Dave Hockaday WB4IUY, Youngsville NC USA FM05
The Vintage Shack

AM Operation @ WB4IUY/AC4QD

This is a pic of the primary AM station here at WB4IUY. I've always operating AM as a mode. I like the 'warm' sound of a properly modulated AM signal, and the typical 'Tech Talk' heard in the AM roundtables. Many AM ops are equipment collectors, restorers, and/or builders like myself. I often enjoy listening for hours (while tinkering in my workshop) to AM roundtables without actually transmitting.

My current AM transmitter is a modified Heathkit SB-401. I plan to write a little piece on how the mods were made and upload it here for others. The gist of the mods are:

- Install a 4pdt switch to control the "AM Override Mode" while in CW

- Use one pole of the switch to activate a small relay to bypass the TX crystal filter with a .01 uF cap (to widen the audio bandpass). I could have wired this directly to the switch, but used a small relay to keep the leads to and from the filter as short as possible.

- Use one pole of the switch to open the CW Sidetone circuit during transmit

- Use one pole of the switch to reconnect the PTT circuit in CW mode

- Use one pole of the switch to disable to the CW delay when in "AM Override" mode.

I set the transmitter to CW mode, set the new "AM Override" switch to "ON", adjust the mic gain/CW level front panel control to set plate current at about 75-100ma. This provides about 10-15 watts out. I feed this into my old Gonset GSB-201 MKIII amp for about 175 watts of carrier, and it plays well. I did build an electret condenser mic element into an old Turner Desk Microphone housing, and bias it with a 9v battery... this really improved my low frequency response. This setup sounds great and has an audio response from about 85hz to over 5khz.


- Interested in AM Operation?
- Working AM On 80 Meters
- AM Spotting Tool
- AM Operating Event
- lack of Respect for AM Windows

Heathkit SB-Series Station

I got this equipment back in 1991 at the Greensboro NC hamfest. It was mostly built in the '60's by various hams, and refurbished in my home workshop by me. It consists of: SB301 receiver, SB-401 Transmitter, SB-600 External Speaker, SB-620 Receive Panadapter, SB-630 Transmit Scope, and SB-500 2m Transverter.

Heathkit SB-Series Station

Yaesu FT-901DE

I bought this old rig back around 1992, in need of much TLC. It had been owned by a CB'er who changed crystals in the 10m band to 11m, turned every slug in the radio with metallic tools and cracked them, and otherwise butchered the radio. I bought it for nearly nothing, and took it on as a project radio. it was a fun job, and I was able to totally restore it to it's original operations and performance. It is a permanent fixture in my shack, and I'm using is in Studio A as my AM rig. It has beautiful receive audio on AM, and does a pretty decent job on transmit. You can see it in this pic with a JPS NIR-10 DSP audio filter. Here's an audio file recorded by Dave W9AD in Ill. of my 901 with the D-104 mic on a snowy NC nite (2/12/2010).

Yaesu FT-901DE

Gonset GSB-201 MkIII

This amp was purchased from M.M. Preston K4SQC (sk) in 1992 and was non-functional. I repaired the power supply in it, and put it back on the air. It uses 4ea. 572B tubes, and is built like a tank. It took a lightning strike in 1998, and was off the air until January 2010 when I made more repairs and put it back on the air. It looked pretty bad when I got it in '92, and I put a poor-boy's paint job on it to dress it up a little. It is in need of a decent paint job now, but works pretty darn good! I run it at about 175 carrier on AM, and it doesn't even get warm :-)

Heathkit HW-101 Transceiver

AM Filter for the IC-756 Standard (non-Pro)

I've located a source for the nearly non-existant AM receive filters for the IC-756, and have it installed (with a little mod from the manufacturer's recommendation). You can see it by clicking HERE.

756 AM Filter Location


ALC Source This is one of the older IC-756's, not a "Pro" model. I use it on AM with a little circuit I learned about from W8JI's Website.

From W8JI (and edited a little for my specific rig): Some newer HF transceivers are excellent on AM, with much less distortion and better fidelity than many older amateur AM rigs. The Ten-Tec Orion and the Yaesu FT-1000D are two examples of very good AM transceivers.

IC-756 Peak envelope power, with 100% modulation, is four times carrier power. For 100% modulation in a 100-watt radio, a 100W PEP radio must run 25 watts or less carrier. With a 100-watt radio, peak power on voice peaks should be held to 100 watts or less.

I use an IC-756 ICOM (non-Pro model) on AM. The problem with that ICOM, like with many HF SSB radios, is it uses ALC to limit output power. Turning the output power down or mic control up will not increase the percentage of positive peaks. This is because the ALC system in most SSB rigs almost always detects peak power. ALC Source If we adjust carrier power to 25 watts and try to modulate 100% (100 watts), the peaks cause the radio to reduce gain until peaks are back at the 25 watt carrier power level. The positive peaks stay at 25 watts or so...and the carrier drops to 7.5 watts when modulated!

The cure is to run the power level all the way wide open and apply an external stable negative voltage to the external ALC input. Adjust the external negative ALC until the carrier is 20-25 watts, and then the mic control until we have 100 watts on peaks using a good peak reading meter.

