rev PM Feb23/03
HF Amplifier Stability at VHF
A great deal of empirical engineering surrounds stabilizing amplifiers. While other forms of troubleshooting and engineering follow logical steps, it seems very few builders actually follow planned steps to test for amplifier stability. It is difficult to find detailed information describing suppressors and how they function.
Cause of VHF Oscillations
As in most planned oscillators, the most sensitive control element in the tube generally has the largest influence in determining oscillation frequency. The normal mode of VHF oscillation in HF PA's is at a frequency where the tube becomes a tuned-plate tuned grid oscillator. The control grid to anode path generally has the highest possible gain in the amplifier system, and that is why this part of the system is (by far) the most problematic area of the amplifier system.
The control grid system behaves like it is connected to a parallel tuned circuit. The stray capacitance is primarily between the grid element inside the tube and ground, generally via the filament and other connections. The inductance is via the grid leads inside the tube through the socket to the actual chassis connection. At some frequency, the grid capacitance will parallel-resonate the total inductance in the grid to ground path.
The anode is the second most common problem area. If the anode also has stray capacitance to ground. The path from the anode to chassis has inductance. At some frequency, anode capacitance parallel tunes the path to ground. This resonance greatly increases anode impedance at some very high frequency.
The grid also has capacitance to the anode, and this is the feedback path.
With all of this, the circuit has everything needed to become a tuned-plate tuned-grid oscillator.
If feedback loss (attenuation) from anode-to-grid is less than tube gain at some problem frequency, the tube may oscillate. The final requirement is the phase of unwanted feedback must be a value that causes regeneration, or positive feedback. These requirements are the same in any oscillator.
Once again, the conditions required for instability are:
 | Gain must exceed attenuation in the feedback path |
| The grid must have a sufficiently high impedance for the amount of available feedback to cause a stability problem, |
| The anode or other element involved in the oscillation process must have a sufficiently high impedance at the same frequency as the grid to cause a stability problem |
| Feedback phase must be within the correct range to obtain positive feedback |
If any one of these four requirements are not met, the tube will not oscillate! This is true no matter how high or low Q is in any individual path, or if the tube has suppressors or not.
Claims have been made that tubes will remain stable for years, and a "sudden event" will make the tube break into an uncontrolled oscillation. That absolutely can not happen, unless one or more of the four important system parameters above change. Once one or more parameters change, the tube will oscillate continuously until operating voltages are removed. Quite often, in fact most of the time, oscillations are not damaging. The most common effect of VHF oscillations are creation of spurious signals; not bangs, pops, or arcing bandswitches. Bangs and pops are caused by gassy tubes or other problems, while arced bandswitches (if caused by an oscillation) are generally caused by oscillations at or near the desired operating frequency!
Location of Suppressor
Suppressors are normally found in anode systems, even though outer locations will work. A VHF suppressor must be located between the tube element and a low-impedance path to ground at VHF. This is because the suppressor must be able to load or "de-Q" one or more portions of the unwanted oscillator circuit. The actual working circuit causing a VHF oscillation is almost always entirely different than what appears on the actual component-based schematic. The cathode, an element commonly involved in low-frequency instability, is rarely involved in VHF oscillations, other than supplying electrons and stray capacitance to ground.
A VHF oscillation, if it happens to occur in an HF PA, is almost always rooted in the system behaving like a "tuned-plate/tuned-grid" oscillator.
Most of our modern PA's are grounded grid (cathode driven). Cathode driven operation requires one or more grids be directly grounded to the chassis (at least for RF) with the lowest impedance possible. This is necessary to shield the output from the input, and assure operating frequency stability and purity of emissions.
Using the anode for suppression generally works best because the grid or grids can remain well-grounded for RF, provided the best operating frequency performance.
The Most Unstable Tubes
The most problematic tubes for VHF oscillation have relatively large elements and long thin leads. Tubes of this type have low gain or are unusable at VHF, because elements in the tube (combined with lead inductances) are actually resonant at VHF.
Leads are a particular problem. The longer and thinner the leads, the less stable a tube becomes. Long thin leads move the self-resonance lower in frequency while increasing element impedance. This allows even a tiny amount of anode-to-grid feedback to cause oscillations.
Tubes most troublesome are 811A's, 572B's, 833's, 4-1000A's, 3CX1200A7's and 3CX1200D7's.
Tubes of moderate instability are 3-500Z, 3-1000Z, and 4-400A's.
Tubes having virtually unconditional VHF stability are the 3CX800, 3CX1200Z7, 3CX1500/8877, 3CX3000, and 3CX5000/3CPX5000/YU-156 series.
