Tech Talks and Tips by K4KYV

Discussion in 'Amplitude Modulation - AM Fans' started by N6YW, Jun 6, 2016.

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  1. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    Don Chester is a favorite when it comes to discussing AM and its many associated subjects that impact our hobby. It's a privilege to have Don contribute in a style and manner that is easy going and fun.
    Sit back and enjoy! There will be some great stuff on this thread so be sure to check in often.
    This is a read only thread; if needed, contact Don directly.
    Thanks!
    73 de Billy N6YW k4kyv.jpg
     
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  2. K4KYV

    K4KYV Subscriber QRZ Page

    Table of Contents
    (by posting #)

    1. Introduction

    2. Table of Contents

    3. Some Thoughts on Modulation Transformer Impedance / Turns ratio

    4. Enhanced AM (EAM)

    5. RF ammeters, Common mode currents and Balance in open-wire transmission lines

    6. RCA's Recommended Break-In Procedure for Power Tubes

    7. Removing the B+ from the primary of a single-ended driver transformer

    8. AM Broadcast Ground Systems

    9. Modulation Reactor Winding Polarity

    10. The weirdest radio problem I ever had.

    11. Possibly a good Class-B driver tube or replacement

    12. Coil winding off-by-one error

    13. Peak, Average and RMS

    14. Signal Bandwidth and "Wide" Reports

    15. The Physical Reality of Sidebands

    16. Efficiency of the AM Linear Amplifier versus Plate Modulated Transmitter

    17. Ground Radials

    18. Synchronous Detection

    19. Purely Resistive versus Highly Reactive Loads: a mechanical analogy.

    20. Restoring Bakelite knobs and dials.

    21. Overmodulation Indicator and Transformer Protector

    22. Rane Notes Audio Links

    23. Preserve Your D-104 Crystal Element

    24. A Tip for Safely Mounting Heavy Equipment in a Rack

    25. Removing enamel insulation from fine enamelled wire

    26. Caution When Repurposing Small Transformers

    27. The Graëtz Bridge


    This section is read-only; if you have questions or comments please message me directly, or start a separate thread under that specific topic.

    I don't claim to be an expert on these topics, so if you spot an error or misinformation, please let me know and the error will be addressed. I stand to learn as much as anyone else from these discussions.

    Thanks & 73,

    Don k4kyv
     
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  3. K4KYV

    K4KYV Subscriber QRZ Page

    Some Thoughts on Modulation Transformer Impedance / Turns ratio

    The optimum design of a modulator and final is more involved than simply calculating the modulation transformer impedances and turns ratio based on the published impedance properties of the tubes.

    If a common power supply is used for modulator and final, regardless of the impedance level, the modulation transformer needs to have a turns ratio of about 1.4:1, or a 2:1 impedance ratio. The only case in which a "multi-match" transformer, with a choice of several turns ratios, is useful is when separate supplies are used for modulator and final, and different plate voltages can be applied to the modulator and final tubes. Otherwise, an appropriate fixed-impedance transformer is to be preferred, and actually gives better performance because all the turns in both the primary and secondary windings are in use.

    The reason for this turns ratio is simple. In a class-B or AB circuit, the peak audio voltage capability of each tube is roughly 80% of the DC power supply voltage. It is very difficult to drive the grid of a tube hard enough to bring the instantaneous plate voltage below about 20% of the DC plate voltage. One of the reasons is that, to get the tube to conduct to saturation, substantial positive voltage must be applied to the grid, and this peak grid voltage is likely to be close to 20% of the plate voltage. With a triode tube, it is impossible to bring the plate voltage below the grid voltage, no matter how hard the tube is driven.

