The AM Forum

Hi, I am trying to get on the AM frequencies on 80 and 40 meters and having just acquired a Swan 500CX, I was wondering if it would be sufficient to start out with. It does SSB with Carrier. I can hear others using AM with this rig, but I am wondering if my transmission would be sufficient. Thanks for any advice or comments. N2IU

You certainly can get on AM with about 150 Watts (and good finals) and many people have been able to do AM with 100 to 150 Watts. How well you can communicate depends on the conditions of propagation.

Atmospheric noise in the AM mode requires a good modulation percentage and a bit more power of at least twice what you have there.

But try it and see what kind of responses you get.


Phil - AC0OB

Using carrier inserted SSB nobody can tell the difference without a scope, however the difference lies in the audio. A transmitter designed for SSB has an audio chain designed for SSB, what Timtron WA1 Henry Yell Arr would call "telephonium audio". Unfortunately modifying it for AM Gangsta audio makes SSB sound muddy. I'd leave it alone and join the gang, but first invest in a good mic that can make all the difference in the world.

Be aware that this transceiver is rated at 125 watts INPUT in the AM mode, not output. That input level is based on about 150ma final cathode current. I would not recommend running at that level, as these finals are 6LQ6 sweep tubes that are under a high level of stress in this mode.

Your carrier output will not/should not be high, efficiency will be roughly 60% at 150ma. at tune up. However, at these lower power levels, your steady state (un-modulated) carrier output will be closer to only 25 watts (about 20% efficiency) at the 150ma level.

If you want to save those $$$ finals, running 100ma cathode current is recommended. Doing that will further reduce your steady state carrier to about 10 watts.

The carrier balance control sets the carrier insertion level and resulting cathode current.

Low power AM with current band conditions will not be too effective, however, if you intend to use this rig for extensive periods, I would highly recommend installing a fan behind or on the side of those finals. Of course a linear amp is a power option.

Modulation peaks should only be set (via mic gain) 10ma max above the carrier level. Also, keep in mind that your audio will be restricted by the 2.7Khz network, great for sideband but obviously not the bandwidth/audio you will typically hear from the guys in the AM window. Audio with desired lows (i.e < 300Hz) or highs (>3000Hz) will not be passed anyway, so your microphone requirements may not be so critical...

FWIW, I have a box full of sweep tubes that I driven into oblivion over the years, learning the hard way...ka8gef

Dale,

GEF's note mirrors the recommendations in the Swan Compendium so you could certainly start out conservatively and see how it goes.  You might be surprised at how well you do with 10W carrier when the band is in decent shape.  I agree with Phil, try it and make some QSOs.

What mic are you using?  I use a Shure 444D with my 500CX, but I have not yet tried it with carrier insertion beyond testing into a dummy load.

Let us know how it goes.

73,
Brad

Thanks to everyone who has offered up their thoughts on this. I actually don't have a mic yet for this radio and am open to any suggestions as to what would make a good high impedance companion mic for this rig.

Dale - N2IU

If you haven't already...

Here's the manual:
http://www.radiomanual.info/schemi/Surplus_Radioamateur/Swan_500CX_user.pdf

And here's some more info on the Swan's:
http://www.radiomanual.info/schemi/Surplus_Radioamateur/Swan_tube_transceiver_compendium_Rev4_2005.pdf


And, this thread suggests replacing the crystal filter with a .01 capacitor -OR- diode switching the filter OUT on AM transmit:
http://amfone.net/Amforum/index.php?topic=10725.0

Interesting possibilities.

Give it a go on AM. We're good listeners :-)

Don

I have used only the Shure 444D with my 500CX, but I have received complimentary audio reports using it. I have read of others using Turner high-Z mics with the Swan transceiverse with good results.  I'm guessing that the Astatic non-amplified D-104 would be a good choice or any of the Heathkit-branded high-Z mics from the early SB tansceiver era (which I think were made by ElectroVoice). I have seen a lot of these mics at hamfests through the years for reasonable prices.

I never was a fan of TV sweep tubes some manufacturers used because at the time they were fairly inexpensive and available at any TV repair shop. Remember TV repair shops? Here's a joke, the D&A Phantom 500 used as a CB audio masher and TVI generator. Funny thing, a 6DQ6 cost about $11 while the ubiquitous 6146 cost $4.50. A good test is give the sweep tubes a few atomic yaylos into a dummy load (only Timtron has license for on air yaylos) (;->) and if the anodes show any color back things down. They were never meant to cherry up, not even Rat Shack gold pin guaranteed for life tubes. After a few exchanges the manager said "You're not using them for sweep tubes." and kicked me out of the store. Actually it was a home brew AM transmitter not for Amateur use.........

Microphone, I should have been more specific and said NOT a cheap crystal mic that sounds like a tin can. (I once made one for a conversation piece.) Like they said a Shure 444 or Astatic d-104 on a plain G stand will mate up with that transmitter very well. When you graduate to AM Gangsta status with a proper AM broadcast quality transmitter like a modified BC-610 and an Orban AM Optimod processor you'll need an EV-664 I've seen at a few Collins stations, an RCA ribbon mic or my old Shure 330 Uniron. I'm fond of those mellow sounding ribbon mics, the broadcaster's favorite. Ah, here I am describing my dream station...........