Last Modified: April 3, 2014
Contents: Basics; Speech Compression Use; Amplifier Kits & Foreign Imports; Metron 1000B; TenTec Hercules; Ameritron; Henry Radio; SGC; Tokyo Hy-Power; Odds & Ends;
The best amplifier advice in the world—fix the antenna first!
Mobile amplifiers have been around for many years, and with the advent of high-power solid state devices it has become common place. But running high power mobile isn't as easy as most amateurs think. There are the obvious things of course, and there are other less obvious ones which are often overlooked. These include, but are not limited to alternator amperage ratings, amplifier brand, antennas, batteries, mounting, placement, remote controls, and wiring.
What appears here are amplifiers that were designed for mobile amateur service, and meet the FCC requirements at the time of manufacture. Amplifiers manufactured before 2006 may or may not have 10 meter coverage. After 2006, some manufacturers require a copy of your license before modification instructions and/or parts are supplied. Other have made sure their amplifiers will not amplify input signals between 26 and 28 MHz, as the new rules dictate.
It should be emphasized, that adding an amplifier shouldn't be a hit or miss scenario; It should be well planned, and the plan well executed. If you don't, you're going to have problems, some of which will be very difficult to overcome. A good example of this is the lowly ground loop, manifesting itself as RFI. This, and other pitfalls, will become evident as you read the articles. Further, Common mode current problems are exacerbated when using an amplifier, further complicating antenna installation. If you're using a remotely tuned antenna, and/or an automatic controller, the requisite motor and/or reed switch leads choke(s) must be very robust. The highlighted articles explain what is needed to assure an RFI-free installation.
If you want to get the best out of your amplifier, then read the Amplifier Care & Feeding, and the Wiring articles. If you don't, you'll most likely have serious electrical problems. Remember, a decent mobile amplifier, and the transceiver driving it, will have a peak amperage draw hovering between 70 and 100 amps! This fact shouldn't be taken lightly!
While it's tempting to purchase a tube type amplifier (P&H Spitfire and SBE SB2LA are examples), unless you're a collector you shouldn't consider buying one. The sweep tubes most of them used for finals, and their respective DC power supply switching transistors, are nonobtainium! Non existent owner's manuals, and the fact they use grid-block keying adds insult to injury. For this reason, they are not covered here.
Speech Compression Use
In the Audio Transmit article, there is a caution against using any form of speech compression, and there are several reasons for the recommendation. As noted in the article, speech processing, however it is done, not only increases the average output power, is also increases the average current draw! Depending on the configuration, it could in fact double! This can easily tax the heartiest of electrical systems when amplifiers are being used.
As noted below, most made-for-mobile amplifiers use metal-film resistors in the input circuitry. These attenuators act as both a matching device, and to dissipate excess drive power. Speech compression use can easily overtax these resistors. If one or more fail, excess drive can be applied to the finals, with predictable results—blow finals! But there is another reason.
Most mobile amplifiers are hard pressed to deliver their advertised power ratings (nominally 400 to 500 watts PEP) at a nominal 13.8 volts DC, yet maintain a reasonable level of FCC mandated IMD levels (spectral purity). The level of IMD is related not only to drive power (typically 50 to 65 watts PEP), but voltage stability as well. Even using a second battery and large wiring to increase voltage stability, higher current draw equates to more voltage sag, resulting in increased IMD!
Lastly, contrary to published specs, precious few HF mobile antennas will actually handle 500 watts PEP, unless there is a preponderance of ground losses (poor mounting for example). Almost be definition, 500 watts PEP is equal to about 175 watts average depending on individual speech patterns. Thus even moderate use of speech compression and high power, can destroy or damage most commercial screwdriver antennas.
Amplifier Kits & Foreign Imports
Starting about 1970 to present day, there has been about 200 companies making solid state mobile amplifiers with the CB crowd in mind. Admittedly, some of these units were well made and a few had switchable bandpass filters. All of them are technically illegal under Part 97 due in part to their lack of spectral purity, and lack of FCC certification. This includes the so-called amateur amplifiers made by Worth-More, Skywalker, and RM of Italy, regardless of what their respective web sites proclaim.
