Tech Talks and Tips by K4KYV

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

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  1. K4KYV

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    Overmodulation Indicator and Transformer Protector

    Years ago, dating back before I had an oscilloscope to monitor modulation, I used an 866A as a negative peak overmodulation flasher. Using a 2.5 volt filament transformer insulated for high voltage, the midtap of the filament winding is connected directly to the modulated HV plate lead to the final. The 866A plate is connected to ground. As long as the plate of the final remains at positive potential with respect to ground or at zero volts, the 866 will not conduct. As soon as the final amplifier tube plate is driven negative (the condition with negative peak overmodulation), the 866 conducts and flashes with the characteristic blue glow.

    I mounted the 866(A) inside a small box with the interior painted flat black, with a small window for viewing the plate area of the tube. The blue flashes were highly visible and this made a very effective overmodulation indicator. As a refinement, I later replaced the 866A with an older version, the original type 866 without the "A" suffix. This version of the tube is rated at only 7500 PIV instead of 10K, but was not problematic with my 2000v to 2500v nominal plate voltage. The advantage of the older version of the tube (if you can find a good one) is that the filament is fully exposed to view, lacking the shield that surrounds the filament in the 866A. This makes the blue flash even more visible to the eye. Because of the lower PIV, I would recommend using only the "A" version at plate voltages above 3000 volts, especially if positive modulation peaks extend very far above 100%.

    Although the circuit described above works as an overmodulation indicator, it puts a momentary dead short across the modulation transformer whenever the 866(A) conducts. It occurred to me that this could conceivably generate unwanted transients or even damage the transformer. This was corrected by placing a small wirewound power resistor equal to the modulating impedance (PA plate volts ÷ plate current in amps) in series with the 866A, from the rectifier plate to ground. This will maintain the proper constant load on the modulation transformer for the duration of any negative peak that exceeds 100%. With the resistor in series with the diode, the modulation transformer sees normal load impedance throughout the entire audio cycle, even while the final amplifier plate is being driven negative. Since this peak occurs over a very small portion of the audio cycle, a small resistor, 10% of the nominal modulator power or less, is sufficient. For example, a 50-watt resistor would be more than sufficient when modulating a full kilowatt DC input to the final.

    Besides serving as an overmodulation indicator and maintaining a constant load on the modulator, this circuit offers some protection to the modulation transformer. Normally, whenever a plate modulated final is overmodulated in the negative direction, the modulation transformer operates momentarily without a load for the duration of the overmodulation peak, which could conceivably damage the transformer if an unexpected transient spike happened to occur during this peak while the final amplifier tube plate is negative and not conducting. Unlike other schemes such as "negative cycle loading" and "ultramodulation", this circuit allows protection for the modulation transformer without introducing audio waveform distortion and does not waste a significant amount of audio power in a resistor.

    Due to inherent characteristics of MV rectifiers, the 866(A) plate must be driven approximately 15 volts positive before the tube conducts, so this circuit doesn't actually flash until slightly over 100% negative modulation. As a further refinement, the plate of the 866A may be biased slightly positive, using a small power supply rated at a few milliamps, to make it flash at or just before 100% modulation. With the 866(A) plate biased approximately 15 volts positive, the flash occurs at exactly 100% modulation. Further increasing this bias causes the 866(A) to flash just before 100%. For example, your final runs 2000 volts on the plate. Bias the 866(A) plate 50 volts positive, and the 866(A) will flash at 97.5% negative modulation.

    The positive bias may be set to cause the tube flash at considerably less than 100% if desired. However, at percentages between the flasher threshold and 100% negative modulation, the resistor is effectively in parallel with with the class-C modulating impedance, reducing the load on the modulation transformer to half its normal value, much in the same manner as negative cycle loading. This may introduce unwanted distortion and splatter at negative modulation percentages between the flasher threshold and 100%, as the modulator tubes abruptly look into half their plate-to-plate load over a substantial portion of the audio cycle. This may be avoided by increasing the resistor to approximately 10 times the modulating impedance. The 866(A) would still flash and the resistor would maintain some load on the transformer and still offer a degree of protection during negative overmodulation peaks, but the circuit no longer provides a constant load to the modulation transformer throughout the entire audio cycle.

    See attached schematic.