    With two tubes in push-pull, the total peak a.c. (audio) voltage across the primary of the modulation transformer is the sum of the peak voltages generated by each of the two tubes. If we assume the figure of 80% of the DC plate voltage, that means that the total peak audio voltage developed by the modulator tubes cannot exceed 1.6 times the DC plate voltage. Therefore, for exactly 100% modulation capability, the turns ratio, total primary to secondary, would be approximately 1.6:1, or 2.56:1 impedance ratio. But to avoid distortion near the 100% modulation point, we need some head-room, so that the tube is not being driven to saturation right at the instant that 100% modulation occurs; therefore it is preferable to have a little less step down, maybe 1.4:1 or 1.5:1. If you are looking for extended positive peaks, a ratio of 1.3:1, 1.2:1 or even 1:1 would be necessary.

    Where the plate-to-plate impedance comes in, involves how much current is run on the final. By Ohm's law, modulation impedance = plate voltage/plate current. This impedance is reflected back to the modulator tubes via the transformer. So you need to choose a plate current that will give the proper modulating impedance that, when reflected back to the modulator tubes through the transformer, will allow the modulator tubes to work into a satisfactory plate-to-plate load.

    There is nothing sacred about the p-to-p load recommendations given in the tube charts; they are just that, recommendations. With most good tubes, the p-p load they work into can be varied considerably, maybe as much as 2 to 1 and still get good results. When working the tubes into a lower p-p load impedance, the peak plate current will be higher, the plate dissipation will increase and the stage may become less efficient. Taken to extreme, the linearity of the tube may suffer. But within reason, the main thing to watch for is plate dissipation and maximum peak plate current. The maximum peak output from the tubes may be reduced when one veers too far from the recommended p-p impedance, but if peak plate current and plate dissipation are kept within the manufacturers ratings, the tube will perform just fine. The same goes for running at a higher-than-recommended p-p load. In this case, for 100% modulation capability at the full power rating, the DC plate voltage will be increased. Again, this causes no problem as long as the maximum plate voltage rating of the tubes is not exceeded. The tube may run more efficiently at a higher p-p load, but if too much step down is used in the transformer, the peak output capability and therefore modulation capability will be reduced.

    The third factor to be considered is the nominal impedance rating of the modulation transformer itself. This is determined by the amount of iron in the core, the type of iron used, and the number of turns in each winding. Most good transformers can be run at least +/- 100% of the nominal value, again as long as maximum current and voltage ratings are not exceeded. If you stray too far away from the nominal impedances, frequency response may be affected. Running a transformer at a much higher impedance than normal will limit low-frequency response due to the lower inductance of a low impedance winding. Conversely, running a transformer at a much lower impedance than recommended may limit the high frequency response due to the combination of stray inductances and capacitances in the windings. Also, the core is more likely to saturate on peaks due to higher currents through the windings. But within reason, the transformer should work OK.

    In fact, this is the principle of operation of the multi-match "universal" modulation transformers. The same turns ratios are used for many different sets of impedance values. The CVM-5 for example is rated for something like a range of 2000 to 20,000 ohms on both the primary and secondary. Looking at the charts, you will see the same set of turns ratio connections repeated over and over to transform widely different impedance levels.

    So, getting back to the topic, if a common power supply is to be used, a modulation transformer turns ratio of somewhere between 1.2:1 and 1.4:1 should be used. Tube types, DC plate voltage, final amplifier plate current and nominal modulation transformer ratio can be juggled for the best fit. Some compromise may be necessary, regarding both performance and power output, when one is limited to using components on hand.

    Calculating the modulation transformer turns ratio based on published modulator impedances and final amplifier plate voltages/currents listed in the tube charts may or may not meet the above criteria. If not, it is better to push the load and modulating impedances a little beyond the tube chart recommendations, or live with slightly less power output capability, than to veer too far from the optimum modulation transformer turns ratio as described above when using a common power supply for the modulator and final.
     
