability can easily damage band switches and other tank components.

 

Cathode Driven Power Amplifier

Many people think grounded grid HF PA's never require neutralization. In many cases this is true, but in some cases it is not true.

Tubes with low impedance compact grid structures and grid connections that come out of the envelope with very short leads, like the 8877, have very little feedthrough capacitance. The 8877 is unconditionally stable all the way up to UHF. With the 8877 grid ring grounded directly to the chassis with a very low impedance connection, the 8877 will not require neutralization or parasitic suppression.

Some tubes are much different. Tubes like the 3CX1200A7 or D7 have significant feedthrough capacitance, and exhibit "out of neutralization" behavior above 20 MHz. This behavior is characterized by maximum RF output occurring well off the plate current dip, and in some cases (i.e. open circuit input terminations) by actual HF instability.

Tubes generally not requiring neutralization in GG HF amps are the:

8877/3CX1500A7   8873 8874 8875  3-500Z  3CX800A7  3CX1200Z7  3CX3000 series 3CX5000 series 3CX10000 series

 

Tubes generally benefiting from neutralization in HF GG amps are the:

810,  811A, 833, 572B,  100TH, 304TH,  8005, 3CX1200A7, and 3CX1200D7.

Tetrodes and pentodes generally have very low feedback when their grids operate at RF ground potential. Connecting a beam forming plate, screen grid, or control grid to the cathode changes things. With a grid or beam forming plate tied back to the cathode, feedback can increase to the point of instability. Some amplifiers, such as the Amp Supply LA1000 or Dentron sweep tube amps, were unstable on ten meters because the control grid was tied back to the cathode.  While these amplifiers could have been stabilized through neutralization, the customer was left to simply load them heavily enough to stabilize them.    

  • Tubes not requiring neutralization in GG circuits are generally those with conical grid supports and grid connections made via a very short wide internal grid lead or leads. Stable tubes (tubes with low internal feedback) often have compact control grid structures inside the tube.

 

  • Tubes benefiting from neutralization are those with long thin (often single) grid leads to single pins, widely-spaced grid wires, and poor or no internal shielding from anode-to-cathode.

Tubes with better internal shielding, short wide grid leads, compact grid structures, and close spacings not only work better at upper high frequencies, they are also significantly more stable at VHF. Such tubes rarely require neutralization or parasitic suppression! The most stable tubes are designed to work at VHF and higher, the least stable tubes generally make poor VHF amplifiers.

How Do We Neutralize a Grounded Grid Amplifier?

 

Electrical Equivalent Grounded Grid Amplifier

neutralizing amplifer grounded grid

In the circuit above, T1 inverts phase 180 degrees. Cneu approximately equals Ckp, the cathode plate capacitance (or feedthrough capacitance) of the tube. Unwanted feedthrough capacitance, Ckp, varies widely with frequency. This capacitance is not frequency linear. It has less reactance at higher frequencies, and higher reactance at lower frequencies. The absolute equivalent value of Ckp varies more than a pure capacitor would with frequency because all stray inductances, including Lint (internal lead inductance) and Lext (external lead inductance), cause Ckp to have a reactance vs. frequency slope much more rapid than a normal fixed capacitor. This means we can really only neutralize a PA perfectly over a small range of frequencies.

In the Ameritron 811H amplifier, neutralization is almost perfect on fifteen through ten meters. The typical feedthrough null is 35 to 45 dB. The 811H neutralization does a good job from 7 to 45 MHz, where feedthrough is less than -20 dB. Feedthrough capacitance is so low perfect neutralization is not required below 10 MHz. Above 45 MHz the parasitic suppressors load the circuit enough to greatly decrease gain and stabilize the stage.

The AL-811H is perfectly stable and will not break into oscillation on any band if we remove the antenna or exciter, key the PA without drive, and rotate the tuning and loading controls throughout their range.

If we repeat this test with a Clipperton L, Yaesu FL2100, or a Collins 30L1 (all un-neutralized amplifiers) most amplifiers (if not all) will break into self-oscillation on 15 and 10 meters. This instability occurs because 811 and 572 tubes have similar poor construction. The tubes have very poor shielding from anode-to-cathode. Both tube types exhibit very high amounts of feedthrough capacitance, enough feedthrough capacitance to make un-neutralized amplifiers unstable near the operating frequency on higher bands, such as 15 and 10 meters.

 

neutralization circuit typical of AL811H and AL1200 by W8JI

The circuit above is a typical neutralization system for a grounded grid amplifier. The ferrite core is a 1 to 2 inch diameter, 1/2 inch thick, using a higher Q (low loss tangent) 61 or 65 material.

 

Test Setups

Neutralization adjustments are best done on a cold amplifier. To adjust neutralization, three basic test configurations can be used:

 

In all configurations except "C", input and output ports can be reversed. The source should be variable, and capable of supplying a few watts. Many transceivers will work OK.

The detector should respond to very low levels, but be capable of withstanding some reasonable power in the event of a circuit or component defect that accidentally allows full source power to couple through.