    Attached Files:

    WZ5Q and (deleted member) like this.
  2. K4KYV

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    Rane Notes Audio Links

    A good source of information regarding audio equipment and how to properly interconnect it, is available at the following website:

    A wealth of technical data on balanced vs unbalanced lines, interconnecting balanced and unbalanced inputs and outputs, mixing together balanced and unbalanced audio equipment, preventing and dealing with ground loops, rfi problems in audio equipment, Pin 1 problem and much more.

    Here is an index of their entire library of RaneNotes by number and subject. Click on the link for an HTML version for the note. At the bottom of the last page of each note is a link to a PDF version. I find the PDF version more attractive and easier to read.

    I would particularly recommend the following:

    Note 151:

    Note 110:

    Note 165:

    Note 166 Rfi and shielded balanced lines:
  3. K4KYV

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    Preserve Your D-104 Crystal Element
    Probably the worst enemy of the D-104 and other crystal microphones is high humidity. For those of us unfortunate enough to live in a muggy humid climate, we can run the air conditioner in summer, but that's an imperfect solution at best, since we probably wouldn't run the A/C 24/7 while out of town or even all day while at work.

    Rochelle salt (potassium sodium tartrate) crystals are a deliquescent, which means they have a strong affinity for moisture and will absorb relatively large amounts of water from the atmosphere, forming an aqueous solution resulting in the destruction of the crystal as it literally dissolves. The greater the exposure, the faster the deterioration in high humidity or damp conditions. The D-104 crystal is coated with a waxy substance to isolate it from the humid atmosphere, but even tiniest crack or defect in the coating will allow the moisture to get through, one of the reasons why the mic should be handled carefully and never allowed to drop onto the floor. Obviously, to protect the crystal and assure a long life, it must be kept away from humid conditions as much as possible.

    Here's a use for those little silica gel bags that come with about everything you buy. Don't throw them away, as the instructions on the bag tell you to do. The idea is to provide something to compete with the Rochelle salt for sucking moisture out of the air. If your D-104 has the usual removable head with the 3-pin plug on the bottom, simply remove it from the boom or stand and place it in a zip-lock bag with a few of those silica-gel packets and seal the bag closed whenever you expect to be out of the shack for an extended period of time. If your mic is of the older variety with the cord permanently attached to the mic head, a plastic bag with the silica packets can be placed over the mic head with tape or an elastic band around the neck to seal and keep the outside air out as much as possible.

    When the packets inevitably appear saturated with moisture, they can be re-used by placing them in a warm oven for a few minutes to dry out the crystals.

  4. K4KYV

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    A Tip for Safely Mounting Heavy Equipment in a Rack
    If you attempt to mount a heavy piece of equipment in a rack or rack cabinet, especially without a helper, holding or balancing the unit while you try to get rack screws started can be frustrating or even dangerous. This can be done easily with the following technique: using a hacksaw, cut the heads off a couple of 1 to 1½ inch-long screws the proper size and thread to fit the holes in the rack (usually 10-32 but older racks may be 10-24). Then use the hacksaw to cut a slot in the end where you just removed the screw head. This will permit a screwdriver to screw those modified bolts part way into the rack, into the holes where the upper rack mounting screws will be located. Once the bolts are installed in the rack, slide the equipment in place. The protruding prongs of your modified bolts will serve to support the equipment while you screw in the lower rack screws, and then you can remove the modified bolts and replace them with the upper rack screws.

    --From Radioworld, Nov 21 '18, p. 14
  5. K4KYV

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    Removing enamel insulation from fine enamelled wire
    Scraping fine enamelled wire is apt to damage the strand and may not clean off all the insulation. Heating with a flame will leave the copper so brittle that it will break easily. The following method is described in a 1949 publication "Electronics manual for radio engineers". It works with single strands and with Litz wire as well.

    Apply a small quantity of a paste of zinc chloride and water to the strand and heat with a soldering iron, immediately followed by tinning with rosin core solder while the resulting mixture of zinc chloride and enamel is still boiling. Wipe off the residue and excess solder with a damp rag quickly, while the wire is still hot. This will clean off the residue and leave a clean tinned end to the strand.

    Zinc chloride is available from $10 plus shipping for 100g/3.5 oz jar.

    A few words of caution; this product should be handled and used with extreme care. Zinc chloride is a skin irritant. Avoid contact of the eyes. If ingested, the lethal dose in humans is said to be as little as 3 to 5 g. Zinc chloride is extremely detrimental to the lungs, and pulmonary exposure to zinc chloride smoke has reportedly resulted in fatalities.