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  4. K4KYV

    K4KYV Subscriber QRZ Page

    EAM

    At his forum at the 2015 Huntsville AL hamfest, Bob Heil gave a demonstration using a portable PA system with one of his microphones. His forum topic was "What is wrong with our audio". He ran an A/B comparison between a flat frequency response setting on the amplifier, and incorporating a broad bump in response in the vicinity of 3000-4000 Hz. With the "presence rise" bump in response, his voice was far more intelligible. In his explanation, he evoked the Fletcher Munson Curve, demonstrated in 1933 at Bell Laboratories. Their measurements showed that the human ear is most sensitive to frequencies in the range of 3-4kHz, but the human voice has more energy in the lower frequency scales, dropping off rapidly at frequencies above 1000 Hz or so. Then Bob went on to describe how those principles should be applied to amateur radio audio, including SSB, contrary to the long-promoted "communications quality" 300-3000 Hz frequency response (aka the space shuttle sound).

    Bob Heil's discourse had much in common with an article published in 1970 by George Bonadio, W2WLR (now SK), describing what he called EAM (Enhanced Amplitude Modulation), essentially a response curve that extends with a flat response down to 80 Hz, and begins a steady rise starting just below 1000 Hz, extending up beyond 2800 Hz, with a sharp cut-off just below 3000. His basic premise is a curve that is an inversion of the natural response curve of the human voice, resulting in an audio signal with uniform density from below 100 Hz to just shy of 3000 Hz, giving the signal more "talk power".

    George also presents an interesting theory of how the human voice works, saying that apparent pitch of a person's voice is less a product of the actual range of frequencies produced, but that the voice is like a frequency-modulated signal that scans from the low to the upper range, and that the apparent pitch depends on the scanning rate; therefore male and female voices have nearly the same frequency range.

    From my own experience, in agreement with what Bob stated at his forum, George's 2800 Hz cut-off is a bit too low. Intelligibility components of the human voice extend beyond 4000 Hz, and limiting frequency response to under 3000 Hz impedes readability. I have used with good results a modified version of George's system since the mid 1970s with simple microphones like the unamplified D-104 and the ElectroVoice 670, getting signal reports of quality just as good as when using (borrowed) professional quality microphones costing many times more.

    The greatest difference between my audio and George's system is selectable high-frequency cut-off frequencies at either 3400 Hz or 5000 Hz. More information on my audio system can be found in Electric Radio, issue # 306/ November 2014. Click on the attachment to view George's original article on Enhanced Amplitude Modulation.
     

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  5. K4KYV

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    RF ammeters, Common mode currents and Balance in open-wire transmission lines

    The classic configuration for reading line current and balance in an open-wire transmission line is to insert a thermocouple RF ammeter in series with each feeder. Unless due care is observed, this can give deceptive results.

    From my experience, it is not unusual for the calibration between two identical RF ammeters (same range, same manufacturer, same type number) to vary considerably. Even if the calibration of each meter is within specifications (usually plus/minus 5%), if the two instruments happen to fall at opposite extremes of tolerance, the difference in readings could be as much as 10%. If you see an unbalance in readings, transfer the meters to the opposite feeders to see if that affects the difference. One way to check for sure is to put one of the meters in one side of the line and measure the current, then replace it with the other meter and measure again, to see if the readings agree. Best to do this several times since random line voltage variations and even shifting component parameters may cause the actual line current to vary a few percentage points in a matter of seconds. Another way is to put the two meters in series on the same side of the line and see how they compare, then swap meter positions to make sure the readings stay consistent. Another approach is to connect the two meters in series and feed 60~ a.c. through them, using a variac, filament transformer and a suitable current limiting resistor, and compare readings. The meter reading should be the same at 60~ as at, say 4 mHz; at 60~ you wouldn't get phase variations from inadvertently moving the insertion point of the meter a few inches along the line. Running all of the above tests and comparing results will give a still better idea of how well matched the meters are.

    If the meters are slightly off calibration with one another, make note of the variance and use a conversion factor or make up a calibration chart to determine what are actually identical readings. Once you are sure the meters are properly calibrated (or readings corrected with conversion factor) don't worry if the absolute readings are off by a few percentage points, or even 10%-20%, as long as the two meters have identical calibration errors; what you are seeking is any difference in currents in each conductor at a certain point along the line.