 

Grid Driven  

See Grid driven tetrodes

Grid driven tetrodes like 6146, 807, or 4CX250's have high power gain. High gain systems require very little feedback to become unstable, so they are generally neutralized. The also often require some form of grid loading resistor to reduce or stabilize gain. The following circuit shows a commonly used tetrode grid-driven amplifier with neutralization:

tetrode neutralization

L1/C1 is the normal input tuning coil. Being resonant on the operating frequency, it inverts phase 180-degrees from end-to-end. C2 is a voltage divider to control the feedback voltage ratio and provide a return path for grid excitation. Cneut is adjusted so its voltage feedback equals the voltage fed through Cgp from plate to control grid inside the tube.

Note that this system depends heavily on L1/C1 being resonant at the operating frequency. This proves the tube is only neutralized at the frequency where C1/L1 is set. It does not stabilize the tube on any frequency except where L1/C1 is resonant.

Lp,Lsc,Lk, and Lg are inductances of leads inside the tube. Lp1,Lg1,Lk1, and Lsc1 are lead and component inductances that occur outside the tube.

While the feedback adjustment setting of Cneut holds true for multiple bands near the initial adjustment frequency, it only actually neutralizes the tube on the band in use at any moment of time!

In a 160-10 meter PA, Cneut generally only works properly over two or three bands. It is usually set near 15 meters so it has the most effect where it is needed most. By the time we get down to 40 meters and lower, feedback voltage through Cgp is generally through such a high reactance that the lack of proper balancing is meaningless.

Additional stability can be added by loading the grid with a broadband termination resistance. This makes neutralization much less critical, and may at times even eliminate the need to neutralize. This resistor would go from the control grid to ground and ideally be added right at the tube. Unless the resistor is an integral part of the bias system, it must be "dc blocked" with a low impedance series capacitance so it does not affect grid bias.

Neutralization

Neutralization generally only affects operation near or at the desired operating frequencies. Neutralization is normally optimized near the upper frequency end of operation, perhaps between 15 and 30 MHz in a 1.8-30 MHz transmitter or amplifier.

Neutralization is sometimes needed because tubes have unwanted internal capacitances. The capacitance between the output element and the input element inside the tube will cause the output circuit to couple back to the input. If large enough, this regenerative feedback could cause a loss of efficiency. It might cause the output maximum to occur off the plate current dip, reducing efficiency. It might increase IM distortion or in rare severe cases may cause the amplifier to oscillate someplace the operating frequency. (This problem is common with grounded grid amplifiers using 572B's like the Dentron Clipperton L,  or quads of 811A's, like the Collins 30L1. Yaesu has this problem is some FL2100's.)

While a need to neutralize does occur in some HF grounded grid amplifiers, it is more common in very high gain grid-driven amplifiers.

Neutralization Adjustment Methods

Neutralization is generally accomplished by adding an external capacitance that is excited exactly 180 degrees out-of-phase with the feedthrough capacitance. One typical adjust procedure is to disable the PA stage by removing anode and screen or filament voltage. A sensitive RF detector is connected to the transmitter output. Neutralizing a totally cold tube is perfectly fine, because there is very little capacitance shift in a tube with temperature changes.

Normal drive is applied, and the neutralizing capacitor is adjusted until feedthrough power is minimum. The tuning controls are continually peaked for maximum power on the sensitive detector throughout the process.

A second less accurate method is to watch the plate current dip in a properly tuned normally operating transmitter. The neutralization capacitor is adjusted until maximum power output and minimum plate current occur simultaneously as the plate capacitor is tuned.

The best method  varies with the PA design, but in general the most accurate method is by applying drive to a cold PA stage (generally either screen and plate or filament power is removed) and feedthrough power is measured with a sensitive detector.

What Happens If We Don't Neutralize a New Tube?

Many times nothing noticeable occurs if we don't neutralize a PA. The results really depend on how much different the internal capacitance is in the new tube(s) when compared to the capacitance of the tube(s) being replaced.

If the PA requires neutralization and we don't neutralize or re-neutralize it, we could find IM distortion higher. We would probably find maximum output power occurs well-off the plate current dip. The un-neutralized stage, in severe cases, might oscillate somewhere near the operating frequency under certain conditions of tuning and loading.

Neutralization is generally only accurate over a limited range of frequencies, but fortunately it is almost always at the higher frequency end of the operating range where the PA needs neutralized. The manufacturer probably knows the optimum adjustment point. In the AL1200 and AL811H, the optimum null frequency is 21.5  MHz.


Unrelated Problems are sometimes Blamed on Neutralization

Since neutralization is the canceling of feedthrough capacitance, and since capacitance doesn't change over the life of a tube (or even much from a hot tube to a cold tube), neutralization won't "drift out" with certain tube types.

A tube will either neutralize right from the start, or it won't. If it appears to drift out of adjustment something other than the neutralization is at fault. Tubes in HF PA's cannot drift in and out of neutralization because the capacitance is set by the tube's physical construction... not by emission, age, or any other time-variable parameter.

People sometimes blame neutralization for problems when they really have gassy or defective emission. Gassy tubes can go into current runaway or even flash over inside. Doing this for 35 years for a living, I've never yet seen a tube in a HF or lower VHF amplifier  "drift" or age out of neutralization.

The capacitance is for the most part related only to the physical characteristics of the tube, like internal lead length, size of the elements, and spacing of the elements. That's why it is perfectly acceptable to neutralize a cold tube (no filament voltage). The change in feedthrough is very small when the tube is operating compared to when it is cold.

VHF Stability

HF Stability

Text drawing added Jan 1, 2012

 

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