General Interest (Vintage) Article from BE: Using 4-400's in AM Transmitters

Discussion in 'Amplitude Modulation' started by AC0OB, Jul 31, 2020.

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  1. AC0OB

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  2. K4KYV

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    Those were the days, my friend. I still have a collection of those old BE magazines, which actually carried detailed in-depth technical articles like the one above. I believe BE morphed into a rag called Radio (unrelated to the pre-WWII west coast ham magazine) which related more to engineering business management than technical content (much the same has happened to Radio World), and which recently went 100% "digital". As far as I know, it has fizzled entirely, or at least has the free subscription offer, since I no longer receive it by e-mail nor did I get any renewal notice for my free subscription..

    One thing it mentions in the article is that the plate efficiency of the 4-400 final is about 70%. That means losses in the plate tank circuit, ATU and feedline are not considered in the nominal figure; the actual total efficiency of the RF reaching the antenna is more like 60-65% or probably less, compared to the DC input to the final. YMMV.
  3. W7TFO

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    Back in the early 70's I was CE at KAWC-AM in Yuma, Az.

    Our local consulting engineer Morris Courtright wrote a feature article "Keeping up really means staying ahead" for BE magazine and featured me and some studio shots.

    I might still have that issue somewhere.

  4. W1BR

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    Didn't RCA play some tricks with harmonic shaping in some of their PA designs to get efficiencies much higher than 70%?
  5. W2VW

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  6. K4KYV

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    Gates also came out with their version circa 1965.

    As I recall, those 3rd harmonic traps have to be carefully tuned to attain precise phase relationship to the fundamental. Like a Doherty system, these would be highly impractical for a ham rig, unless you were rock-bound to one frequency, since it would require extensive re-tuning every time you QSY.
  7. W7TFO

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    Myself and Sam KF7WWW killed a local BTA-5T a couple years back. It had set dormant for many years in a 'squalor' of a site. It was chock full of insulation, junk, and of all things, playing cards brought in by mice living in it. Good thing our hot climate precludes any Hantavirus here.

    Many good parts have gone on to hams from that old beast.

    W1BR likes this.
  8. K4KYV

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    Too bad you live so far away. Hope you could use those gigantic oil caps and the 851.
  9. AC0OB

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    I saw that and questioned his figures.

    According to Eimac tube data, 2500VpX0.275Ip = 688W. Output power is stated as 510 Watts at the plate.
    510W/688W = 74% efficiency. Now let's add 13W of screen power for a single stage and the efficiency is 73%.
    Now even assuming typical output network losses of 2%, the efficiency of the final stage would be 71% minimum.


  10. AC0OB

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    As did Collins. I don't know who first patented the concept but here is a simplified diagram of the Collins 828 D/E series transmitter. Stage efficiency of this final was touted as 88%.

    At 1 MHz a 333 ns pulse was applied to the final's grid which contained harmonics and the correct phase.

    At the plate the 3Fo waveform combined with the Class C waveform to produce a quasi-square waveform similar to a Class D switching waveform.

    To change frequencies in the field a complete kit of components had to be swapped out.


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    Last edited: Aug 2, 2020

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