    With a symmetrical, balanced open wire line, tuned or untuned, feeding a balanced load, unbalance in the readings is caused by common-mode currents superimposed on the differential-mode currents. With no common mode currents, the line current has to be the same in each conductor, since the outgoing and return currents in any closed loop must be identical. If no common mode current exists, the voltage loops and current loops on balanced tuned tuned feeders will occur at the same points along the line. If a common mode current on a transmission line (sometimes called "antenna current") exists, it will shift the voltage/current loops and nodes of one conductor relative to the other so that they are offset from each other along the line. At certain points along the line the currents may read identical with the RF ammeters, but if the meters are shifted up or down the line a significant fraction of a wavelength, the current readings could be quite different. Imbalance at the load will cause unbalanced readings for one reason and one reason only: common mode current induced onto the transmission line.

    Take my quarter-wave 160m vertical as an example. I use a 450-ohm UNTUNED open-wire transmission line from shack to the base of the tower, feeding the base of the vertical through a coupling coil wound over the cold end of a parallel tuned circuit whose bottom end is grounded to the radial system, and the lead to the insulated base of the vertical tapped down on the coil to a point to achieve optimum match. Although I tried to eliminate electrostatic coupling between the coupling coil and the main coil as best I could, some common mode current still occurs, so that right at the coupler at the base of the tower, RF current meter readings in both feeders are the same, and a neon lamp lights up equally bright when brought near either one. Further back towards the shack, I can find points along the line where the neon lamp is very bright when brought near one feeder, but I can practically touch the other feeder with it and it won't light up, at least at lower power levels. OTOH, using a 450-ohm non-inductive resistor as a dummy load, the currents read the same in both feeders and at any point along the line the neon lamps glow with equal brightness at each feeder.

    A few years ago I met the retired chief engineer at WSM, and mentioned the balanced two-wire feedline they once used with their big Blaw-Knox tower (the feed-through insulators are still mounted on the walls of both the transmitter building and ATU shelter). He said when they used that system, there always was some unbalance in the two-wire transmission line, but it never caused them any great concern.

    Although not a problem in my case, it is possible that common-mode current in a nominally balanced transmission line could be a source of RFI from the transmitter, since the supposedly balanced OWL acts like a single-conductor long-wire antenna as far as the common mode current is concerned (the reason why common mode is sometimes called antenna current), thereby increasing the RF field in the vicinity of the transmission line.

    Despite the residual common mode current in my transmission line, the OWL is still more efficient than a piece of fresh RG-213 feeding the vertical through a matching L-network. Running the same DC input to the final and an rf ammeter in series with line running to the base of the tower, I get a noticeably higher RF current reading with the OWL than I get with the coax.

    Don k4kyv
     
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  6. K4KYV

    K4KYV Subscriber QRZ Page

    RCA's Recommended Break-In Procedure for Power Tubes

    From Application Guide for RCA power tubes

    The following "break-in" treatment is recommended for new or used tubes which have been in storage for an extended period, before placing such tubes in service. This "break-in" treatment preferably should be in equipment in which the tube is to be used when new circuits are tested or when adjustments are made.

    Step 1: Make sure that the cooling system and protective devices are functioning properly.

    Step 2: With no other voltages on the tube, apply voltage to the filament or heater at the prescribed typical operating voltage for 15 minutes.

    Step 3: Apply reduced value of rf drive power and grid-No.1 voltage (approximately three-quarters normal drive power) for 15 minutes.

    Step 4: Apply reduced value of plate voltage and grid-No.2 voltage (approximately one-half normal values) until stable performance is obtained.

    Step 5: Increase rf drive power and grid-No. 1 voltage to normal.

    Step 6: Increase plate voltage and grid-No.2 voltage to normal, gradually or in steps. Operate the tube until stable performance is obtained at each voltage level.

    After the tube is given the above treatment and is operating normally to give the desired output, it is suggested that the readings of the meters and the control settings be recorded for future reference.


