811As ---> 572Bs? Not so fast

Discussion in 'Amplitude Modulation - AM Fans' started by K5UJ, Oct 5, 2018.

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  1. AC0OB

    AC0OB Subscriber QRZ Page

    In just about every circuit I have built that has an inductive load, a Plate Parasitic Choke of 100 ohms wrapped with about 5 turns of #16 stopped that nonsense.


    Pheel
     
  2. K5UJ

    K5UJ Ham Member QRZ Page

    I had been told that before by someone and forgot about it. I've heard before that an operator is lucky to make it to 90% with one. This was in the days when rigs were made to work with a D104 and nothing else, most hams had no oscilloscopes and no idea of their modulation level, and FCC was monitoring all the time. Rigs had to be made idiot proof.
     
  3. K4KYV

    K4KYV Subscriber QRZ Page

    But the idiot proofing didn't accomplish much; if they turned up the audio gain too high, it would simply flat-top at 90% modulation, causing just as much splatter as overmodulation in excess of 100%. Back in those days a lot of hams overmodulated because they had no oscilloscope or modulation monitor, and no way to tell exactly when they hit 100%.

    The diode flasher using an 866 with the plate grounded and the filament connected to the modulated +HV line to the final made a simple and cheap overmodulation indicator, but few hams ever actually built one.
     
  4. K5UJ

    K5UJ Ham Member QRZ Page

    True, and that sometimes goes on even today by the few who for whatever reason, refuse to purchase a used scope for 50 bucks and use it. That diode flasher was always a cool idea.

    Unfortunately, if someone took a Globe King 500 and put it in a bigger rack with room for a good 2 KV supply for both PA and modulator, the higher v. would probably produce a chain reaction of crapouts that would eventually result in almost rebuilding the rig.
     
  5. K4KYV

    K4KYV Subscriber QRZ Page

    One of the problems would be the final tank capacitor arcing over on modulation peaks. As it is, the plate spacing is just barely adequate if at all.
     
  6. N2DTS

    N2DTS Ham Member QRZ Page

    A lot of them are still on the air and most do not sound that bad.
    I would just use one as is, everything in the transmitter was marginal.

    It used to be fun trying to make old ham gear work long term and sound good, now days maybe just use it as is and
    if you want good audio get a broadcast rig or buy a K7DYY rig, or run an sdr into a big amp.

    Nothing in any ham gear was much good except for maybe a few Collins pieces and all you are going to do is ruin any resale value and cause the next weak link
    to blow when you mod it for hifi.
    I like hearing WRL transmitters on the air, they have their own sound (including some power hum).

    I have even heard stock Valiant's that sounded ok if the clipper was adjusted correctly...
     
  7. W2NBC

    W2NBC Ham Member QRZ Page

    Rest assured, "resale value" is a moot point as the SDR jockeys and store-bought Class D transmitters take over the AM landscape..

    It's been a while, but I own a GK 500B and made some "mods". As KYV mentioned, voltage discrepancies of the MOD deck vs the RF deck inherently limit any possibility of > than 90 %. I replaced the MOD deck HV transformer (higher voltage), increased capacitance, and removed and replaced all "couplates" with "Hi-Fi" RC stages.
    The RF tank was also modified with some stout components, including a rewound tank coil. The hummmm was traced to the VFO physical connection path.

    My take is that finding unmodified AM gear is actually rare these days.. After all, isn't it all about sounding GOOD?

    Long live Angel Music
     
    AC0OB, K5UJ, W2VW and 1 other person like this.
  8. K5UJ

    K5UJ Ham Member QRZ Page

    Some like the challenge of improving an old ham rig, but as far as I'm concerned, the improvements have to be reversible or no-brainer improvements like better components. Hacking up the cabinet for example is not my style. But putting the chassis in a taller rack so bigger and better iron can be placed outboard while saving the stock rack is okay.
     
  9. W2VW

    W2VW Ham Member QRZ Page

    The last half dozen old AM Transmitters I acquired were all heavily modified beforehand.

    On average the modifications were a big improvement.
     
  10. K4KYV

    K4KYV Subscriber QRZ Page

    It depends. On something like a 75A-4 receiver I am reluctant to make irreversible mods, and I hate to see a well sought-after classic piece with extra holes blasted in the front panel or cabinet for some hammy hambone modification of questionable worth. But on rigs like the Valiant, DX-100 or GK500, I never thought they were all that great nor irreplaceable in the first place, so I'd say if you think you can improve the performance of one, then hack away.

    As far as re-sale value, I don't think much of anything is going to have a lot of that for very much longer, since most "vintage" and "classic" radio enthusiasts are rapidly becoming "vintage" ourselves, and in the not-too-distant future there won't be many left on this side of the turf who are interested in that kind of stuff. Treasure troves of unobtanium will once again be hauled to the landfill after the owner goes SK, because his heirs won't find anyone who wants it.

    This has been evident at Dayton the past few years, as I have seen nice old collectors items sit unmoved on vendors' tables all weekend, to be loaded back up to return home at closing time. I had a hard time giving away a rough but restorable RCA AR-88, no-one would give me $75 for a non-working but repairable 75A-1, and my mint condx BC-221 frequency meter sat both days, marked $30, with no takers.
     
    WA5VGO likes this.

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