For those in the I just can't help myself category, Ameritron offers the ARF-1000 RF Harmonic filter for $160. Although rated at 1000 watts PEP (400 watts CW), that rating depends on the amplifier it is used with (harmonic content), and the SWR of the load. As Ameritron states in the manual, power out may drop 40% or more with the filter in place. If this is the case, then the harmonic content would still be higher than Part 97 rules dictate. If you're contemplating buying an ARF-1000, download the manual from their web site, and read the warning therein.
Unfortunately, far too many amateurs using non-filtered amplifiers justify their use by saying, "...I'm just using it mobile..." While that may be true, the IMD these amplifiers generate pollute the amateur bands we all share. This is especially true of larger models using up to 16 transistors. Just remember, it is the individual amateur who is responsible for his/her spectral purity, not the manufacturer! While rare, the FCC has levied fines against amateurs for using these unregistered devices. From my point of view, the low cost isn't justifiable in any situation.
A Caveat On Finals
Older model solid state amplifiers often use power transistors that are no longer in production. For example, honest-to-john, Motorola® manufactured, MRF454 transistors are nonobtainium at any price. However, it is not uncommon for replacements from other manufacturers to be dual labeled. For example, the 2sc2290 (which has also been discontinued) is a very close replacement part for the MRF454, and they're often labeled with both part numbers. However, the input, and output impedances are different. As a result, they are not plug and play, and some circuit modification is universally required. Sometimes this is easy, sometimes not. The bottom line is, before you buy a used amplifier which uses Motorola® MRF454s or 2sc2290s, make sure it works properly (this eliminates ebay® as a source). You should also note which finals any particular amplifier uses, as some, like the SG500, have used more than one type during their production cycle.
The question remains, should you purchase an older model solid state amplifier knowing that final transistors are no longer available? Unfortunately, it is a question with a questionable answer! If the amplifier works perfectly on all bands it was designed for, if the price isn't too steep for your budget, and you're not into overdriving and/or speech processing, you're probably okay buying one.
The Metron 1000B amplifier (aka Transworld MA1000), uses four, two device modules with combiners and 6 bandpass filters, and could be fully remoted. Although the WARC bands were not listed on the bandswitch, it will cover them handily. It had several drawbacks besides the use of Motorola MRF454 finals. There were no cooling fans and unless you wanted to replace the finals on a regular basis, you had to add fans. The input circuitry used swamping resistors to reduce the drive power to the finals. Overdriving will cause these resistors to fail, possibly resulting in failed finals. I never did replace any of the 454's, which proves how rugged they are. If you find one of these used, make sure it works perfectly in every respect.
Used prices hover around $225. Depending upon the serial number and when it was made, some models of this amplifier may be refit with current production finals (2sd2290s if you can find them). Unfortunately, Datron World Communications, no longer services these amplifiers, including manuals, parts, etc., so save your e-mails and phone calls.
By the way, the optional factory remote control is all but impossible to find these days. Duplicating it isn't difficult, but some provision for keying with modern made-for-mobile transceivers is a requirement. The switching voltage is a nominal 13.8 VDC, at about 600 mils. An Icom IC-706, IC-7000, or IC-7100 will thus require a keying interface.
I don't know the actual introduction dates, but the TenTec Hercules I and II are still very popular mobile amplifiers many years after their production has ceased. They are remotable, they have cooling fans, and they're relatively compact. They use MFR458 finals (MRF422s in the Hercules I) which is a drawback. Another drawback is its power connection. The Cinch-Jones style connector is not up to the task of handling 80 plus amps, and as a result you often see used ones which have been modified for a more robust connection.
Whether the Hercules II can be modified to use the Toshiba 2sc2879's (MRF458's closest replacement) depends on the color code of the original transistors. It is a moot point, however, as the upgrade cost is prohibitive.
Used prices vary widely with $400 being the low for fully functioning Hercules Is, to upwards of $850 for Hercules IIs. Lastly, TenTec is no longer providing factory service on the Hercules models because some important parts are no longer available. For more information, contact them directly. TenTec, 1185 Dolly Parton Pkwy, Severville, TN, 37862-3727. Their number is 865-453-7172.