    For tubes that have been out of service for several years, I follow the above procedure, but with increased time for each step. I normally run the filament for about two hours (Step 2), then apply reduced grid drive for 30 minutes to a couple of hours (Step 3), reduced plate voltage (Step 4) for 1-2 hours, and finally let the tube run into a dummy load with full grid drive and full plate voltage for a couple of hours.

    With questionable tubes, I have been known to run the filament overnight before continuing with the procedure.

     
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  7. K4KYV

    K4KYV Subscriber QRZ Page

    Removing the B+ from the primary of a single-ended driver transformer



    Q: Is it recommended to remove the B+ off the input of an interstage transformer?
    This would be in a Johnson Valiant. I thought the interstage transformer removed DC off the circuit feeding the modulators. Someone told me I should put a capacitor in series with the primary side of the interstage from the 12AU7, then bring the 300vdc back to the plate of the tube with a shunt feed. This doesn’t make sense to me. I thought the coupling caps on the stages of the audio amp section would block the dc as well.

    Is this a worthwhile endeavour to clean up the pre amp stages of the audio on the Valiant?



    A: That modification is highly recommended for all transmitters that use a single-ended driver stage feeding push-pull class AB1 modulators. This includes the Ranger and other transmitters that use a single tube and inter-stage transformer to drive a push-pull class AB1 modulator stage. This circuit may not work satisfactorily in the DX-100, Viking I and II, and other rigs that operate the modulator tubes in class AB2 or class-B; see the discussion below that follows, regarding the use of a choke instead of a resistor. It did work fairly well with a Valiant that belonged to a friend of mine; I assumed those modulators operated in class AB1, but on second thought they may be in AB2. I would like to hear from readers who have tried it with the Valiant.

    Yes, the driver transformer removes the +DC off the circuit feeding the modulator grids, but in the stock circuit the driver transformer itself is the issue. Removing the B+ off the primary winding, or more accurately, removing the plate current to the driver stage from passing through the primary winding, improves transformer performance by eliminating the saturating effect to the iron core, caused by the unbalanced DC flowing through the winding. This is the same reason that better quality plate-modulated transmitters, including most broadcast transmitters, use a modulation reactor and coupling capacitor to keep the unbalanced DC to the final from flowing through the secondary winding of the modulation transformer.

    With this modification, the paralleled plates of the 12AU7 driver stage are coupled to the transformer through a capacitor; the +300 vdc is brought to the driver tube plates with a shunt feed. Ideally, a high inductance plate choke would be inserted between the +300 volt supply and the plates of the 12AU7, with a 1- or 2- mfd coupling capacitor wired between the paralleled plates and the top end of the primary. Problem is, such a high inductance choke is difficult if not impossible to find from the usual parts sources, in addition to the space problem of where to mount the choke. Instead of a plate choke, a plate resistor may be used. As I recall, with the paralleled sections of the 12AU7 driver tube, the plate resistor should be about 15,000 ohms. A method for calculating the value of the plate resistor is described below. Make sure the resistor wattage is high enough to carry the driver plate current. Several smaller resistors may have to be wired in series or in parallel if a larger size resistor is not available. Since this resistor will result in a drop in the plate voltage feeding the driver plates, the resistor must be returned to a higher voltage source, which would be the same +HV supply that feeds the modulator and final plates. With this higher source voltage, the voltage drop in the resistor brings the voltage back down to +300 at the driver tube plates.

    It is best to return the bottom end of the primary winding directly to ground, rather than leaving it connected to the +300 supply. Completely removing the +300 volts from the primary side of the transformer protects the transformer, since the insulation between windings will no longer be subjected to the voltage differential between +300 volts on the primary and the DC grid voltage that appears on the secondary winding. With some transmitters, the modulator tube grids operate at zero DC volts, using cathode bias for the modulator tubes, while with others, an external negative bias supply is used that further increases the DC voltage differential between windings. One precaution: make sure the 1 to 2 mfd plate coupling capacitor is rated at high enough working voltage to withstand the high plate supply voltage, preferably with some safety margin. If possible, use a non-electrolytic oil or paper capacitor, rated at something well above the +HV supply voltage. An alternative, if a suitable high-voltage coupling capacitor cannot be found, would be to leave the transformer primary connected to the +300 volt supply, decreasing the DC voltage differential across the coupling capacitor, but sacrificing the additional protection to the transformer gained by removing the +300v from the transformer altogether.

