Frequently Asked Questions

1. What type of power supply do you recommend?

Astron RS-70 will work fine for the Skywalker 500, Skywalker 400-6m and Skywalker 160-6m. Two RS-70's in parallel are required for the Skymaster KW.

2. Can I use a switching supply?

Yes, just be sure it is designed and marketed for communications use. Most switching PS's found at ham fests were designed to power computers, not communications equipment. The RFI produced by these PS's will show up as increased background noise in your receiver, sometimes several S units of noise. Another problem with switching PS's is "overshoot", which is a transient voltage spike that occurs when the load is suddenly removed. CW and SSB operation require the PS to go from full load to zero load instantly and may cause a poorly designed switching PS to overshoot, which may exceed the breakdown voltage rating of the amplifier's transistors and lead to failure. Switching PS's designed for communications use will contain circuitry to eliminate both RFI and overshoot.

3. Will the amplifier work off a 12V battery?

Yes, as long as the voltage during keydown remains above approximately 10V. Below 10V, the T/R relay will not remain engaged and will "chatter". You should understand that the further the voltage drops, the lower the power output of the amplifier will be. The output power drops because the input power drops. Power (P)= voltage(V)squared divided by resistance (R). If the PS voltage drops from 14 to 11 V and R remains unchanged, we can calculate the drop in input power as P=1-(11)²/(14)²=0.38. This shows that you can expect a 38% decrease in input power with a 3V drop. Since output power is directly proportional to input power (at 50% efficiency, P_out=P_in/2) the output power will drop by 38% as well.

4. What does the filter do?

All broadband amplifiers generate harmonics. Harmonics are spurious emissions that occur at 2,3,4,5,6...times the fundamental (your transmit) frequency. Many solid-state amplifiers (like ours) use "push-pull" design which allows the even order harmonics to mostly cancel each other out in the output transformers. With these amplifiers, most of the spurious energy is contained in the odd order harmonics with the 3rd order harmonic being the strongest. As the fundamental frequency is increased toward the upper end of the amplifier's operating range, the amplifier's gain decreases and the 3rd harmonic occurs further and further above the max. design frequency, resulting in very low emissions. Conversely, as the fundamental frequency is lowered toward the lower end of the amplifier's operating range, the amplifier's gain increases and the 3rd, 5th, 7th, etc. harmonic may fall within the amplifier's operating range, resulting in very high emissions. For example, if our 500W HF amplifier is operated at 29 MHz, the 3rd harmonic is at 87 MHz. Since the maximum operating frequency limit of this amplifier is 30 MHz, it will have almost zero gain at 87 MHz and therefore 3rd harmonic emission will be very low, even without any filtering. Conversely, if this amplifier were operated at 1.8 MHz, you would have harmonics all the way out to the 15th falling within the operating range of the amplifier, resulting in a high level of harmonic emission and requiring good filtering to meet FCC requirements. The filter attenuates these harmonics down to a level which exceeds the FCC requirements.

5. I have a lousy sounding CB amplifier that I want to use on the amateur bands. Will your filter kit help clean up the output so it sounds good?

No. This filter will not do anything to clean up a lousy sounding amplifier that is splattering. High intermodulation distortion (IMD) is the cause of splattering. What causes high IMD? In a good quality amateur amplifier, high IMD is usually caused by the operator over driving the amplifier into the saturation region which causes a non-linear condition known as "flat topping". Another operator error that causes high IMD is having the audio speech processor turned up too high, essentially flat topping the audio amplifier circuit. In a poor quality CB amplifier (that some hams attempt to use on the amateur bands), the bias circuitry may not be sufficient to keep the amplifier operating in class AB. As the drive power is increased, the "static" bias circuit (usually consisting of a couple of resistors) allows the amplifier to drop into class C which creates a tremendous amount of "cross over" distortion. Also, the material used in the input/output transformers of CB amplifiers is selected to provide maximum performance on the CB band. This material does not provide sufficient reactance to maintain amplifier stability when operated on frequencies lower than about 14 MHz, which creates even more distortion. Fortunately for the amateur community, these cheap CB amplifiers will usually destroy themselves after a few hours of use, ending the splattering problem. In the meantime, I get emails from amateurs quite often who have purchased a cheap CB amplifier, only to find out that nobody will talk to them with it because they sound terrible and would like to purchase my filter kit to clean up their signal. Sounds like a reasonable plan, right? Wrong! Why? Low pass filters are designed to remove harmonics which are at frequencies of 2, 3, 4, 5 times the fundamental transmit frequency. IMD occurs within a few kHz of the fundamental frequency and is therefore well below the cutoff frequency of any harmonic filter. In fact, there is no such thing as a filter that will remove IMD products and turn a distorting, lousy sounding CB amp into a clean sounding amateur amplifier. So please do all amateurs a favor and throw the CB amp in the trash or mail it to the FCC along with the information on where you bought it, so they can keep other amateurs from spending their money on this illegal junk.

