Copper Talk » Open Forum » Archived Messages » 2004 » 01/01/2004 to 01/31/2004 » Heathkit SB220 Amp « Previous Next »

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Stitches
Posted on Tuesday, December 30, 2003 - 9:17 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

What would be a fair price for one of these w/manual, and in good shape if a guy/gal wanted to buy one?
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Crafter
Posted on Tuesday, December 30, 2003 - 7:38 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

$350.00 to $450.00 Gave 375.00 for mine with a guarantee on the transformer on e-bay imagine that! Bought from another ham though. Beauty is in the eye of the beholder. Weight is 50 lbs so figure shipping WHEW!
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Stitches
Posted on Wednesday, December 31, 2003 - 5:30 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

I can only imaagine what it must have cost to ship a box of iron like that.
Have you hooked it up to anything yet? And if so, was it a good investment?
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Crafter
Posted on Wednesday, December 31, 2003 - 9:49 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Cost me 50.00 bucks using UPS ground. Yet works good requires 100w drive but gives legal limit plus. I use a yeasu 101 or my jb76 to drive it.
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Wayne
Posted on Monday, January 05, 2004 - 1:26 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

I have a SB220 got it new in 1977 from Heath-kit its still just like new, and I do mean just like new. You see its still in box never been unpacked,just can't find the right tech. to put it together. Oh yea for the young ones might not understand when I say put it together,it is a kit.
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Crafter
Posted on Monday, January 05, 2004 - 9:19 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Well get to it they come with directions if not I got some. You can do it yourself.
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Stitches
Posted on Monday, January 05, 2004 - 2:45 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Hey Wayne.....that's one sweet amp from what I hear. And still in the box? NICE!
I've got an XYL and a pick-up truck I'll trade ya for it, you want me to send you a picture of the truck? hahaha.
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Adshar64
Posted on Saturday, January 10, 2004 - 6:14 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

The SB-220 has some excellent design features and a few design weaknesses that are easily corrected.
HV Power Supply
Before the arrival of the SB-220, there was a popular notion that a legal-limit SSB amplifier needed a heavy-duty power-supply that required two grown men to move it about. Heath engineers knew that this idea was based more on amateur radio folklore than on sound electrical engineering principles. They also knew that the average duty-cycle of a human voice was only about 15% when no carrier was present, as is the case for SSB operation. So, why build a 100% duty-cycle, AM, "lock-to-talk" power supply when one was not required? Thus, they designed a power-supply that would competently do the job that was needed. This resulted in a considerable size, weight and cost savings, which they happily passed along to the buyers of their product. At first, some people in the ham community had negative comments about the SB-220's "wimpy" power supply. With the passage of on-the-air time, it became apparent that the power supply would do the job, and do so with a low failure-rate and with no detectable on-the-air ripple. This was no accident. Heath engineers had wisely specified a HV-transformer design that had an exceptionally low secondary-resistance of only 12.2-ohms. This minimized the voltage drop under full load in the capacitor-filter fullwave-voltage-doubler rectifier circuit. Such circuits have an extremely high peak-current to average output-current ratio. So, minimizing the transformer winding resistance is essential for good voltage regulation and to minimize the I2R heat loss in the transformer's windings.

Many hams initially labeled as "inferior" the capacitor-filter fullwave-voltage-doubler rectifier circuit. They did not realize that this circuit has some advantages over the traditional, fullwave-bridge rectifier circuit. These advantages are:

1. Low ripple-voltage. This is due to the fact that, as one capacitor bank is charging, the other capacitor bank is simultaneously discharging, thus, cancelling each other's 180º out-of-phase, sawtooth waveforms.

2. It allows the transformer to have only half as many secondary turns which yields a more efficient transformer design. Here's why: Since a layer of insulating-paper is required between each layer of wires, fewer turns means fewer layers of paper. This allows the transformer to use less paper and more copper. The net result is a transformer that has a high ratio of copper to paper. This makes for a very capable transformer.

3. Excellent voltage-regulation during current-transients, due to the fact that no swinging-inductance filter is used. This is exactly what's needed for CW and SSB modes of operation.

Cooling
Since about half of the power that is consumed by a linear amplifier is converted into heat, another important amplifier design consideration is cooling. Most of the heat that a 3-500Z, or any other internal-anode tube, dissipates is carried away by radiation from its anode.

