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Distortion on Receive

Discussion in 'Amplitude Modulation' started by K0OKS, Aug 10, 2021.

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  1. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Back in 1959 after I had just upgraded to General, I threw together a modulator that used a 6AQ5 driver. The thing sounded crappy but I could modulate with it and the signal was readable enough that the other station could understand my callsign. Problem was, the 6AQ5 would overheat and after a couple of minutes would go into thermal run-away. The plate would turn red and the signal would become extremely distorted. It was built on a small chassis, so I turned it upside down and let the 6AQ5 poke down into a glass filled with water. That kept the tube cool long enough that I could make a complete transmission as long as it wasn't too lengthy.
     
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  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Ah, one of the first water-cooled Modulators. :cool:

    Pheel
     
  3. W7TFO

    W7TFO Premium Subscriber QRZ Page

    6J5? My fav oscillator is an 802.

    These whippersnappers...

    73DG
     
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  4. W9BRD

    W9BRD Ham Member QRZ Page

    "Thermal runaway" = grid emission. The 7BZ basing has two grid pins to help heatsink the grid; the idea is to use them both in parallel.

    Also to use the tube within ratings. :)

    The higher-transconductance, transmitting-tube-good grid-plate C 6DS5 is also a 7BZ tube, but because its heater runs at 0.8 A [!] instead of the 6AQ5's 0.45 A, the DS5's plate dissipation is derated to 9 W instead of the 6AQ5's 10. Avoidance of grid emission is also why we see maximum grid-return resistance values given for tubes.

    The legendary "6L6s upside-down in oil" canard has just one itty-bitty problem: Plate dissipation is only part of the tube-ratings story, and of course with a glass tube we can see the plate (and sometimes even other elements) glow when they shouldn't. The operative long-term limiting factor with tube use is cathode emission; that's what we "use up" over the lifetime of a tube, and I think cathode emission was the basis for ICAS (Intermitent Commercial and Amateur Service) v CCS (Continuous Commercial Service) ratings (RCA) and CCS v ICAS v IMS (Intermittent Mobile Service) ratings (GE), as for such tubes as the 8156 and 7984; 8156 ratings:

    8156_ratings.jpg
     
  5. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Regarding IMS rating, the 6C21 triode transmitting tube is essentially a 450T class of tube, repurposed as a radar pulse modulator, capable of thousands of watts of peak output power. Eimac rates the filament at 8.3 volts to achieve the necessary high peak emission. After WWII many hams acquired those tubes to use in transmitters, usually running them at the recommended 8.3 v on the filament. Much greater tube life could be had by running them at the normal 7.5 volts, since in amateur service the extreme filament emission is not needed. The published 6C21 ratings were a compromise between peak emission and tube life. For wartime service, it was assumed that the useful tube life needed for a radar installation would be a matter of a few months, not several years.
     
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  6. K5UJ

    K5UJ Ham Member QRZ Page

    I can't put my finger on an example this minute, but I think the marginal ham gear designs of the 1950s were firstly to cut costs, but also were based on the assumption replacement parts were cheap and plentiful.
     
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  7. KA4KOE

    KA4KOE Ham Member QRZ Page

    If a noise blanker is on or turned up too high, then that can cause issues on receive as well with adjacent strong signals/static.
     
  8. K0OKS

    K0OKS Premium Subscriber QRZ Page

    That is very true. This receiver has no noise blanker, though.
     
  9. W9BRD

    W9BRD Ham Member QRZ Page

    That reminds me of the "ANL" (automatic noise limiter) circuitry in older receivers; those circuits operated -- post-detector -- during AM reception only, either by clipping above an average level or by failing to conduct on peaks that exceeded an average level. In the receivers I used that included ANL circuitry, more distortion was always present with ANL turned on.
     
    Last edited: Aug 24, 2021
  10. W2WDX

    W2WDX Premium Subscriber QRZ Page

    As for me running a little bit of asymmetry on occasional peaks is fine, as long as there is nothing above 99% negative. Occasional 105%-110% positive peaks seems to be OK for most detectors. What is more important to me is more average signal within those limits as far as audio is concerned, as long as it sounds transparent on receive and not thumping and pumping. I like dynamic multi-band compression, mild peak limiting, and making sure the audio is in alignment with the tilt of the transmitter. Much more important than running cray high positive peaks, like 150% or higher, which is nothing more than making you inaudible to a larger audience or even people you are conversing with. Forcing people to change receivers just to QSO because you want to claim you have a big peeny makes no sense whatsoever, not to mention is a great way to waste power.
     
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