From: K2DH To: Wayne Heinen Subj: RE: QUESTION AM-6155 Phil- You raised a good point. Undoubtedly, there are some who are unfamiliar with the so-called "FAA" amplifiers, so here is some info about them. They were rated for 50 Watts output in AM service, but use an Eimac 8930 (or its equivalent by several other mfrs) and have power supply capability to run considerably more power. They are tetrode amps, so require little drive. The 8930 is a beefed up version of the 4CX250 series, with 350 Watt plate dissipation (instead of 250W as in the '250) and full performance specs to 500 MHz. The AM-6154 "FAA" amp was designed to tune 118-136 MHz. Easily modified to 144, they will produce about 500W out with about 5-6W drive. The AM-6155 version has the same chassis, but a slightly different RF cavity, allowing it to tune, in stock form, 225-400 MHz. Again, easily modified for the 222 band at 400-500W out. Almost as easy, but not quite, is the mod for 432. Again, with careful work, a good tube, and about 11W of drive, >400W output on 432 is common. They are a good buy for medium power in a small (but heavy) desktop amplifier. They were pulled from ground to air service starting in the early '80s, as the FAA and others upgraded to solid state transmitters. Fair Radio has been selling them for many years, having acquired a tremendous quantity of them from FAA. They actually ran out at one point, and what they had left were so bad that Fair was gutting them and selling off the important pieces inside (tubes, HV transformer, etc). But they got more and are again selling them complete. In my earlier post, I indicated that the current run seems to be much less clean than the originals 10-15 years ago. I bought two of mine direct from the FAA out of Oklahoma 14 years ago, and they were pristine, and the third from Fair Radio. I also know of some that appeared at an electronics junk yard in Northern California a couple years ago that were NEW, still in the sealed styrofoam packing from ITT. So, they may still be available clean, with some searching. Several hams, including N3AHI and K0TLM wrote up a series of mods for them and Fair Radio included (may still- I don't know) the mod documentation with each sale. Hope this helps. They're great little performers and treated right, will last a long time. In terms of watts per dollar, they are unbeatable. 73 Dave K2DH
Jason, I saw your request and some of the responses you received. I agree with most of what I have seen posted however here is some additional information. 1. Most of the amps are sold by Fair Radio in Lima Ohio. They adverize in QST. Sometimes, I've seen them at hamfests and I did get a couple from a guy that got some from a government surplus sale (where Fair Radio gets them). 2. Most of them (likely all of them) were made by ITT. 3. The 6154 and 6155 are nearly identical except for the input cavity. One only tunes the low band but that is not necessarily a problem since mods have been done that essentially don't use the input cavity for either 144 or 432. 4. They use an 8930 which as far as power goes is a 300 watt 4CX250. The plate diameter is larger so you can't use a 250 unless you make a solid metal collar that will fit around it. The amp has a finger stock connection to the plate and the Russian 4CX400 will also fit it with a little bending of the finger stock. 5. You can't use one amp for 144 and 432 because of the input circuits. You can change the output cavity from one setting to another in 5 minutes or quicker. 6. I've gotten over 450 watts out on 144 and lots of people get 350 or so out on 432. There are some other mods that most people make to some of the metering circuits and biasing. I've never tried an amp without these mods and there is vairous thoughts by people on how necessary they are. 6. The most common 432 mod uses a replacement input circuit so the imput cavity does not matter. 7. Most of the 144 mods do use the input cavity (with some mods) but K1WHS has a mod that does not use it and he also gets the same power that everyone else does. 8. There is a mod for 222 that uses the small input cavity version (6155). I've never seen a non cavity mod for 222 but it probably can be done. They're great amps - not 8877's but the price is hard to beat. They are very rugged with power supplies that can be run on 120 or 220. I forgot to say it but the rf part of them is a separate plug-in from the front. Four screws does the job in a minute or two. Also, all tuning comes out to the front panel -input tuning, input coupling, output tuning and output coupling it's great. If you get the chance, buy Ian's book that was recommended. It tells you almost everything you need to know about vhf and it tells you WHY - something missing from most books. If you are interested, I have a package of comversion articles that I have accumulated over the years. 73, harry, W3IIT
You will, no doubt, get a dozen replies; here's mine. The AM-6154 and 6155 amplifiers were used by the FAA in the early 1980's as ground-to-air AM transmitters. The 6154 was designed to cover 118-136 MHz and the 6155 to cover 225-400 MHz. Both models were set to 50 watts output, and amplified an AM exciter (which hams rarely see, nor is it of much interest to us being AM). Both models were rack-mount, 7" high, 19" wide, and about 24" deep; they weigh about 75 pounds and have an internal AC power supply that can be used on 120/240 VAC. They are built so they have a slip-in RF drawer that contains the RF amplifier itself; the main chassis holds the power supplies and a small blower. No T/R switching is included. Because they are intended to be used with the exciter, the amplifier circuitry expects to get some DC control signals from the exciter which have to be simulated by some modifications. The reason the amplifiers are so interesting to V/UHFers is because they are capable of outputting over 400 watts on 2, 222 or 432 MHz with only a few hours work and almost no extra parts. This is because they use the 8930 tube (or the Amperex equivalent, the DX-393), which is basically a 4CX250R with a 350-watt anode. 