Here's how it works: during normal operation, the anode becomes so hot that it glows a bright orange color. The surrounding objects are relatively much cooler, so the anode looses heat to the surrounding objects by radiation-cooling. Unfortunately, the anode also heats up other parts of the 3-500Z that are sensitive to heat. This includes the critical glass-to-metal seals and the solder that is used to fasten the pins to the base of the tube. These heat-sensitive parts must be cooled with forced airflow.

Heath's engineers came up with a deceptively simple method of effectively cooling the 3-500Zs. They realized that the conventional, uncheap, air-system-socket / glass chimney cooling method had some serious trade-offs, such as: 1. It was difficult to force enough air through the restrictions to airflow in the "air-system" to adequately cool the filament-pins and filament-seals of the tubes. 2. The horizontal fins on the standard anode [plate]-cap coolers were obviously not designed to be cooled by the vertical airflow from the air-system chimney. 3. The restrictions to airflow with the air-system cooling method dictated that a high-pressure centrifugal blower be used. All high-pressure blowers have one thing in common: High Acoustic Noise.

What was needed was a cooling system that would quietly move high-velocity air past the hot filament-pins [1], filament-seals, the anode-seals, and the glass envelopes.

The Heath engineers knew that when horizontal air flows across vertical cylinders, such as a 3-500Z envelope and its pins, the airflow will follow the curve of the cylinders. This provides fairly uniform cooling to all areas of the cylinders, eliminating hot spots. They concluded that, with horizontal airflow, the cooling air would have a direct path the heat-sensitive parts of the tube. Horizontal airflow would also allow the anode-cap cooler's fins to lineup properly with the flow of cooling air, allowing it to function efficiently.

Since the filament-pins are below the chassis, and the filament-seals and anode-seals are above the chassis, the Heath engineers used an open-ended chassis so that a single, 15cm [6"] diameter fan blade could simultaneously blow cooling air above and below the chassis.

To position the four, hot filament-pins optimally in the under-chassis airflow, the pair of tube-sockets were mounted with the two pairs of filament-pins (#1 and #5) facing each other. This positioned the hot parts in front of the tips of the fan blades where they protruded under the chassis.

The cooling system design was brilliantly simple. It was relatively quiet and it worked well. Reports of 3-500Z-pin solder melting in SB-220 amplifiers are very rare - with the exception of cases where the fan motor bearings eventually seized because they were never oiled!

On the other hand, I have heard of many 3-500Z-pin solder melting episodes in other amplifiers that used air-system chimney/centrifugal blower cooling.

One weakness in the SB-220's cooling system was the failure to reduce the heat-reflectance of the bright aluminum surfaces that were adjacent and parallel to the anodes. This would have reduced the radiant heat that was reflected back at the hot anodes. The best color for this purpose is black. [2]

This deficiency is easily corrected: After the tubes have been removed, black, liquid shoe-polish can be applied to the vertical, aluminum surfaces, that are near the anodes. The built-in felt applicator, that is attached to the bottle cap, works well for applying the black coating.

One weakness in early models was the failure to provide oil-holes for the fan motor bearings. This problem can be corrected by drilling a small hole, no more than 1/4" [3mm] deep, above the front and above the rear oilite bearings. [3] The fan should be lubricated yearly with a thin, non-gumming [4] oil such as Hoppe's 1003. [5] / Two or three drops of oil needs to be inserted in each hole. What isn't absorbed into the felt wicks, that surround the oilite bearings, runs out the bottom. More oil is not better, just messier. The easiest way to get the desired amount of oil in the holes is to apply the oil with a disposable insulin syringe. Each "unit" on the syringe equals about one drop of oil.

Premature Filter-Capacitor Failure
Aluminum electrolytic filter-capacitors are very sensitive to heat. For every 10ºC increase above room temperature, the life expectancy of the capacitor is approximately cut in half.

The electrolytic filter-capacitors in the SB-220 are subjected to a high degree of heat during normal operation. The major source of this heat is from the eight, 30k-ohms, voltage equalizer resistors that are adjacent to the eight filter-capacitors. During transmit, a minor source of capacitor heating, results from the 60Hz ripple-current flowing through the capacitor's internal resistance.

The capacitor heating problem is compounded because cooling air does not reach the capacitors due to the molded plastic capacitor holders that surround and insulate each capacitor. In some cases, the heat present will partially melt the ends of the capacitor holders that are nearest to the 30k-ohms resistors.