6. If I install your filter kit I can put my antenna right next to my neighbor's TV antenna without any problems right?

Wrong! The low pass filters only attenuate harmonics. If harmonics are not the cause of your problem (they almost never are) removing them won't help. What causes 99% of all RFI problems? YOUR FUNDAMENTAL SIGNAL!!! A high power fundamental signal in CLOSE PROXIMITY to ANY type of receiving equipment (TV, radio, telephone, PA system etc.) will blow right past any tuner or filtering on this equipment and enter the amplifier stage along with the intended RX signal where it will cause RFI. This type of RFI is called "fundamental overload". All the harmonic filters in the world won't help in this very common situation. What will help? Increase the standoff distance (vertical, horizontal or both) between your antenna and what you are interfering with. This is one of the many reasons high power FM radio station antennas are located on high towers. A 100 KW radio station with a 20' antenna would cause a lot of RFI even though the harmonic levels meet FCC requirements.

7. Your amplifiers have more fans than the others, aren't they noisy?

More fans ensure that the heatsink temperature remains as low as possible during operation. Lower heatsink temperature allows lower transistor junction temperature which results in longer transistor life. The fans operate at 9V. The lower voltage reduces the fan RPM which reduces noise. In fact two 120mm fans operating at 9V are much quieter and have higher airflow volume than one 120mm fan operating at 14V. When the fans are operating at 9V our amplifiers are quieter than the average PC. Reducing the RPM also reduces wear on the fan bearings resulting in longer life. A switch may be installed to supply full PS voltage to the fans if required for demanding applications such as RTTY. At full PS voltage these fans will be somewhat noisy but will provide better cooling than ANY of the other solid-state amateur amplifiers on the market.

8. What does the switch below the on/off switch do?

It switches in a mica capacitor in the output matching circuit to provide a better impedance match into a 50 ohm load on the lower frequencies. The switch may be left in the up (off) position during operation on the lower frequencies, however the amplifier gain will be reduced slightly due to the less than ideal impedance matching. The switch must never be left in the down (on) position during operation on the higher frequencies.

9. Where did your amplifier designs come from?

The theory for my designs came from the many Motorola Applications notes (AN762, AN749, AN758, AR164, AR347, AR511 etc.) written by the late great Master of HF solid-state design, Helge Granberg/K7ES-sk. The only physical similarities between my designs and Helge's will be in the type of input/output transformers used. The rest of the design will look quite different. However the theory outlined by Helge is still alive and kicking in my amplifiers. So the answer to the above question is that the designs are mine but are based upon the work of Mr. Helge Granberg.

10. How many hours will it take me to build the kits?

A rough estimate would be 16 hours for the Skywalker 500 and 32 hours for the Skymaster KW.

11. What special tools are required to assemble the kits?

A drill press is required to get the holes used to secure the PC boards and transistors to the heatsink drilled in straight. These holes must also be tapped to accept 4-40 machine screws. A digital VOM is required to set the bias current for each module. A wattmeter and dummy load are required to confirm proper operation.

12. Are there any transformers to wind?

Yes, there is an input and output transformer on each module that must be wound with 4-7 turns of teflon wire. The number of turns depends on whether it's an input or output transformer and which amplifier you are building. A typical transformer takes me about 2 minutes to wind. Remember these are RF transformers and have only a few turns, NOT the huge AC transformers found in PS's and vacuum tube amps which have hundreds of turns.

13. What's the hardest part of building one of these kits?

If by "hard" you mean hard to understand or tricky there is nothing hard about building these kits. All the guess work and rocket science has been taken out and replaced with very detailed instructions that tell you exactly what to do and also what NOT to do. If by "hard" you mean time consuming, I would say drilling and hand tapping the heatsinks. There is some manual labor required here that some of the younger hams are not used to. Older hams remember when most amateur projects required you to be able to work with your hands. To those hams I say "Welcome Home!" this is your kind of kit!

14. Are there any tiny unmarked surface mount chip capacitors etc. that may get lost in my carpet if I drop them?

No, I wouldn't do that to you! Yes, they are cheaper than leaded parts, and yes my PC boards are designed to accept surface mount components if that's what you want to use. However, every part included in the kits is leaded and marked. The cost advantage of surface mount parts is far outweighed by the frustration that occurs when a customer drops his bag of chips and loses half of them in the carpet. If you have built many kits, chances are you have lost a few of these tiny unmarked parts. You won't with these kits.