  6. K4KYV

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    Caution When Repurposing Small Transformers
    I was recently given a non-working 75A-4 that had a burnt-out power transformer. The previous owner couldn't find a suitable replacement that had the special 120 volt bias tap on the HV winding. I found a near-exact replacement transformer in my junk collection, except it has no bias tap nor a 5v filament winding for the 5Y3. I installed the new transformer and replaced the 5Y3 with solid state diodes. For the bias tap, I used a small 6.3 volt 1.2 amp filament transformer, wired backwards with the intended 6.3v secondary winding connected to the filament line in the receiver. After clipping out a shorted (and superfluous) ceramic capacitor in the plate circuit of the last audio stage, the receiver came to life, but still had problems. It was pulling way too much current from the B+ line, and had serious 120~ hum. The first thing I checked was the B+ voltage, and found it to be about 60 volts too high. No wonder the 6AQ5 tube and the filter chokes were running hot to the touch. That's the first note of caution when changing to a solid state rectifier. A high-vacuum rectifier tube has a certain amount of voltage sag, but a solid state rectifier may have less that 1 volt drop under full load, causing the HV to run too high. This will eventually lead to other component failures, so I fixed the voltage drop by adding a 300-ohm 10w wirewound resistor between the rectifier and the 1st filter choke to drop the B+ voltage back down to normal.

    That got rid of the loud hum and abnormally high voltage, but the receiver still acted quirky, still pulling excessive current from the +HV supply. I did some voltage checks, and found the control grid voltage to the 6AQ5 to be -5.5 volts; the manual says it's supposed to be -11 volts. Low bias voltage will cause the tube to pull excessive plate and screen current. The 6AL5 bias rectifier tested good, so I measured the a.c. voltage from the transformer tap to the cathode; instead of 120 volts a.c., it measured less than 90. This didn't make sense; the transformer is rated at 120 volts primary to 6.3 volt/1.2 amp secondary. The filament voltage in the receiver measured slightly above 6.3 volts, so with this winding connected to the receiver filament line, approximately 120 volts should appear across what was originally the primary. With a transformer, the turns ratio determines the voltage transformation, and it doesn't matter which winding acts as primary and which as secondary; the voltage ratio will always be the same. Thinking I had a bad transformer, I pulled a couple more similar transformers out of the junkbox, clip-leaded the 6.3 volt winding to the receiver filament line, and measured the voltage on the other winding. In each case it was low, somewhere in the vicinity of 90 volts.

    Then it occurred to me what was happening. Those little transformers would have a noticeable voltage sag when the full 1.2 amps is pulled from the secondary winding, so the manufacturer added a few extra turns to bring the voltage up high enough to compensate for voltage sag in the transformer plus that of the leads from the transformer to the tube filaments. I checked the output voltage by connecting 120v from the power outlet to the primary. Without any load on the secondary, the voltage measured a little over 8 volts. Other similar transformers yielded exactly the same result. So, when only 6.3 volts is applied the the LV winding, the voltage appearing across the other winding will be low.

    I was able to fix the problem by digging into the "6.3v" secondary winding of the transformer, which fortunately is usually the outermost winding and easy to get to simply by pulling away the outer layer of paper insulation that covers the winding. I was able to tap down a few turns from the end of the winding and find a spot that yields close to 120 volts when filament voltage from the receiver is applied to the low-voltage winding. The connecting lead was moved over and soldered to this tap, clipped ends of the original turns were safely positioned not to short together, and the transformer was re-sealed with epoxy to hold everything permanently in place. The a.c. voltage to the bias rectifier now measures correct.

    It's always best to temporarily wire or clip-lead in a substitute component to see how well it works, and make any necessary corrections before making the change permanent.
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  7. K4KYV

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    Graëtz Bridge

    You may encounter the term "Graëtz bridge" for the full-wave bridge rectifier in some engineering texts, named after German physicist Leo Graëtz (1856-1941). Graëtz and Polish electro-technician Karol Pollak (1859-1928) each independently invented the bridge rectifier circa 1896.

    It's easy to remember the circuit for the FW bridge without even looking at a schematic. Each output terminal from the a.c. source (the power transformer) connects to the cathode of one diode and to the anode of another. The positive d.c. output terminal of the rectifier connects to the two remaining cathodes joined together, and the negative output terminal connects to the two remaining anodes joined together.

    For a more distinct view of the animation, first start the video and immediately click on the YouTube logo to view the site directly, then click on the little square box to the right for full-screen mode.

    For complete table of contents of Tech Talks, go back to page 1, message #2
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