The heat that is dissipated can be reduced by c.70% if 100k-ohms, 3W, metal oxide film resistors are used to replace the original 30k-ohms resistors. This simple modification will prolong the life of the (8) electrolytic filter-capacitors.

Other resistance values may be used, up to roughly 150k-ohms, provided that the resistors can withstand the voltage that is applied to them and the resistances are within ±5% of each other. I do not recommend using ancient-type 2W carbon-composition resistors for this application. They do not maintain their rated resistance tolerance with age.

Note: Increasing the equalizer resistance will also increase the capacitor bleed-down time after the amplifier is shut off.

Since this amplifier has a shorting HV-interlock, that grounds the HV positive when the perforated cover is removed, it is advisable to wait until the voltmeter indicates nearly 0V before allowing the interlock to short the HV to the chassis.

Here's why: When the HV-positive is shorted to ground, the stored energy in the filter-capacitors is applied directly to the grid-current meter shunt resistor, R3 {0.82-ohms}, which is the only HV-negative path to chassis. The peak discharge current can be substantial.

For example, if the filter-capacitors are at the 100V level when the interlock shorts, the peak-current through R3 is: I c. E/R c. 100V/0.82-ohms c.100A. If a substantial voltage exists in the filter-capacitors when the interlock shorts, R3 can be literally blown away by the discharge current-pulse. If the multimeter happens to be in the grid-current position, the meter can also be crispy-crittered.[6]

It is for this reason that I removed the interlocks from both of my Heathkit amplifiers. Another consideration was that the interlock does not prevent the operator from contacting the potentially fatal voltage from the electric-mains when the amplifier is plugged in and switched off. The interlock will not prevent the operator from being electrocuted. In other words, the safety-interlock does not make the amplifier safe.

Another advantage of removing the interlock is that it allows the perforated cover to be removed while optimizing the tuned-input circuits. This allows better access to the tuned input circuits.

>>>> There is no safe substitute for pulling the electric-mains plug before putting fingers inside any amplifier.

Intermittent Meter Readings
There are at least two problems that can cause intermittent meter readings in the SB-220. If this occurs only on the voltmeter, the most likely problem is with the (3) 4.7M-ohms, 1W voltmeter multiplier resistors [R6, R7, and R8]. These resistors, which are rated at 350v maximum per-unit, are subjected to c.1000V-per-unit in the Heathkit circuit. This can lead to resistor deterioration, which causes fluctuations and/or inaccuracies in the 0-3500V meter indication. [7] The abused resistors can be replaced with modern, 2W flameproof spiral-film resistors that were designed to handle this voltage.

The other source of trouble lies inside the meters. Here's why: Different metals are used for the various parts of the meter. These parts, which conduct current to the meter armature, are fastened together with screws. Over a prolonged period of time, moisture in the air causes electrolysis to take place at the junctions of the dissimilar metals. This increases the resistance at the junctions, which causes intermittent meter indications.

This problem can be corrected by prying off the meter face, carefully removing the meter scale, and applying small dabs of conductive-paint to all of the dissimilar metal junctions that carry current to the armature.

The conductive-paint can be thinned with acetone to facilitate penetration into the narrow areas between the parts. The conductive paint should be allowed to dry around 15-minutes before the plastic meter faces are replaced.

Transceiver Relay Contact Failure
During receive, the voltage across the Antenna Relay Jack rises to about +115V. A bypass-capacitor, C52, is connected in parallel with this jack, so the capacitor charges up to 115V during receive. During transmit, the transceiver's relay (if one is used) places a short-circuit across this jack and the fully-charged C52. The SB-220 relay coil current is only about 0.025A, but the peak discharge current, produced by placing a direct short on the charged capacitor, can be surprisingly large. This action is like an electric spotwelder. In time, the contacts in the transceiver relay can become pitted and fail to make contact, or become spotwelded together, causing the amplifier to key-up continuously.

This problem can be corrected by placing a 100-ohm to 200-ohm, 1/2W resistor [to limit I] in series with the center pin [blue wire] on the Antenna Relay Jack. C52 must be connected to the blue-wire end of the resistor. See Diagram 1.

The voltage lost in this resistor will c. 5V, which is insignificant to the 110VDC relay coil.

Filament Circuit
The most popular, published modification for the SB-220 has been filament inrush-current limiting. It is true that a large number of 3-500Zs in SB-220s suffered from filament-to-grid shorts, so some people began to theorize that excessive filament inrush current was the villain.