Circa 1990 Ameritron brought out their ALS-500 mobile amplifier. It uses four 2sc2879's and has a built in fan, high SWR and thermal protection. On early models, only the on/off operation was remotable with band-switching done at the amplifier itself. Later models can be fully remote controlled with the addition of the remote head. Retrofit kits are available for some early models.
One drawback to the ALS-500 is the power connections. Early models used pigtails (2 for B+, 2 for ground) which is not an ideal scenario. Later models utilize a Cinch-Jones type connector, but only two pins for each connection is used. This isn't ideal either. If you own one of these, you might want to connect up the four unused pins thus making the power connection more robust.
The input circuitry uses 12 metal oxide swamping resistors, in series with the 2 input transformers; a common practice. If you overdrive the amplifier (>60 watts), in time, these resistors increase in value, resulting in blown finals. If you haven't already, you should read the Amplifier Care & Feeding article to avoid the scenario.
Weight is 7 pounds; the lightest of the lot. The two examples I've bench tested only delivered 425 watts PEP into a dummy load (at 13.8 VDC) which is a little short of their rating. The MSRP is $899 with a street price of about $790. The remote head is $50, and the owner-installed 10/12 meter mod is $30. It was marketed under the Vectronics name as well. I recently saw a Vectronics-labeled one advertised for $295. The finals sell for $48 in matched pairs. Ameritron, 116 Willow Rd., Starkville, MS 39759. Their number is 800-713-3550 for sales, 662-323-8211 for service. Incidentally, as of Spring 2014, the ALS-500 is the only currently available, designed-for-mobile amplifier (See Odd & Ends below).
The ALS-500 has gone through several iterations since the company was founded by Denny Had (K8KXK, now W4USR). He didn't design the amplifier, that was Tom Rauch, W8JI. In spite of its light weight, it will perform well as long as you follow the guide line in the Amplifier Care & Feeding article. Although it has an internal fan, it still needs adequate ventilation. The remote control option isn't all that good, if for no other reason than is is too large (plus the ammeter is worthless). With few exceptions, a keying buffer (ALS-704 or similar) is required.
Henry Radio used to make two rather robust mobile amplifiers, the SS750 and the SS1200. Both cover 160 through 10 meters. The SS750 uses eight 2sc2879's and runs on a nominal 14 VDC. Retail price was about $1,400. If you can supply enough current it will put out an honest 750 watts PEP. Like its big brother, it is remotable, has built in fans and is thermally protected. In my opinion, it is also ugly. Used SS750s sell for about $650 to $800 depending on accessories.
The SS1200 runs on 28 VDC ,and used eight MRF422's. Retail price $2,400. This price did not include the remote manual or automatic band change mods. While Henry's base amplifiers used to set the standard, the reader can set his/her own opinion whether this is the case with their mobile amplifiers. I've never seen or heard an SS1200 on the air, and considering the cost of a 12 to 24 volt inverter, I might not ever!
As of February 2005, Henry has quit making amplifiers of any kind. They still imported some VHF ones, but that ceased in 2009. Their repair facility is reportedly still in operation, but only on select models. Henry Radio is located at, 2050 S. Bundy Drive, Los Angeles, CA, 90025-6123. Their number is 310-820-1234.
This is the latest news from SGC, and the redesign of the SG500 Received April 2, 2014 (sic):
There has been progress in the rework of the SG-500. We have finals that are working but we are now developing software to support the self protection features. This is time consuming as we first do an adjustment then have to test it in 10-12 units to verify overall results. Then we do another step and retest again. We verify heat readings on the four modules as well as current readings, input drive, frequency, etc. Once loaded in spread sheets we then compare readings on multiple units to develop an average reference that can be used to produce the required self protection activation points. We still do not have a firm completion date but again when it is available we will pot it on our web site.
SGC Technical Support
My favorite mobile amplifier is the SGC SG500 Power Cube. It uses eight 2sc2290's and covers 160 through 10 meters. Its MSRP price is $1,425. It is thermally, over current, under voltage, high SWR, overdrive, and module imbalance protected. A fan kit is available (MSRP $220), and is recommend especially for trunk mounting. A remote control head is also available (MSRP $65), but really not needed as you can build your own remote control.