    Eliminating the core saturation caused by the DC effectively increases the amount of iron in the transformer, thus increasing the primary and secondary inductance of the transformer, improving the low frequency response and reducing phase shift distortion. Once this modification is successfully achieved, the modification may be taken a step further for still more improvement in winding inductance and low-frequency response. Remove the driver transformer from the transmitter, carefully tagging the wire leads so they can be wired back the same as originally. Carefully pry the metal frame from around the core. Insert a thin blade into the gap in the core, and carefully pry the I-shaped part of the core loose from the E-shaped part of the core that holds the windings. Now, using a plastic or wooden tool, gently tap with a hammer to force the E-shaped part of the core from the coil. Some transformers are wax impregnated, and the transformer may have to be warmed to about 140 degrees to soften the wax and loosen the core from the winding. Once the transformer is disassembled, re-assemble the core by re-stacking the laminations so that with each adjacent layer, the E- and I- laminations face the opposite direction, the same way that a power transformer core is stacked. This procedure, sometimes called "cross-laminating", eliminates the gap in the core and further increases the effective amount of iron in the core, further increasing the inductance of the transformer windings and improving the low-frequency response. Place the frame back over the transformer core, re-mount the transformer and re-connect the primary and secondary leads as original.

    Why get rid of the gap in the core? The gap was designed to reduce the saturation effect of the DC flowing through the primary winding, but as explained above, this also reduces the effective amount of iron in the core and thus the inductance of the windings. Since DC no longer flows through the winding, the gap is no longer necessary.

    I modified my Johnson Ranger in this manner and helped a friend modify his Valiant. This resulted in a substantial improvement in audio quality. This modification may be applied to a number of popular vintage transmitters, but in each case the plate resistor will have to be calculated as described below.


    How to calculate the value of the plate resistor:

    To figure out this resistance, measure the +HV and the low voltage from the power supplies in the transmitter, and measure the total plate current drawn by the two 12AU7 sections (or other driver tubes in other transmitters). An easy way to measure the plate current of a tube is to measure the voltage drop across the cathode resistor, and use ohms law to figure out how much current is passing through the resistor. Once you have determined the current pulled by the driver tube, use ohms law to calculate how much resistance you need to drop the +HV down to the normal plate voltage on that tube, and use that for the plate resistor. Then use ohms law to calculate the watts of power dissipated in the resistor. If you don't have a suitable resistor in the junkbox, order a new one from a source like Newark. I would double the wattage of the resistor from the dissipated power you calculated.

    Don k4kyv


    Rob k5uj responded:

    Well, I'm not sure what "shunt feed" means in this context -- that term seems to be used for a variety of contexts such as shunt feeding a tower, but I found this choke for sale:

    https://www.tubesandmore.com/products/choke-one-electron-audio-prc-2-shunt-feed-se-output

    I'd just connect the 300 v. to a junction with the tube plate and one side of the coupling cap to the transformer primary.

    You need some kind of impedance between the tube plate and the +300v supply. Otherwise, the filter capacitor in the power supply acts like a dead short to ground as far as the audio is concerned. That 48 H choke looks interesting, but I'm not sure it's enough inductance for this application, since unlike a single-ended power amplifier driving a speaker or a class-B or AB2 modulator, this transformer is looking into a very high impedance load, the grids of a class AB1 amplifier. A resistor is a more practical solution; you just have to return it to the +600v or whatever the final amp/modulator plate voltage is, rather than to the +300 supply, in order to make up for the voltage drop in the resistor.