Another theory was that the filament-to-grid shorts were caused by a "manufacturing defect". Neither theory turned out to be true, even though Eimac®, as usual, generously made good on the tube warranties.

Curiously, none of the authors, who wrote the SB-220 inrush-current limiting articles, ever bothered to measure the actual filament inrush-current. So, I decided to measure it with my trusty old HP-1706A oscilloscope. After all, my name is Measures, so why not?

Here's what I found: The maximum, peak inrush-current through the 3-500Z filaments in an SB-220 is only 60% of what Eimac® allows. This esoteric feat was accomplished by the use of a special current-limiting core in the Heathkit filament-transformer. The core is similar to the cores used in current-limiting neon-sign transformers. Externally, this core appears to be substantially different than the core used in the HV-transformer.

About a year after I measured the inrush-current, the fact that the SB-220 used a current-limiting filament-transformer, was mentioned in QST Magazine.

The actual cause of virtually all grid-to-filament shorts in the 3-500Zs was later discovered to be a very brief, and usually very noisy, VHF parasitic-oscillation at roughly 110MHz. As will be discussed later, the large grid-current-pulse, that accompanies the parasite, was creating a large magnetic-pulse inside the 3-500Zs, which pulled the hot filament-wires off-center, causing them to touch the grid wire cage.

Another interesting feature concerning the SB-220 filament circuit is the fact that they normally use a filament-voltage at the low end of the recommended voltage range; typically about 4.85V. This may not seem important, but, according to Eimac®, each 3% reduction in filament-voltage, provided that no drop in PEP output occurs, will double the life expectancy of the amplifier-tube. Thus, the amplifier-tubes in a SB-220 can be expected to last at least 4-times longer than the tubes in some other, much more costly and prestigious brands of two 3-500Z amplifiers.

Just because something costs more does not automatically mean that it is better.8

For example, the contemporary, c.4db more expensive, Henry Radio Co. 2KD-5 was generally considered to be a better designed, higher power output, and more rugged, amplifier than the SB-220, This amplifier typically had a filament-voltage of more than 5.90V with a line voltage of 240V. 5.90 volts clearly exceeds the maximum filament-voltage rating of 5.25V and reduces the useful emission life of the two 3-500Zs to only a few-percent of what could have been realized if the same tubes had been used in an SB-220.

Another example is the Trio-Kenwood TL-922. In this amplifier, about 5.3V is applied to the 3-500Z filaments when the amplifier is operated from 120V or 240V. This reduces the useful-emission life of the tubes by more than 75%. Of course, if your electric-mains voltage is 216V, the filament-voltage will be perfect.

Obviously, the filament-circuit in the SB-220 needs no stepstart circuit to limit inrush-current, but there is still a good reason to add a stepstart circuit to the SB-220.

Here's why: If the amplifier is started up when powered from a stiff, 240V source, the inrush-current, through the Power Switch and other components, is considerable. A stepstart circuit will eliminate this potential source of trouble.

An easy-to-build, stepstart circuit is shown in Diagram 2. In this circuit, the stepstart relay can close only when the filter-capacitors in the +110V and +3000V power supplies have charged up to about 2/3 of their normal voltages. If the stepstart relay closes before the HV reaches 2/3 of its normal potential, the 2k-ohms nominal-value resistance should be increased. If the relay has trouble closing, the resistance should be decreased, which will increase the current through the relay's coil. If the circuit is functioning properly, the stepstart relay will close in about one-second, as the voltmeter passes the 2000V level. The amplifier may be operated at full-throttle 1-second after the relay closes.

The two, 20-ohms, 10W, resistors and the stepstart relay can be glued directly to the bottom of the chassis, directly under the filter-capacitor bank, using silicone-rubber adhesive. [9] The resistors should be held up off of the chassis a few mm, by the silicone-rubber. This mounting method is appropriate because drilling mounting holes in this area could harm the filter-capacitors.

Since the stepstart relay adds to the current burden on the +110V power supply, it is probably a good idea to replace the stock, halfwave rectifier [D16] with a full-wave-bridge rectifier unit. Note: The grounded red-wire, on the transformer's 80VRMS winding, must be ungrounded and connected to the input of the full-wave-bridge rectifier.

Adding A Standby-Switch
Another popular modification for the SB-220 is the addition of a standby-switch. A standby switch is really not necessary in this amplifier because it uses instant-on type tubes and a current-limiting filament-transformer.