Looking at the above tubes, it is the tubes with the thinnest and longest leads that are most troublesome. These also are tubes with the poorest VHF performance when used in amplifiers intended to operate at VHF.
The most troublesome tubes above tend to oscillate in the lower-VHF range, between 30 and 100 MHz. The typical instability frequency of an 811A or 572B is around 80-100MHz, assuming all leads are short.
Moderately stable tubes tend to oscillate at 100-200MHz. 3-500Z's, for example, generally are most unstable from 180-200 MHz.
Anode Circuit Layout
Anode circuit layout can contribute to VHF instability. Long thin leads from the tube anode connector to the chassis at VHF are a problem. Problems can occur when thin (and long) plate blocking capacitor leads, thin and/or long wiring, and poor mounting of the plate tuning capacitor are used. Remember, this is a VHF path also, even if the amplifier only intentionally operates on HF.
To maximize stability:
| Use wide anode circuit leads from the tube to the tuning capacitor |
| Mount the tuning capacitor directly on the chassis, or on a large metallic groundplane area that is thoroughly bonded to the chassis at many points |
| Use a low-inductance plate blocking capacitor |
| Keep all leads as short as possible, even if it is at the expense of having wiring "look pretty" with all perfectly aligned 90-degree angles |
| Use the chassis as a groundplane and an input/output shield, not a front panel |
Grid Circuit Layout
The grid circuit layout is probably the single most important area for insuring a stable design. Long thin leads from the tube grid connector to chassis at VHF are a problem. This can be from thin (and long) grid bypass capacitors, thin and/or needlessly long wiring, and failure to ground grids directly to the chassis by mounting ground lugs directly on the chassis near the grid pins.
To maximize stability:
| Use wide low-inductance grid leads from the tube socket directly to the chassis, Connect the ground leads at the closest possible point, using ground lugs right at the grid pins (rather than using socket mounting screws) for grounding |
| Use low-pass Pi-network or parallel tuned networks as input matching circuits |
| Mount any swamping or load resistors near the tube, with short leads |
| Mount the low-pass or bandpass input matching system near the tube, or use exceptionally low-impedance transmission lines to reach the input matching system |
| Keep all grid connections as short as possible, even if it is at the expense of having wiring "look pretty" with all perfectly aligned 90-degree angles |
A Bad Grid Idea
One of the very worse things in modern grounded-grid triode PA's is the inane engineering claiming floating grids on capacitors adds useful negative feedback. This is similar to what Collins did in their 811A amplifier, and Japanese manufacturers copied the bad idea into their power amplifiers. Heathkit was also a victim of this engineering gaff.
When I was designing PA's in the late 70's and early 80's, an employee of Eimac and author of many articles and a handbook put considerable pressure on me to float the grids of 3-500Z PA's through small mica capacitors.
The theory presented was pretty simple. He claimed floating grids through small mica capacitors added negative feedback, making the amplifier "work better". The basic idea was that filament-to-grid capacitance formed one part of a capacitive voltage divider, the grid-to-ground capacitors forming the other half of the divider.
The alleged "idea" was this capacitive divider would float the grid partially up from ground, and reduce grid-to-cathode (grid-to-filament) driving voltage. I quickly concluded that no one ever actually measured or calculated feedback over a wide range of operating frequencies and grid currents. Since this is a C1/C2 divider, the sampled feedback should be constant in both amplitude and phase regardless of frequency, power levels, and tuning.
The basic circuit is:
The grid connects at the junction of C1 and C2, while the cathode connects to the top of C2.
C2 is the internal stray GK capacitance of the tube
R1 is the time-varying grid impedance
R2 is added to allow us to see the input impedance change of the divider on a probe model.
Sweeping the system from 100KHz to 30MHz shows us the following:
What we see is a huge spike in grid-to-ground impedance at 2MHz, and very uneven response above that range. We did not even include the time-varying grid resistance and phase error, since this would take up to much space on my web site.
By manipulating the value of L1 (the grid chokes) we can move the spike around, but we are ALWAYS left with some frequency where the grid isn't grounded! This is a serious violation of good engineering practices in a grounded-grid PA, and is actually at the root of stability problems in some PA's. Collins, for example, had a series of field modifications to the 30L1 grid system. The best idea would have been to abandon the bad idea that this system adds controlled negative feedback, and changed to a true grounded grid.
If they wanted negative feedback, the PROPER method would have been adding a resistor in series with the cathode by placing a series resistance immediately at the drive point to the cathode!
There are obviously major flaws with the super-cathode drive concept, when it uses a capacitor divider. Grid current causes grid-to-cathode impedance to constantly vary. When grid current is absent, the grid-to-cathode impedance is nearly an open circuit. Grid-to-cathode capacitance dominates the upper half of the divider, and everything appears to work as planned.
