General Instructions.

Constructing And Retrofitting Low VHF-Q Parasitic Suppressors

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Someday, you may be able to buy amateur radio amplifiers that use low VHF-Q parasitic-suppressors. For the time being, if you prefer to have a more stable amplifier, you must install the low-Q parasitic-suppressors yourself -- or install them with the help of a friend who is familiar with amplifiers.

„ Please check all of the parts in your suppressor retrofit-kit for shipping damage, missing parts, and for correct values. If you find a problem, please telephone me collect or write me a postcard describing the problem. I will send you the needed parts as soon as possible.

Capacitor marking codes: There are 2 capacitor marking codes. Both codes give the value in pico (10--12) Farads, or pF for short. For values less than 100pF, a 2-digit code is used. For example "25" = 25pF. For values above 99pF, a 3-digit code is used. As in the ±5%-tolerance resistor color-code system, the 3rd digit is the number of zeros that are added to the first 2 digits. For example "471" means 47, with 1 zero added = 470pF. "103", which does not equal 103pF, means 10 with 3 zeros added, or 10,000pF = 0.01µF = 10nF. "102"=1000pF = 1nF.

The porous ceramic-cement coating on the MOF and MF resistors is used mainly to provide a smooth surface for applying markings. Don't be concerned if the coating chips during handling or installation. It won't affect the performance of the resistor unless the conductive film has been damaged sufficiently to show up on an ohm-meter. The blue-colored coating on the Matsushita resistors will normally discolor to brown and/or light-blue in the presence of RF. This does not affect the performance of the resistor. If you have doubts about a resistor, you can measure the resistance. This usually requires unsoldering one end of the resistor in question.

Safety Notes

Amplifiers that are plugged into the electric-mains contain lethal voltages, whether they are switched on or switched off! If you don't pull the electric-mains plug before working on your amplifier, your amplifier may be for sale at a 'silent key' estate sale!!

The water-soluble solder flux that is used for soldering nichrome contains zinc-chloride, hydrochloric-acid and acetyl-acid. It is corrosive, poisonous if swallowed, and it splatters during soldering. The flux is especially bad-news for eyes and it is irritating to skin. Either wear safety glasses or keep your face away from the soldering operation. The vapour that is given off during soldering contains hydrogen-chloride gas.

It is advisable to open a window and to spread several layers of newspaper on the table where the nichrome soldering and tinning operation will take place. The newspaper catches any drops of flux that are too large to stay on the joints -- plus it soaks up splatters.

The corrosive flux residue and the flux splatters must be removed from the suppressor assemblies with warm water and a brush. In order to remove the flux residue from the pores in the ceramic coating on the film-resistors, the completed suppressor assemblies should be soaked for about 5-minutes in a bowl of warm water. A spoonful of baking soda [sodium-bicarbonate] in the bowl of water helps to neutralize the acids. After a good soaking, the suppressors should be scrubbed with a toothbrush under running warm water and then dried with a towel. If this is not done, the corrosive flux residue will eventually do bad things inside the amplifier.

Soldering with the messy nichrome-flux, inside the amplifier, should be avoided. Connections that need to be soldered inside the amplifier can be pre-tinned with silver-solder outside the amplifier so that the flux can not contaminate the inside of the amplifier. After tinning, the flux residue should be removed with water. Once the end of a nichrome wire has been tinned with the silver solder, no additional nichrome flux will be needed for soldering it inside the amplifier. If flux is needed for silver soldering inside the amplifier, liquid rosin flux may be used.

Tin [Sn]-Silver [Ag] Solder

The 94%-Sn, 6%-Ag solder supplied in the retrofit-kit is about 3.5-times stronger than ordinary electronics solder. It melts and flows at 430ºF/221ºC. Silver usually improves the flowability of solder. This solder is highly corrosion resistant, so it is ideal for antenna and ground system construction. It contains no toxic hazards such as lead. It is approved for soldering food utinsels and it is excellent for general electronics use provided that liquid rosin flux is used instead of the nasty flux. For electronics use, brush liquid rosin flux on the outside of the solder and let dry. Before soldering with Sn/Ag silver solder, remove any existing tin/lead solder with solder wick.