This transformer is gentle to the filaments, so the amplifier may be switched on or off as often as you like with no problem. The exception is: if you have just made a long RTTY or FM transmission, the glass-to-metal seals in the 3-500Zs should be allowed to cool down for about 1 minute before switching off .

Silicon Rectifier "Protection"
In the early 1960s, silicon-rectifier manufacturing technology was hit and miss. There was considerable variation between individual rectifiers of the same type. The variation between rectifiers led designers to use resistor-capacitor equalizer circuits in parallel with series rectifiers.

Today, silicon-rectifier manufacturing technology has improved, so that same-type rectifiers are very uniform in their parameters. Modern silicon rectifiers do not need to be equalized. Unfortunately, old habits die a slow death and many hams are still clinging onto 1960s design techniques.

Much has been written about adding "equalizing" resistor-capacitor protection networks across the rectifiers in the SB-220's HV power supply. Unfortunately, these "protection" circuits not only do not perform as advertised, they can lead to premature rectifier failure.

Here's why: The 1/2W resistors that are typically used are rated at 250V maximum. How can a 250V resistor be trusted across a 600V or a 1000V rectifier? If anything breaks down in a series-rectifier circuit, it is like dominoes falling. One resistor failure can wipe out the remaining good parts in the circuit.

The most frequent failure mode for HV power supply rectifiers is too much reverse-current. This problem can be eliminated if the total PIV capability of the series connected rectifiers substantially exceeds the peak voltage in the circuit.

In any series circuit, the current in all of the elements is exactly equal. The rectifiers are all in series. So, the reverse-current burden is exactly the same for each rectifier unit. How is it that something which is already exactly equal needs to be "equalized"?

During the half-cycle application of reverse-voltage, it is important that all of the rectifiers in a series leg have similar junction capacitances. If they don't, then the reverse-voltage across the lower capacitance rectifiers will be excessive. Here's why: in a series circuit, smaller capacitors charge-up faster, and to a higher voltage, than larger capacitors.

Approximately 10nF [0.01µF or 10,000pF] of bypass capacitance across each rectifier is probably a good idea if, for example, 1A rectifiers are placed in series with 6A rectifiers. This is the case because of the wide difference in junction capacitance between 6A and 1A rectifiers.

If all of the rectifiers in a series leg are similar, they will all have similar junction capacitances, so no external capacitors, or resistors, are needed.

Long ago, before they knew better, some commercial high-voltage silicon rectifier-stack manufacturers used "equalizer" R/C networks. These manufacturers stopped using "equalizers" for the same reasons that were previously outlined above. I don't know of any commercial HV-rectifier manufacturer who has not abandoned the malpractice.

Silicon Rectifier Failure
When a silicon rectifier fails from excessive reverse-current, the rectifier will short-circuit. This failure mode is very rare in SB-220's because the per-leg total rectifier PIV-rating used {>4200v} is more than 1000V higher than the actual PIV {c.3100v} in the circuit. This represents a conservative design because, during a voltage surge, the eight electrolytic filter capacitors, which are rated at 3600V-max, would likely fail before a >4200PIV rectifier string.

A much more common type of rectifier failure in early production SB-220s is rectifier-opening. This is caused by a defective spotweld inside the silicon rectifier. Eventually the weld breaks and the rectifier opens. The forward voltage jumps the gap at the open weld. When this happens, a 60Hz arc can usually be heard from inside the amplifier when current is being drawn from the HV supply. It is important to switch off the amplifier immediately when this noise is heard. Here's why: In a fullwave voltage-doubler rectifier circuit, there are two, series, filter capacitors.[10] One capacitor charges on the positive half of the cycle. The other capacitor charges on the negative half of the cycle. The two capacitors discharge in series. When current is being drawn from the supply, and if one of the filter-capacitors is not being charged by its rectifiers, the capacitor that is being charged will force reverse-polarity current through the capacitor that is not being charged. If unchecked, reverse-current will cause electrolytic capacitors to discharge their corrosive electrolyte through their safety vents. In other words, reverse-polarity current will destroy polarized electrolytic capacitors in short order.

RL1, The Antenna and Bias Relay
See Diagram 1.

1. The added, reverse diode across the coil absorbs the reverse-voltage spike when current stops flowing in the coil. This prolongs the life of the coil insulation and quenches the magnetic pulse from the coil. If the magnetic pulse is unchecked, it can retrigger the transceiver's VOX circuit if a nearby, high-Z dynamic microphone is being used.