Unfortunately, a problem appears whenever the grid draws current. Even the tiniest amount of grid current causes grid-to-cathode impedance to decreases rapidly. With only a few dozen milliamperes of grid current, grid impedance drops to a few hundred ohms or less. As grid current is drawn, the decreasing grid impedance dominates the upper leg of the voltage division circuit!
There are also new potentially destabilizing resonances added in the grid path.
This system causes three major problems:
| Grid drive is effectively reduced as operating frequency becomes higher, just where we need the gain to flatten normal gain roll-off |
| Feedback starts to show significant phase-lag with increased drive and reduced operating frequency |
| Grid-to-chassis inductance at VHF and LF is increased, making the amplifier less stable |
When I tested several amplifiers with and without this alleged "super-cathode" system, I found IMD performance decreased significantly under some operating conditions. Stability also significantly decreased.
Ground the grids either directly with short heavy leads or a low-inductance high-value capacitor with very short leads in any grounded-grid PA!
What Does the Parasitic Suppressor Do?
The parasitic suppressor normally has two components in parallel, a resistor and an inductor. At low frequencies, the path through the inductor dominates the system. At very high frequencies, the resistor dominates the system (assuming it is a low-inductance resistor).
One common problem is people assume brown carbon resistors are non-inductive. That isn't the case. For an example, look at the following resistors:
All of the spiral-conductor resistors above have significant inductance at VHF, and make very ineffective suppressors unless the reactance is cancelled. Only the true carbon composition resistors are useful in non-resonant standard suppressors.
This is a typical suppressor system, including inductance of the anode lead:
In this case V1 represents the tube. The following is a simulation of currents in the suppressor:
Starting at 30MHz, the ratio of current in the inductor to current in the resistor is:
Frequency -I(L1)
-I(R1)
30MHz
0.0047
0.0015
60
0.0041
0.0026
90
0.0034
0.0034
120
0.0029
0.0037
160
0.0024
0.0041
190
0.0021
0.0042
220
0.0018
0.0043
This tells us something very important. The INDUCTOR dominates only at low frequencies. At 30MHz, current in the inductor is three times current in the resistor.
At 190MHz, in the range of the instability frequency of a 3-500Z, the resistor has twice the current as the inductor.
This tells us any changes in INDUCTOR design or inductor Q (such as use of nichrome wire) mainly lowers low frequency Q. It would have virtually no effect on very high frequency Q of the system.
| The dominant factor in controlling VHF Q is the resistor value, and any reactance in the resistor path |
| The dominate factor in determining HF Q and performance is the inductor value, and any changes in inductor Q |
This has been my point all along with the Measure's nichrome suppressor. Measures claims, incorrectly, his suppressors provide lower VHF Q while, in fact, they do exactly the opposite! A typical Measures hairpin suppressor actually produced significantly higher system Q in the anode of a 3-500Z (nearly twice the VHF Q), because the equivalent Rp of the suppressor in series with the anode lead was lower!
The reasons HF PA's arc are explained at other pages of this site, and include incorrect relay sequencing, load faults, as well as improper tuning and exciter transients.
Reducing VHF Q
If we want a lower VHF Q, while maintaining high LF Q and efficiency, the system must shift current into the resistor faster as frequency increases. The suppressor must also have higher Rp, so it dominates the anode path inductance that is in series with the suppressor.
While Measures openly touts his "low-Rp suppressor", the fact is a low Rp suppressor results in higher anode system Q!
A Truly Improved Parasitic Suppressor
In order to reduce VHF Q, we must have a resistance dominate the anode system. This means, in a frequency sweep simulation, the ratio of currents in the resistance to current in the inductance must be as high as possible. Let's call that slope the rate of transfer.
The rate of transfer can be increased by adding a small value of capacitance in series with the resistor:
The old suppressor was:
Frequency -I(L1)
-I(R1)
Ratio
30MHz
0.0047
0.0015
3
60
0.0041
0.0026
1.6
90
0.0034
0.0034
1
120
0.0029
0.0037
.78
160
0.0024
0.0041
.58
190
0.0021
0.0042
.5
220
0.0018
0.0043
.42
The new one:
Frequency -I(L1)
-I(R1)
Ratio
30MHz
0.0069
0.0026
2.6
60
0.0050
0.0055
.9
90
0.0027
0.0052
.52
120
0.0019
0.0050
.38
160
0.0013
0.0048
.27
190
0.0011
0.0047
.23
220
0.0009
0.0047
.19
Graphically we see the currents are:
The green curve is current through the inductor, the red curve shows current through the resistor. Notice how flat current is in the resistor, and how sharp roll off of current in the inductor becomes.