94/6 Sn/Ag solder is ideal for resoldering filament pins on 3-500Zs. To do this, unsolder the pin and remove as much of the old solder from the pin and the wire as is possible. Scrape the wire clean. Apply liquid rosin flux. Re-tin both parts with Sn/Ag solder. Reassemble and resolder. Remove the flux residue with alcohol. Lightly polish the pin with fine steel wool. {Do not use nichrome flux for soldering tube pins!!}

Soldering Nichrome Alloys

It's best to use a soldering iron with a tip temperature of no more than 600-700ºF. Higher temperatures may cause the flux to decompose into a brown, useless mess. To facilitate soldering, clean and polish the nichrome wire with fine steel wool or 400 grit abrasive paper before cutting and forming.

Wires that are to be joined together should be wrapped [about 0.8-turn], one around the other, so that the parts will be held in place during soldering. Tin the iron with 95/5 silver solder. Warm up the joint that is to be soldered by touching the molten solder on the iron to the joint. Remove the iron and hold a small hanging drop of flux against the joint using the single-drop dispensing plastic bottle. The flux should boil and sizzle -- activating the flux ingredients. Be careful not to melt the tip of the bottle. Touch the joint again with molten solder. The silver solder should flow into the joint. If the solder does not flow properly, reflux while hot and try again. After soldering each connection, wipe the flux residue from the tip of the soldering iron on a cellulose sponge dampened with purified water.

Retrofitting

Grid fuse resistors are optional. They have no effect on stability. When a grid-to-ground RFC is present, a grid fuse resistor is used to replace it.

1. At the anode-connections ["plate-caps"] on top of the amplifier-tubes, remove the original high-VHF-Q anode [plate] L/R parasitic-suppressors and the leads that accompany them.

2. In the anode-circuit only, where practical, remove any copper or silver straps, braid or heavy buswires, that exceed 0.8 inch [20mm] in length [see Figure 1A]. The anode-circuit is the circuit between the anode-connection and the tune-capacitor. No modifications should be made to the HF tank-circuit. The HF tank-circuit is the circuit between the tune-capacitor, the HF-tank-inductor and the load-capacitor.

3. Build Low VHF-Q Parasitic-Suppressors that will mechanically replace the original parts you removed

Constructing Low VHF-Q Suppressor Assemblies

A. Leave enough nichrome wire at the ends of Ls to reach the solder or screw connections. If the span is small, Ra and Ls/Rs may be bent into a "dogleg". If the electrical connection in the amplifier will be soldered, pre-tin the end of the nichrome wire using the nichrome flux and silver solder.

B. Illustration on page-5. Connecting the suppressor-resistors, Rs, in parallel with the nichrome suppressor-inductor, Ls:

Since the suppressor-resistors, Rs, are supposed to be the lower-inductance VHF current-path in the Ls/Rs parasitic-suppressor, it is important that their lead-lengths be kept short. To accomplish this, the width of the U-inductor, or the coil-inductor, is made about equal to the length of the resistors.

Leave at least one inch [25mm] of nichrome wire protruding beyond each connection to Rs to allow for fitting the suppressor to the anode circuit. Wrap the copper wire leads of the resistors around the nichrome wire to keep them in place during soldering. 0.8-turn is enough. Clip off the extra copper wire and solder the connections as described above. Be sure that an appropriate cooling air-gap exists between the resistors.

C. Solder Ra to the end of either nichrome wire that protrudes from the suppressor. Tin the other end of the nichrome wire with silver solder. After about 10-sec., put the assembly in a bowl of warm water to soak.

D. Construct dual, unequal L, nichrome wire and/or ribbon [if specified] mechanical replacements for the original, high VHF-Q, anode-circuit straps and/or buswires and braid that you removed earlier. See ^ on the diagram.

4. Remove all of the flux residue and splatters from the assemblies under running, warm water, using a toothbrush. This is a good time to remove any flux splatters from your skin. Dry the assemblies. Install your handiwork in the amplifier.

In 2-tube amplifiers it is important that suppressor U-inductors are NOT positioned in the same plane. If U-inductors are in the same plane, they will couple to each other and a push-pull parasitic oscillation is possible. To decouple them, the plane of #1 U-inductor is tilted 45º to the left and the plane of #2 U-inductor is tilted 45º to the right. This places them at an angle of 90º. It is also helpful to install the suppressors on adjacent amplifier-tubes well away from each other. Due to the effect of nearby objects, 90º orientation is not always the best orientation for U-inductors. If an amplifier shows signs of instability , experiment with a different orientation of the U-inductors. In some situations, a coil-inductor type suppressor will provide better isolation. [see page 5]

5. Install the cathode, RF negative feedback resistor(s) [Rc], the [optional] grid fuse-resistor(s), their grid-to-gnd RF-bypass capacitors, and the wirewound HV glitch resistor in the HV positive lead. NOTE: installation of the HV glitch resistor often necessitates that a standoff insulator be added. A suitable standoff insulator can be a small rectangle of fiberglass perfboard that is glued edgewise to the chassis with silicone rubber adhesive.