50 watts average power of AM is actually four times that power peak; or 200 watts; and these things were designed to do that all day, every day. That's why they used such a high-power tube for only 50 watts output power. The RF drawer of either model can be modified for ANY of 2, 222 or 432; but as it comes from the factory, modifications HAVE to be made for any of the three bands. In general, the mods involve redesigning the RF grid circuit to be more efficient and to tune one of the ham bands (nobody has yet admitted to wasting time trying to get the grid to cover more than one band using the same mods). The plate circuit in either model can be used on any of the three bands, although use at 400 watts on 432 MHz places a big strain on the stock plate DC choke and plate blocking capacitor, which almost always have to be reworked for serious 432 MHz use; they have apparently proven able to withstand high power on 144 and 222 MHz without modification, although the mods do not hurt. The power supply chassis is composed of three smaller drop-in chassis, an additional metering PC board, and a 400 Hz 120VAC blower which is powered by a DC-to-AC converter (one of the three drop-in chassis). The high-voltage power supply uses a very compact and lightweight transformer, a dual-section oil-filled filter capacitor, two screen voltage dropping resistors and a string of three zener diodes to regulate the screen voltage. Another drop-in chassis houses the filament transformer which also provides grid bias power and another winding to provide power to the DC-to-AC converter which drives the very small, compact 400 Hz blower (which is able to cool the large tube due to the high speed, 5500 RPM, of the blower). A front-panel-mounted meter with 12-position switch is also wired to a PC board which contains circuitry that originally monitored the output power and antenna VSWR in addition to the blower current, filament voltage, grid, screen and plate voltage, and plate current. The RF drawer contains a directional coupler and low-pass filter which are almost always removed from the output of the amplifier (sometimes rewired to the input side) because they don't handle 400+ watts very gracefully (read: they tend to stop working after a short time when overloaded by some 600%). I don't know for sure why Fair Radio asks $235 for the AM-6154 and $285 for the 6155, because the internal hardware is identical other than the frequency coverage, and either model must go through much the same mods to cover any ham band. There have been very small quantities of the amps available from other places on occasion; but Fair has by far provided the lion's share of them, probably well over a couple thousand to US hams alone. One that I have has the serial number around 14,100; so it is obvious that many thousands of these amplifiers were made. The manufacturer was ITT Optical in Pennsylvania. Many people claim output powers of 550 to 600 watts on 144 or 222 MHz. I've gotten 450 from one of mine on 432, and when the original 8930 is replaced with a Svetlana 4CX400A, I saw 550 watts out of another one that I modified for a friend (but we don't run it that hard because the plate circuit, even modified, is horribly inefficient on 432 and blows very easily above 450 watts or so). Hope this tells you more than you ever wanted to know about the 6154/6155s! They are extremely popular among US V/UHFers and usually command a $300+ price tag, whether modified or not (because the shipping alone totals around $35 for UPS Ground!). Recent estimates by Harry Brown, W3IIT of the Packrats, are that there are OVER several hundred in the Philly area alone; and Ev Tupis, K2?? of the Rochester VHF Group, estimates ALMOST that many in northwestern NY state! 73, Steve Ko0U/1
When you look at the specs, you have to realize that they were constructed to last a long time and put out the power with the tube loafing. The tubes used have a 300 watt plate dissipation and when Svetlana tubes are installed 400 watts. The amps are not perfect but they are much more well built than anything ever sold as an amateur product. They will put out 300 watts without any problem and compared to solid state amps, their linearity is superb. Sure, some of the guys did a poor job of converting them and they do have to be tuned but the results are well worth it. I don't know what Fair Radio is selling them for these days since I got mine a long time ago. They don't all come from Fair Radio though but Fair is the only one commercially that I have seen selling them. There must have been thousands made. In the Philadelphia area alone there are at least a couple hundred. The conversions take an evening and they work great. If you run into one at a hamfest take a good look at it. I think you'll be impressed. The comments on the reflector have been with problems that some have had with them and is not a reflection on their quality. I doubt that you will find anyone actively on the air that will part with theirs unless they have a really BIG amp and a big backup too. 73, Harry, W3IIT
From: Larry Filby Hello Again: No sooner blasted out my e-mail... and saw your query regarding the FAA amps. AM6154/6155. These were originally AM linear amps... run conservatively at 50W out... from around 120+ Mhz up to maybe 399 Mhz. (The two models cover different ranges.) They were used as amps for FAA air control AM exciters. They are cavity amps using an 8930 (4CX350) external anode tetrode. Some people manage to get up to 400-500 watts out of them. (I wouldn't want to live down the road from those guys.) Used mainly on 2M and 222... where they are easy to get on. Some have managed to get them going on 432 with varying degrees of success. They do require quite a bit of work and conversion. Built in AC supply... and a SCREAMING blower motor. Very popular, and been around in surplus since maybe 1980. Often referred to as Fair Radio Amps... as they were the ones who got them in a surplus bid from the FAA. The are grid driven... meaning easy (Low) drive... 6-10 watts. I owned a couple at one time. I'm personally not crazy about them, but a lot of people are. Fair Radio may no longer have complete units... or anything at all. But lots of them floating around. Price? Not sure... maybe $250-$400 depending on whether or not they're converted. 73 DE Larry/K1LPS (Vermont)
From: "Dr. Gerald N. Johnson, P.E." Subject: Re: AM 6154/55 ? Who what??? J, the AM6154/55 were made for the FAA tower base stations. The two models cover 108-135 or 225 to 400 mhz AM for commercial and military aircraft. Apparantly they work like the Collins units I'm familiar with using a low level AM exciter and then a linear amplifier. The low band one tunes to 2m fairly easily while the high band one makes either 220 or 432 mhz. The Collins one I have made for UHF tunes to 432 reluctantly and efficiency is not what I'd like, but it was cheaper than a solid state brick (MARS)... 73, Jerry, K0CQ