With its optional fan kit its about a foot square and weighs 25 pounds. It offers seamless integration with the SG235 auto-coupler which is a plus for those who like to QSY a lot. Typical output is 500 watts PEP with about 30 to 50 watts of drive depending on the band. Much more than this and the automatic input attenuator kicks in. Although it has an ALC output, it is positive going which means it is incompatible with any transceiver including SGC's. Finals cost $45 for a matched pair (see cautions below).
It is important to note, that earlier versions of this amplifier used MRF454 Motorola transistors. Upgrading an older model to the newer 2sc2290's is possible, but rather costly if you can find Toshiba 2sc2290s. If you find a used SG500 with the older finals (MRF454s), either make sure it operates perfectly (difficult to do), or forget about it. The last used one I saw, sold for $1,000 with the fan kit.
The SG500 has two very unique, selectable features for an amateur power amplifier. One of those is automatic band selection, which is the preferred method, even when using the factory remote control option. Here is how it works. The CPU controlled circuitry detects the presence of RF, measures the operating frequency, selects the proper bandpass filter, and keys the transfer relays. Having automatic band selection is very handy if you QSY a lot, and there is never a chance of using the wrong bandpass filter which could potentially damage the bandpass circuitry. The only drawback is a slight delay when first transmitting (250 ms), although re-keys take about 100 ms
The other unique feature RF keying (factory disabled, but easily re-enabled). It is the only amateur amplifier with this feature. While convenient, it is not the stuff of champions. Besides the filter select delay, RF keying adds another 100 ms to the key down time. Just as important is the 500 ms receive delay after key up (a VOX nightmare!). It should be obvious that PTT keying is the preferred way.
The best part is, the two features are independently selectable. Thus PTT keying may be used along with automatic band selection. The keying voltage is 4.3 volts at 2 mils which means it can be switched directly by an Icom IC-706, or IC-7000 (and select Yaesu models like the FT857) via the HSEND line
PTT keying of the amplifier is a requirement if you are using an SG235 auto coupler. Using the automatic keying circuitry built into the SG235 prevents the amplifier from operating until the coupler has found a match. The coupler's built in PTT circuit has an input current of under one mil, and thus compatible with most miniaturized radios. However, be advised that the SG235 PTT output needs to be buffered if any other amplifier is used in place of an SG500. SGC, 13737 SE 26th St., Bellevue, WA 98005. Their number is 800-259-7331.
As noted above, the SG500 can be remotely controlled. Whether you opt for the factory remote control, or you build one, there is one important caveat. Power for the remote control is taken from a four pin header plug. Pin four of that header plug is supply voltage. Although that pin is protected by an internal 5 amp fuse, if you short the pin to ground, a circuit trace will fail before the fuse opens. Thus, extra care needs to be taken to make sure this pin is not shorted to ground as the repair is both tedious, and time consuming. It's expensive too if the factory does the work!
Tokyo Hy-Power has filed for Chapter 7 bankruptcy, and appears to be gone forever. Replacement parts for their amplifiers, especially ones using the proprietary THP-120 finals, may soon become nonobtainium. This makes purchasing used examples highly questionable.
Odds & Ends
Here is a bit of good news for us high power enthusiasts. A new mobile amplifier will be announced late this year (2013), with delivery hopefully in 2014. The delay from the original planned availability date is due to part shortages, as Japan still hasn't fully recovered from the tsunami.
Nominal power output will be in the 700 to 800 watt range. It will be fan cooled, remotable, with fully automatic band selection, and compatible with most designed-for-mobile transceivers (plug and play). Considering the manufacturer, it will not be inexpensive. Based on their other model, I'd guess in the $2,000 range. I have to admit this sounds expensive, and to be sure it is. However, a bit deeper look is prudent.
It is important to remember that designing any solid state amplifier is not an easy undertaking, especially so when the supply voltage is a nominal 13.8 vdc! Meeting the FCC spectrum cleanliness, as well as today's market-required Plug & Play hookup, dictate a higher selling price than current competition. Considering the other built-in features this new state-of-the-art amplifier will have, it may prove to be a less expensive alternative overall. Stay tuned!