6. ONLY for kits that use 1, or 2, RLC, VHF series-resonant, suppressors between the cathode terminal(s) and gnd: The lead lengths on the R and C components should be kept short. NOTE: Each RLC suppressor adds 25pF to the input capacitance of the amplifier-tubes. This will usually affect the input SWR on the 28MHz, 24Mhz, and 21MHz bands. Sometimes the SWR gets better, and sometimes it gets worse. This may necessitate removing a similar amount of C from the output capacitors in the tuned-circuits on these bands. Another solution is to add roughly the same capacitance to each of the input-capacitors. The tuning slug on the inductors in the tuned-input circuits is then backed-out to reduce L and improve the SWR. In rare cases, one or two turns must be removed from the inductor.

28MHz Operation: If a VHF suppressor does not become fairly hot on 28MHz, you can be sure that it is not doing its job well since 28MHz is almost VHF [30-300MHz]. This is especially the case with continuous-carrier [AØ] operation. The design goal is to come up with a suppressor that gets hot on 10m but not hot enough to fail. The safest procedure is tune up with high speed dits, as discussed below. Using the low voltage position on 12m and 10m usually reduces the suppressor resistor dissipation by about 40%. [see page-5]

Film-Resistor Information:

As previously mentioned, the ceramic coating on the film-resistors will normally discolor to a lighter-blue and/or brown when they are subjected to RF. Even though this condition looks nasty, the resistive film is usually OK. If you have doubts about a resistor, it should be checked with an ohm-meter after unsoldering one end.

DC dissipation test results for film resistors: Individually, at room temperature, with natural convection cooling, the various MOF and MF resistors that are furnished with the retrofit-kits can handle the following dissipation levels for 60-minutes: The 2w-rated, color-banded resistors can dissipate 6w. The smaller, blue-body Matsushita, 2w-rated resistors can dissipate 7w. The blue-body Matsushita, 3w-rated resistors can dissipate 10w with <5% R change or 12W with <10% R change.

It is difficult to burn out [open] one of these resistors. Burn-out will normally not occur even thorugh the resistor is observed to be glowing dull-red in a darkened room. Of course, this abuse causes the resistor to exceed its ±5% resistance tolerance rating. Before resistor-burnout occurs, the solder connections may melt. Thus, it is important to wrap the leads of the resistors about 0.8-turn around the wires that they will be soldered to.

Pre-Blastoff Precautions and Troubleshooting

If your amplifier ruined an amplifier-tube, arced, popped, or went bang, before you installed the suppressor retrofit-kit, there are some precautions that should be taken before the amplifier is operated.

It is not uncommon for a parasitic oscillation to simultaneously damage the amplifier-tube and other amplifier components such as the Zener bias diode and the current metering circuits.

Here's why: An intermittent VHF or UHF parasitic oscillation normally creates a very large, and usually audible, current pulse in the amplifier's grid and anode circuits. This current pulse can damage the amplifier-tube. It can also damage the anode-current and grid-current meters and their shunt resistors. This may change the calibration of the meters. In some cases, a shunt resistor and/or a meter may be burned-out completely during an intermittent parasitic oscillation. This can result in substantive changes in meter accuracy. Before plugging-in a repaired amplifier, the accuracy of the current meters should be checked with a digital, or other accurate, current meter, a resistor and DC source. The currents in a series circuit are always equal. So, to re-calibrate a current metering circuit, connect everything in series. If a meter circuit uses separate meter shunt resistors, the test current must be connected to the shunt R. If the meter indication in the amplifier differs substantially from your accurate reference meter, the meter circuit in the amplifier require recalibration. This can be accomplished by changing the value of the meter's shunt resistor or by changing the value of the series resistor between the meter and the shunt resistor. If no series resistor exists, and the meter in the amplifier is reading high, you can add some series resistance between the shunt R and the meter to decrease the meter reading as needed.