An external ALC carrier control can be a 9V battery across a 500k pot in a voltage divider. The positive battery lead goes to ground, and the ALC output comes from the pot wiper. P1 goes to the EXT ALC jack on the radio. Remember to disconnect the battery when using other modes or when not using the radio! The pic on the left is my first version, quickly assembled to test this circuit.

To use this circuit, I run the radio's normal power control wide open, adjust the pot on the ALC source for 10 watts carrier, and adjust the mic control for 100% modulation. This gives me about 40 watts pep. when modulated at 100%, and about 800 watts PEP when I use this with my Al-82 amplifier, and I get great reports on the air!

New ALC Source UPDATE - 11/1/2010 - From time to time I would leave the battery connected, and it would be drained in 2-3 days. I would also forget to unplug the circuit from the ALC jack when operating other modes. So, I whipped up a little unit for this task that is a bit cleaner and less troublesome. I used a 5k pot instead of the 500k and this provided a smoother ALC adjustment range to the rig. The 500k was originally selected for 9v battery use to minimize battery drain, but since I used a scrap 'wall wart' type 9 vdc power cube to power this newer version, the 5k pot is a better choice. Power input from the 9vdc power cube is fused at .1 amps. I also installed a SPST switch in between the wiper of the pot and the ALC output connector P1. This allowed for an effective disconnect from the ALC jack when not in use. The new unit is much more user friendly when used by forgetful operators like myself. NOTE - You CAN NOT power this from the same DC supply that powers the rig, as the positive (+) terminal of the 9vdc power source is grounded to provide the negative (-) voltage needed at the ALC input. A little power cube is perfect, as it is isolated from the primary 12vdc supply voltage powering the rig.

AM Blog, 5/22/2010

Lack of respect for "Windows" on different bands...

There are gentleman's agreements and suggestions by the ARRL for various "windows" to be used by certain modes or activities. These include things like the "DX Window" found on many bands, call frequencies like 50.125 on 6m, and mode windows like the PSK31 window and the AM window. Many of these "windows" have been around for decades and are well documented in practically every amateur radio publication and all across the internet.

To preface this...I collect vintage Amateur Radio gear. There are lots of people like me, who collect older amateur radio equipment, restore it, and enjoy operating it on occassion. Many of these old rigs are crystal controlled and have been locked to these frequencies for decades, especially since these frequencies were widely known as the frequencies to operate AM on. Most of these rigs operate on AM.

The "AM Windows" have been around since before I became a ham in 1974. One such example of windows are the two frequencies 3.880 & 3.885 mhz in the 75 meter band. There are only a handful of such frequencies in the entire HF portion of our allotted amateur radio spectrum. AM has a typical bandwidth of at least 6 khz, and most of the older receivers were designed to accept this bandwidth for proper reception and reproduction of transmitted audio.

I'm always amazed at the number of people who tune either just outside of the window (or right smack in the middle of the window!) and operate SSB. Their arguements are often "I don't hear the AM'ers" or "I was here first and _they_ can move". I guess either they don't understand the design of their own receiver including it's more narrow passband and ability to null carriers from the oppsite sideband, don't understand that many of the AM Vintage rigs can't move to another frequency, or they simply just don't care where the window is.

AM'ers, for the most part, stay in the windows to enjoy the company of other collectors, experimenters (many AM ops build and/or modify their own gear), and others with similar interests. There are lots of us...the numbers are quite large. There are publications, businesses, and even entire hamfests dedicated to this facet of the ham radio hobby. We don't _HAVE_ to stay in these tiny slivers of spectrum called the AM Windows, but we choose to do so out of respect for the other modes scattered across the bands (among other reasons). We can operate anywhere those of you who operate SSB can operate...but we don't. We keep to ourselves and try to minimize our interference to others by operating in these widely known and universally understood "AM Windows".

Imagine if all SSB operations were be herded into a few tiny slices of spectrum called "SSB Windows", while AM operations ran across the entire phone bands. Imagine if AM'ers decided to "take back" a fair amount of spectrum and began having AM contests, calling CQ, holding nets, etc on AM all across the phone bands with our wider signals. Now imagine if those of you who operate SSB actually behaved as most of us do and gave AM a little "elbow room" around the windows so your SSB signals (and many of them are quite wide as well!) didn't jam the few frequencies that we relegate our own selves to.

I started typing this on this Saturday morning while working (or rather, trying to) the AM Military net on 3.885. A group of hams in the southeast US constantly obliterates operations on 3.885 Am by operating on 3.888 LSB. Their lower sidebands splatter all the way down to below 3.880. One of the guys is regularly heard on a rants that he will jam AM anytime and anywhere he hears it, simply because _he_ doesn't like it. This happens all the time, and is a perfect example of what this blog is about: People who either don't know or don't care how their operations in and around the windows impacts a much larger group of people who are operating in a manner as to minimize their impact on everyone else.

Please folks...stay out of and away from (by few khz) the AM windows with your SSB signals, so AM'ers can also enjoy their favorite part this awesome hobby!

free counters