This means we will have very low anode SYSTEM Q starting at a low VHF frequency of 50-60MHz, and continuing up to UHF. Dissipation in the resistor is still reasonable at HF, efficiency and tank Q at the operating frequency remain high, yet VHF suppression is greatly improved.
Selecting Component Values
Optimum resistor value can actually be determined by measurement, or determine empirically.
If the anode path is long and thin, the impedance will be high. A high anode path impedance (thin or long leads) requires higher values of resistance, because we want the resistor to dominate the anode system impedance. The best value for a resistor is generally one that is approximately equal to, or slightly higher than, the anode path reactance at the frequency of instability.
That impedance can be measured on an impedance test set, or other ways by creative engineers or technicians, but as a general rule long, thin anode leads like 811A's require 100-150 ohms of resistance while shorter thicker anode leads like those in 3-500Z tubes require 50-100 ohms of resistance. Stable tubes with external anodes often can just use anode lead resistance, using brass or other materials, to adequately dampen anode path reactance.
The inductance has to present a significantly higher reactance than the suppression resistor value at the frequency of instability. This causes the majority of current to flow through the resistance at the very high frequency, and not the inductor.
If you look at amplifier designs, you will find tubes like 811A's generally have higher value resistors and many turns of wire in the suppressor. Tubes like 3-500Z's have significantly fewer turns, especially when grid leads are kept very short and direct to the chassis, and lower value resistors.
The more unstable the amplifier tube, the larger the inductor and resistor must be.
One way to view this is to consider the frequency response of a Hi-fi amplifier. Larger values of plate load resistors in amplifier stages reduce higher-frequency gain. The same is true in HF PA's.
Lower frequencies of instability require larger inductors, so the RF path is shifted over to the resistor at a lower frequency.
Uses For Improved Suppressors
Series-resonant suppressors are used with slightly inductive resistor paths, and larger-than- normal shunt inductors. A small capacitor is placed in series with the inductive resistor path, and this capacitor series-tunes the resistor path. This results in a very rapid shift of current into the resistor as frequency is increased. This works well with amplifiers operating at 1/3 to 1/2 the instability frequency, minimizing resistor heat while providing perfect stability.
Typical applications are 3CX1200A7 and D7 tubes, 572B tubes, and 811A tubes.
Shunt suppressors with series-resonant tuning are also sometimes used, the normal application is very high power stages with substantial anode-to-tank currents. These suppressors consist of a series R/L/C system, where the C is normally just stray capacitance to the tube anode. Sometimes these suppressors take the form of a ferrite block placed between the anode and chassis. The inductance of the block series-tunes stray capacitance, and the losses act like a damping resistance in series with that path. I've stabilized 50-100kW VHF transmitter designs using shunt suppression.
Other Instability
Some PA systems are prone to oscillation at low frequencies. Yaesu and Dentron amplifiers using 572B's, and the Collins amplifier using 811A's are good examples of production amplifiers with stability problems.
These amplifiers tend to oscillate NEAR the operating frequency.
All of these amplifiers, except the Yaesu, use tubes with high anode-to-grid feed-through capacitance and no neutralization. Worse, the Collins floats the grids for RF, reducing the already poor isolation of anode-to-cathode feedback path in the 811A.
Yaesu uses one of the poorest engineered feedback systems of all, with a capacitor from the output of the pi section back to the cathode! Phase shift in that path would vary wildly with tank circuit tuning and load impedance on the PA, as would the amount of feedback!
The Yaesu amplifier is a particular problem with Chinese 572B tubes, because grid mu is lower. Negative grid bias has LESS of an effect on cathode current, so the Chinese (and Russian) tubes draw extra quiescent current when the antenna relay is open. This additional current allows the tube to amplify while the amp is in standby. Since the antenna and input source are removed in standby, and the improperly designed feedback path to the tank output remains in place, the PA oscillates near the operating frequency with no load! Voltage in the tank builds up to many thousands of volts, because no energy is extracted to a load. The fact the oscillation is at a low frequency allows the bandswitch to see the full voltage, and it fails.
Amplifiers can create extremely large voltages when RF is applied and a load is not present!
All of the amplifiers discussed above would be greatly improved by:
| Adding a proper bridge neutralization circuit like Heathkit, Ameritron, and Gonset used in 811 amplifiers. |
| Grounding the grids either directly or through low reactance very-short-lead capacitors, directly between the socket's grid pin and chassis. |
| Using the improved suppressor outlined above to de-Q the amp at lower VHF. |
Conclusion
I hope this information is useful, and helps people understand what really goes on in a parasitic suppression system. As time permits, I add more articles about curing unique problems in amplifiers, and diagnosing amplifier failures. I hope these pages are a good start.
Please pass this web address along to others.