A quick way of determining whether damage may have occurred to the current metering circuits is to measure the value of the grid-current meter, and anode-current meter, shunt resistors. If the resistance has increased, the damaged resistor should be replaced. After the new shunt resistor has been installed, it is probably a good idea to check the meter calibration to make sure that the meter accuracy was not altered by the current-glitch during the parasitic. 3A rectifier diodes can be used to protect meter movements.

Zener Diode Testing: When a Zener goes kaput it usually shorts. Using a typical DMM, a healthy Zener will not measure less than 50ohm in either direction. A shorted Zener will measure l<1ohm in both directions.

Bandswitch Problems: If your bandswitch suffered from VHF parasitic related arcing before you installed the improved parasitic-suppressors, there are two areas of concern:

1. Bandswitch contacts can be burned so badly by the VHF voltage and current that they no longer make contact. In some cases the contacts may be missing because they were vapourized by the parasitic-arcing. Fix: Drill out the rivets and replace the burned contacts, or replace the defective wafer.

2. A conductive path may have been formed on the surface of the bandswitch's ceramic insulating material. This conductive path can arc-over during the application of normal HF voltage. Thus, when you fire-up the amplifier, the bandswitch may arc along the same path -- even though the VHF parasitic oscillation voltage is no longer present.

The conductive material on the ceramic insulation comes from the metal contacts that were arcing during the parasitic. During arcing, part of the metal in the switch contacts is vapourized into the arc plasma. Some of the hot metal vapour may stick to the surface of the ceramic insulation. After cooling, the conductive particles are about as difficult to remove as is the glaze from a ceramic pot. If you know where the bandswitch is arcing across the ceramic insulation, it may be possible to remove the metal particles with 240-grit wet-or-dry silicon-carbide abrasive paper, used under water in a bowl of distilled water. Rinse the carbide particles away with distilled water after abrading the ceramic. Dry throughly in a warm oven before reinstalation.

Tune Capacitor Problems: VHF parasitic-voltage related arcing can pit and blister the aluminum plates in an air-variable capacitor. The resulting surface roughness reduces the operating voltage capability of the capacitor which can cause the capacitor to arc-over during the application of normal HF-voltage. The Fix: insert a thin flat-file between the plates. File them smooth and round-off any sharp edges. Realign bent plates with long-nose pliers. The air-gap between the adjacent plates must be equal at any setting. A parasitic-damaged air-variable capacitor can be quite ugly -- and yet perform perfectly if you overhaul it.

NOTE: Bandswitch and Tune capacitor arcing can also be caused by by a too-slow relay. A suitably quick switching circuit appears on page 33 in the Jan. 1994 QST. There is an error in the drawing. The rightmost junction of Dgp and the 1ohm resistor connects to the negative HV. Short out the cap. and all will be well. We carry the parts for this circuit -- except for the high speed vacuum-relay -- for which RF Parts, Inc. is a source.

Here's how to tell whether arcing is VHF parasitic or HF fundamental in nature. If you have installed the glitch protection components in the retrofit kit [the resistor in series with the +HV and the 200a-peak glitch diodes in the metering circuitry], switch the meter to measure Ig (grid-current). If VHF energy is causing the arcing, Ig will increase during the arc. If the arcing is caused by HF energy, Ig typically decreases during the arc. If you discover that the arcing is VHF in nature, additional measures to reduce the VHF gain are obviously needed. An additional parasitic suppressor in the anode circuit may help. For a multi-tube amplifier, separating or changing the position of the parasitic suppressors may help. * *In some cases, it may be necessary to lower the Q of the VHF resonance of the amplifier output enclosure itself. This can be accomplished by suspending a 50mm to 100mm diameter closed loop of nichrome wire inside the output enclosure. One point of the loop can be silver soldered to a ground lug for mechanical support. Do not allow the loop to couple HF energy from a tank inductor or to come close to any HV point. {NOTE: Such devices are used in large, commercial HF amplifiers. }

+ Tuning-Up Your Amplifier

Linear amplifiers are like AC induction motors -- they are designed to run best when fully loaded.

If your amplifier's instruction book says to reduce drive power during tune-up (and many of them do), it is not giving you correct information. In order to be linear, linear amplifiers must be tuned-up with the same peak drive-power level that they will be driven with during actual operation. Reducing drive power during tune-up results in nonlinearity and excessive grid-current when the amplifier is eventually driven with the normal drive power. If you want to operate with reduced power during good band conditions, first tune up your amplifier with maximum drive power, then turn down the microphone gain to reduce power to the level desired.