    This works only in the case of a class AB1 modulator. If the modulator runs class AB2 or class B, drawing grid current on audio peaks, you would have severe audio distortion because the internal resistance of the R-C coupled stage is very high. The choke would be necessary in that case.

    "Shunt feed" is pretty much a generic term. It could apply to about anything where a load is wired in parallel with the output a generator. This includes antennas, audio amplifiers, electric motors, etc.
     
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  8. K4KYV

    K4KYV Subscriber QRZ Page

    AM Broadcast Ground Systems

    Here is a link to drawings of a standard AM broadcast ground system, including configuration of overlapping radial systems for multiple tower arrays. Also shown is the construction of a special tool for ploughing in ground radials. Useful information for amateur installations as well.

    http://www.hatdaw.com/papers/groundsystem.pdf
     
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  9. K4KYV

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    Modulation Reactor Winding Polarity

    When wiring in the modulation reactor, correct polarity of the winding polarity must be observed. Reversed polarity may cause loss of high frequency response and possibly, catastrophic failure of the reactor.

    Transformer and choke winding terminals are often marked "S" and "F". S means start; F means finish. The "start" terminal is connected to the end of the wire where the winding was started, i.e. the layer of wire closest to the core. The "finish" terminal is connected to the end of the wire where the final turn of the winding was finished, the layer farthest from the core and closest to the outside of the winding.

    With a modulation reactor, the Start terminal should be connected to the B+ or to the blocking capacitor that goes to ground. In other words, the "bottom end" of the reactor coil that is effectively grounded at audio frequencies, carrying only unmodulated B+. The Finish terminal connects to the +HV line to the RF final; this is the terminal connected to the "top end" of the winding, carrying the modulated B+. One reason this precaution is important is that the starting layer of wire is separated from the core only by a thin layer of insulation, and thus apt to have more capacitance to ground than would the finish layer. This additional capacitance acts like a capacitor shunted across the winding of the reactor, degrading the high frequency response. The second reason is that the outer (finish) layer of the winding is apt to be better insulated for high voltage than is the inner (start) layer. Many if not most reactors were designed with less concern over HV breakdown at the start layer than with the finish layer, which carries the modulated B+ with peak voltage that may exceed by twice or more the voltage at the start layer.

    This may be less of a concern if the entire reactor is mounted on insulators so that the case and core are floating above ground, but this precaution is still recommended, since the transformer case would have capacitance to ground, and the insulation could possibly break down if subjected to fully modulated high voltage.

    This precaution applies to all types of reactor construction, which includes shell type (E-I lamination construction), and core type (rectangular core with two coils side by side).

    Transformer cores.gif


    If the winding is not clearly marked S and F, or B+ and Plate, the start and finish terminals can often be determined as follows with open-frame (exposed winding) reactors. Some reactors may have a split winding, each half a mirror image of the other, wound so that each terminal connects to the "finish" layer of that half of the split winding. In that case, polarity of the winding is unimportant and these precautions may not be necessary. If you are unsure, the following tests may determine whether or not polarity must be observed. Unfortunately, this test cannot be performed with fully potted reactors since the winding is enclosed inside the metal case.

    To carry out the test, you will need an audio generator and a sensitive audio or a.c. voltmeter or an oscilloscope, something capable of detecting a relatively feeble audio voltage. Ground one side of the audio generator to the core or case of the reactor. Connect the ground lead of the a.c. voltmeter or oscilloscope to the core. Tape a rectangular piece of tinfoil tightly over the paper insulation on one face of the exposed winding. Connect the "hot" lead of the oscilloscope or a.c. voltmeter to the piece of tinfoil. Now try connecting the output lead from the audio generator to each of the two terminals of the reactor. You should get a substantially higher reading on the meter or scope at one terminal than at the other. The one with the higher reading would be the one connected to finish; the outermost layer of the winding serves as one plate of a capacitor and the tinfoil as the other, with the outer layer of insulation as the dielectric. The start layer of the winding is partially bypassed to the core via the capacitance through the insulation to the core, while separated from the tinfoil by all the turns in the winding; the inductance of the winding further isolates the start layer from the tinfoil. The difference should be greater at higher frequencies, since the shunting capacitance effect and winding inductance effect are greater at higher frequencies than at lower. Try taking readings at various frequencies all the way up to the high frequency limit of the signal generator to verify your observation.

