AM notes from the field

Discussion in 'Amplitude Modulation' started by K5UJ, Jul 1, 2020.

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  1. K5UJ

    K5UJ Ham Member QRZ Page

    I liked that Mike Pappas said that with envelope detectors distortion happens at 90% negative. So many AM hams and others do not seem to realize this.

    What seemed to finally kill tube rigs for broadcast was tube supply and cost and the electric utility bill. When you have tubes lit up day and night every day you have to re-tube a rig every 12 to 18 months. When NOS tubes from Richardson put the cost up around $2K it became intolerable for a 1 KW AM. We hams are lucky--we can run our triodes and tetrodes for the rest of our lives and not have any lose emission provided they get treated right and I think most of us know how to do that.
  2. N2DTS

    N2DTS Ham Member QRZ Page

    Yes, how long would these stations be able to get tubes and find people who know what they are doing on old stuff?
    Maybe if they made lots of money they would not care about the electric bill?
  3. N1BCG

    N1BCG Ham Member QRZ Page

    Many, if not most, smaller market stations have a narrower profit margin than restaurants. Replacing a tube transmitter with a solid state transmitter takes a lot of thought because while there's a big difference in operating costs, a new transmitter is a significant investment. The resulting savings will take years to realize.

    And that's a much easier pitch than to replace an audio processor. The difference between a 1980s processor or a brand new one would only be apparent with a critical comparison and few if any listeners could tell the difference or would even care. I can see how a processor manufacturer could make that case, though.

    The New York market leads the way in crappy sounding AM stations. The Telos Voltair installation instructions call for a setting just below the point where the encoding is noticeable. It's supposed to be a transparent function, but the higher the setting, the more the results, thus the sales managers adjust the Telos Voltair, not the engineers.

    Sounds hamateurish, doesn't it? Check these out: (No Voltair - Music) (No Voltair - Talk) (Voltair Hell - Talk) *

    * No, they don't use RealAudio 1.o as an STL

    Yet, their internet stream sounds clean:
    Last edited: Jul 2, 2020
  4. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    We later changed out the BC-500GY for an RCA BTA-1R1 because of a license upgrade and going from omni (ND) to a DA system.

    Power costs for the lower power stations were not an issue back then because industrial customers received a handsome discount from the power company and radio stations were considered industrial users. Later we installed a BE AM-1A only because tube reliability was becoming a concern with the Chinese tube supply, so the RCA BTA-1R1 was relegated as the backup with the BE AM-1A as main which ran 24/7 without a module failure while I was taking care of the station.

    Now in the above I am speaking to local stations only with power PO's of 0.25kW to 1 kW.

    For a large metropolitan 50 kW station power costs became significant and a typical tube type 50 kW transmitter facility needed about 175KVA facility power with three phase 208 or 440V power input. The final Class C stages averaged about 72% power conversion efficiencies. Audio input was 500/600 ohm analog only.

    The newer Harris 3DX-50 and the Nautel NX-50 solid state PolyPhase units still require about 85kW power input alone for 100% modulation, so a 100KVA facility power installation is required. The final Class D stages average about 90% power conversion efficiencies. Audio input is 500/600 ohm analog or AES-EBU Digital Audio stream.

    I always recommended to any station owner or GM that 1% to 5% of yearly earnings be placed into a transmitting facility "upgrade" fund because of impending obsolescence, moves, etc.

  5. K5UJ

    K5UJ Ham Member QRZ Page

    The worst AM I've ever heard was WOWO on 1190 in Ft. Wayne the last time I was there for a hamfest. That station was so over processed it was painful to hear. If I owned it and found that the CE set up the processing on his own I'd fire him. But I'm sure someone made it nasty on orders from management or a PD. You can process and gain coverage but there's a limit beyond which it becomes self defeating.

    I know there are stations still running the 3DX as the main, but not because they want to. That rig can show problems and I believe Harris is now GatesAir and they have suspended AM tx manufacture and I am not confident they still support the older DX line. From what I've heard, Nautel is the way to go, and I think just about every 50 KW is doing MDCL now. If it is done right you can't tell.

    While I love tubes to death, if I were a bill by the hour engineer and got called into an AM that today had a tube rig as main, I'd want to be paid something in advance before doing a thing.
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I am sure we can all name a bunch of crappy sounding stations but the worst I ever heard was KNIM in Maryville, Mo; it sounded atrocious with all highs and no lows. The YL and I were travelling back to Iowa from KC and I actually spoke to the Owner/Manager and offered to stop by and tune his audio free of charge, but he wouldn't hear of it. They were using a BE AM-1A transmitter which is capable of good audio reproduction if only good audio is input to it. They now have an FM translator so hopefully they have made some changes in the audio chain.

    GatesAir has poor to no support for their AM lineup as many of their AM engineers and support personnel have flown the coop.

    Nautel currently has good support and builds the quality Polyphase PDM transmitters.

  7. WA3VJB

    WA3VJB Ham Member QRZ Page

    I was headed to the Deerfield (NH) Hamfest one autumn in the 1990s and was dialing around the AM band daytime just to hear what stations were doing. Some station in Connecticut was just as clean as it could be, with programming of live college basketball play-by-play. I could not believe it. The whole thing, from frequency response, to low distortion, to good audio processing. Just wonderful.

    These days, check out "Absolute Radio," a high powered medium-wave station out of London on 1215Kc. You may hear some echo at times, but the passband and processing are very effective for the music format they're running.
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I have mentioned this before but CFZM 740 out of Ontario is another station with excellent audio.

    A number of community oriented 0.25 kW to 1kW AM stations here in Iowa have great audio as well.

    WA3VJB and N1BCG like this.
  9. N1BCG

    N1BCG Ham Member QRZ Page

    True, largely because they provide unique content in non-competitive markets so the emphasis can be on a clean, natural sound. Most smaller stations throughout the U.S., the Class C (formerly Class IV), typically don't buy ratings and don't need to rely on "juicing" technologies.

    That, and many engineers may simply select a manufacturer's preset that closely matches their station's format. It's hard to run into trouble that way. Others who immediately open the "Expert Settings" menus often define "expert" too broadly in terms of themselves and can get in to all kinds of trouble.
    Last edited: Jul 5, 2020
    AC0OB and WA3VJB like this.
  10. WA3VJB

    WA3VJB Ham Member QRZ Page

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