Tune-up method #1: Set the amplifier's HV supply to the CW-Tune/low-V-tap. If you are not sure where to preset the Load control, set it to 70% of maximum loading [30% of C] just to be safe. Apply 100% of the CW drive power -- or the power level that you intend to drive the amplifier with during actual use. Alternately adjust the amplifier's Tune and Load controls for maximum relative power output. The whole process should take less than 6 seconds. It may sound brutal, but this tune-up method results in good amplifier linearity and it won't damage the tubes -- if the maximum anode/plate-current rating is not exceeded. If the anode-current is excessive, the resistance of the cathode, RF negative feedback resistor needs to be increased slightly or the internal, or external, power adjust control in the transceiver needs to be turned down.

Tune-up method #2: If you would like to reduce the overall stress on your amplifier during tune-up, you can use a reduced duty-cycle driving signal. This can be accomplished by keying the transceiver, on CW mode, with an electronic-keyer, set to send 50wpm dits. This produces a 1/2-on, 1/2-off, or 50% duty-cycle. Using this method, the amplifier may be tuned-up, again for maximum power output, in its higher-voltage, SSB-mode. The CW carrier control should be increased until the transceiver's meter indicates a small amount of internal ALC. This guarantees that the essential, maximum peak drive power is applied during tune-up. The duty-cycle can be reduced to 33% -- or less -- by using a Tuning Pulser.

And, For Curious Skeptics (like me)

Some hams--and some electronic engineers--do not believe that intermittent VHF parasitic oscillations exist - especially in amplifiers they designed. This is understandable because intermittent VHF-parasitic oscillations usually last only a matter of microseconds. For that reason, they are virtually impossible to observe in the act. A major factor with parasitics is the VHF-gain of the particular amplifier-tubes that happen to be installed in the amplifier. Even with new amplifier-tubes, there is a considerable variation in VHF gain. Tubes with below average gain will probably never have a parasitic oscillation -- no matter how poorly the parasitic-suppressors perform. Thus, when low VHF gain tubes happen to be installed in an amplifier, it is easy to assume that the amplifier design is perfectly stable.

Since catching a parasite in the act is virtually impossible with ham-type test gear, a different analytical approach must be used. It is a safe assumption that the resonant circuit which supports the parasitic can be found and evaluated with a dipmeter. If you have access to a dipmeter and you would like to observe the before and after effect of installing different parasitic-suppressors, before any modifications are made, unplug the amplifier from the electric-mains and check the anode resonance. This can be measured on the either side of the HV blocking capacitor. The resonant frequency is usually between 75MHz and 150MHz. You should be able to tune the resonance a few MHz by adjusting the tune capacitor. The VHF dip in many factory-stock amplifiers' anode-circuits is so sharp that it will "suck-out" the oscillator in the dipmeter. For this reason, the dipmeter must be backed away {decoupled} from the anode-circuit conductor to accurately observe the dip. A sharp dip indicates that the anode-circuit has a high VHF-Q. This is not good news unless you happen to need a self-excited VHF-oscillator.

After installing low VHF-Q suppressors, check the dip again. The dip frequency will usually not change appreciably but the dip should now be smoother and it should be necessary to couple the dipmeter coil closer to the anode-circuit to achieve the same degree of dip -- indicating that you were successful at reducing the VHF-Q and the VHF-oscillating ability of your amplifier.

If you make an experimental change to the parasitic suppressors in your amplifier, and you want to evaluate the change, use a plastic ruler to measure the distance from the anode circuit to the tip of the dipmeter coil that results in a 20% dip on the dipmeter. If the coupling distance required for the 20% dip decreases after a change, a lower VHF-Q is indicated and the change was obviously an improvement. If the dip distance increases, the VHF-Q increased and the change in the suppressors was patently a step backward.

+ The only tangible immediate result of your efforts is that amplifiers usually tune more smoothly and predictably after installing lower VHF-Q parasitic-suppressors. The long term benefit is no surprises.

+ If you have a problem with your amplifier, please do not hesitate to telephone me, up to 8pm California time, at 805-386-3734. I have instruction manuals and/or schematic diagrams for most of the amplifiers that have been manufactured during the past 25 years.

 

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