    This is also a concern with the secondary side of the modulation transformer for the same reasons stated above, whether or not a modulation reactor is used. The same tests should work with many modulation transformers as well.


    See attached schematic.


     

    Attached Files:

    Last edited: Nov 1, 2017
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  10. K4KYV

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    The weirdest radio problem I ever had.

    Several decades ago, right after I had just got my transmitter working on 160m (a single 304-TL modulated by triode connected 813s), I started hearing an annoying unmodulated carrier popping up near my operating frequency. It was a very stable, solid, clean carrier with no QSB and would appear day and night, so I assumed it had to be someone tuning up near-by, but they never ID'ed or attempted to transmit code, and I never heard any modulation. Just a continuous carrier out of nowhere that would sometimes stay on frequency for hours. This had me totally baffled for a couple of weeks, as I thought it it had to be someone local, but I knew of no nearby hams in the vicinity. Was someone playing a joke on me, or maybe it was a new ham close by, trying to get a transmitter on the air.

    Then, one time I just happened to bump one of the tuning knobs on the rf exciter, and the carrier went away. Upon further investigation, I discovered that rotating the plate tuning knob on the first buffer stage following the VFO changed the frequency of the carrier! It turned out to be an inexplicable self-oscillation in my exciter that occurred on 160m only; it was a very stable self-oscillation that would have made an excellent VFO. But the strangest thing was , this was happening with nothing on but the tube filaments, with the transmitter in standby mode with the HV plate supply completely turned off by cutting the a.c. voltage to the plate transformer primary. But how could an rf stage go into self-oscillation with no DC voltages on any element of the tube?

    The offending stage used a 6V6 as a buffer stage following the external VFO. The power supply used a 5Z3 rectifier tube. I began measuring voltages with my multi-meter, and discovered that the plate of the 6V6 stage registered about +15 volts. I could short out the B+ line to the stage, and the oscillation would stop. I pulled out the 5Z3 rectifier, and the oscillation stopped, but shorting out the transformer secondary winding with a clip lead did not kill the DC voltage or the oscillation. Therefore, this was not the plate transformer picking up stray 60~ a.c. by magnetic coupling to another transformer. After some head-scratching, it finally occurred to me what was happening. For stand-by operation during receive, I turned off the DC voltage from the power supply by opening a relay in the a.c. line to the primary of the plate transformer, leaving the secondary winding connected. The 5Z3 stayed lit all the time, fed with a separate filament transformer. With the transformer supplying no a.c. voltage to the plates of the rectifier but maintaining DC continuity to ground , there were still enough stray electrons emitted by the heated filament with high enough velocity to randomly hit the rectifier tube plates, causing a charge to accumulate and generate about +15 volts DC output from the power supply.

    My solution to the problem was to change the 6V6 in the buffer stage to a 6AG7, which is a much better shielded tube at rf. That completely killed the oscillation. That 6V6 must have been extremely prone to self-oscillation, which occurred only on 160m because it operated straight through on that band but worked as a frequency multiplier on all others. I still use that rf exciter unit to drive my 8005s modulated by 838s homebrew transmitter that I normally use on 40m.

    That was a typical example of the kind of technical problems I have always had with my radio equipment. Very rarely do I ever see a textbook malfunction with a textbook solution, like you read about in the handbooks. It's nearly always either a frustrating intermittent, or else some weird problem like the one described above, something I wouldn't have thought of in a thousand years.
     
    Last edited: Jan 2, 2019
    NL7W, WA3VJB, W1BR and 6 others like this.
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