2. In its stock wiring configuration, +110V is connected to terminal #2 of the relay. During receive, the relay connects this voltage to the center-tap of the filament-transformer [terminal #8], which is the DC cathode-current path to the filaments of the 3-500Zs. The positive cathode-voltage causes the tubes to cutoff during receive. The tubes cut-off because the grids are relatively 110V more negative than the cathode's electrons. Electrons are negatively charged, and since like charges repel, no electron-current flows in the tube.

A problem arises if one of the tubes happens to develop a filament-to-grid short: Since each grid is DC-grounded, a shorted tube also short-circuits the +110V power supply. This power supply is part of the unfused filament-transformer.

Thus, if a filament-to-grid short occurs, and the amplifier is not switched off promptly, the filament-transformer will literally melt-down and short-out. The black tar that comes out of the overheating transformer makes an unpleasant mess inside the amplifier.

This potential source of grief can be eliminated if the relay is rewired as shown in Diagram 1. The new circuit uses resistor-cutoff bias, utilizing the existing 100k-ohms resistor [R27], which is rewired to relay terminal #5. The current through this resistor, during receive, is normally less than 0.25mA [P>>>> If you have any questions or comments about this article, please feel free to telephone me at 805 386 3734.

Rich, AG6K


End Notes

1 The filament-pins receive a considerable amount of heat through conduction from the c.75w filament. The amount of heat present requires that continuous forced-air cooling be directed at the filament-pins, even on standby.
2 The designers of the Trio-Kenwood TL-922 amplifier, whose circuit looks amazingly similar to the SB-220 circuit, exhibited some original thinking when they decided to incorporate black, low heat-reflective surfaces, adjacent to the 3-500Z anodes.
3 The fan does not need to be removed to be drilled.
4 "WD-40", "LPS", and similar products, are NOT non-gumming.
5 This oil can be purchased in stores that sell fishing reels and/or firearms. Ordinary, 10w or 20w SG-grade motor oil can also be used.
6 Meter damage can be avoided by connecting two, ordinary 1A, any PIV, silicon rectifiers across the terminals on each meter. The two diode arrows should point in opposite directions.
7 When the SB-220 is powered from 120.0V or 240.0V, the no-load HV is close to 3080V.
8 For an extensive treatment of this subject see: "The Emperor's New Clothes" by Hans Christian Andersen.
9 The areas to be bonded should be degreased. After the stepstart parts are in place, the amplifier should not be disturbed for at least 24-hours to allow the silicone-rubber adhesive to cure.
10 In the SB-220, each of these two capacitors is made from four, 200µF, 450V capacitors in series. Thus, the four capacitors act as a single 50µF, 1800V capacitor.
11 To be more precise, after the big-bang, if the anode [plate]-current meter indicates a current flow, when the amplifier is not in the transmit mode, a 3-500Z filament has been pushed against its {grounded} grid. This short-circuit grounds everything that is connected to the filament circuit, including the unfused, +110V relay supply. For more info see "RL1, The Antenna and Bias Relay", #2.
12 If you would like to purchase a Suppressor Retrofit-Kit for the SB-220, they are available from me
13 In later models, the grid-to-chassis capacitors were changed to 115pF.
14 For more information, see "Amplifier Driver Compatibility" which begins on page 17 in the April 1989 issue of QST.
15 In actual use, the metal in the contacts will run hotter because of the increased current burden. This will increase R. Thus, the contact dissipation will probably increase more than 39%.
16 At the instant of maximum peak-current(s), the peak grid-current, per tube, is c.0.5a and the peak anode-current, per-tube is c.1.2a. Thus, the peak cathode-current is 1.7a per-tube. This represents a meter- indicated anode-current of about 800mA for two 3-500Zs.
17 The average input-resistance for a pair of 3-500Zs is about double this value, or c.65-ohms.
18 The capacitors used should be 500V silver-mica type or 1000V ceramic, NPØ type.
19 These suppressors are supplied with the suppressor retrofit-kits
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Bullet
Posted on Saturday, January 10, 2004 - 3:38 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

i cant believe you coppied that much of that article your fingers must be numb. lol

id just posted the url.(im much to slow a typist)

great info though!

i gave $550 for mine including shipping and a couple harbach mods installed. it was in very nice condition but i replaced some things due to thier age and installed new grafite tubes and new 3amp rec diodes and new caps and new zenier diode.