15. Why don't you take credit cards?

The credit card companies charge a 5% fee to process transactions for a small business such as mine. I don't want to charge 5% more, and I can't afford to give up 5% of an already thin profit margin. Also there is the potential for someone to order a kit, not be able to put it together, and decide to cancel the credit card purchase. Also the FCC requires me to have a copy of your amateur license on file before I can ship your order. So the simplest way to purchase an amplifier from me is to send a check and enclose a copy of your amateur license. If you want the order to ship ASAP send a postal money order in an overnight or 2 day envelope. Please call or email to discuss lead times and availability before requesting an expedited shipment.

16. Can I wire the money to you?

Yes, but it will cost you an extra $30. Why? Because your bank will take $15 out of the amount sent as their fee for sending the bank wire and my bank will take $15 out of the amount received for receiving the bank wire. Therefore your payment will be $30 short and I still can't process your order until I receive a copy of your license. If you have a scanner you could scan your license and email it to me. However the best way is to go get a postal money order and send it along with a copy of your license in an overnight envelope. Your order will process just as fast and will cost you a lot less than a bank wire.

17. How do your HF amplifiers compare with the factory amplifiers such as the SGC 500, Ten-Tec Hercules II, Henry SS500, and Ameritron ALS-500M?

In general my amplifiers cost less, sound better on the air, and have more gain than the factory amps. How can all this be true? Simple. I have less overhead than the factories do so I can charge less. My amplifiers don't have all the bells and whistles that come on the factory amps so you save money there as well. If you're like most hams that don't want or need all that fancy stuff why pay for it? The amplifiers actually sound better because the very stable design allows you to use much more bias current (I use 1A per module quiescent current) than the factories which usually use the minimum recommended on the data sheet (about 200 mA per module). Higher bias current allows the amp to operate closer to class A which increases linearity and reduces IMD and harmonics. The result is a better sounding amplifier which requires less drive power (higher gain). Why don't the factories just increase the bias current like you did? Because as you increase the bias current the potential for amplifier instability increases and it costs more to achieve a design capable of remaining stable with high levels of bias. For example, my design has a separate precision regulated dynamic bias circuit for each module. The factories may use one of these circuits to supply as many as four modules. This increases the risk of instability under high bias due to minor variations in beta between the transistors used in the different modules. In short, I have concentrated on achieving the best basic no compromise PA design possible, and left off most of the bells and whistles.

18. How am I most likely to damage this amplifier?

Over driving is the number one cause of transistor failure in our amplifiers. ANY amplifier that does not have an over drive protection circuit (SGC 500 and Ten-Tec Hercules II have this protection circuit, most others don't) is susceptible to this type of damage. Typically an operator will forget to turn down the drive power before turning on the amplifier. The 100W drive signal destroys the transistor base-emitter junctions. High SWR is a distant second cause of failure. There are two typical scenarios that lead to transistor failure due to SWR. One is the use of an antenna that is not able to handle the amplifier output power. The RF power will cause the antenna to overheat resulting in either an open or shorted antenna line and infinite SWR. The other scenario is the guy with the screwdriver antenna who forgets to turn the amplifier off before changing bands and retuning the antenna. The infinite SWR the amplifier works into as the antenna is retuned to the new band can destroy the transistors. Before the transistors die, they will sometimes take revenge on the operator by melting the antenna. The RF power produced by the amplifier will be either radiated by the antenna or dissipated as heat in the antenna, feed line, and amplifier heat sink. If high SWR prevents that power from being radiated things will start heating up fast. If you have the discipline to make sure the drive power and SWR are correctly adjusted before turning on the amplifier you will never have any problems. If you want an amplifier that is "fool proof" as possible you need to pay up for an SGC 500 or Ten-Tec Hercules II that has all the "training wheels" installed.

19. I have one of the RF Applications SWR/Wattmeters that will open an internal relay under high SWR conditions. Will this work with your amplifier?

Yes. Simply run the keying line from the amplifier through the relay in your meter. This relay auto resets so when the SWR is back to normal the amplifier will come back on line. Most factory amplifiers require you to cycle the on/off switch to get the amplifier going again after a high SWR shutdown. That may be difficult if the amplifier is not within easy reach. For this reason, many mobile operators prefer the combination of one of our amplifiers and an RF Applications meter over a factory amplifier with manual resetting protection circuitry.

20. How does your 400W-6M amplifier compare to the competition?

As far as I know there is no other 400W 6M 14VDC solid-state amplifier. Hopefully by the time the competition produces a 400W 6M amp I'll have an 800W design available.

21. How does your 160-6M amplifier compare to the competition?

As far as I know there is no other 14VDC solid-state amplifier that will cover 1.8-30 Mhz at 500W and 50-54 Mhz at 400W.