The Case for a Grounded B+ & other Innovations in the Design of a Vacuum Tube RF Power Amplifier

Discussion in 'Homebrew and Kit Projects' started by W9JEF, Jul 6, 2021.

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  1. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    :cool: :D It was déjà vu all over again; bought these meters for $6 a piece at an antique shop I frequent:

    [​IMG]
    The two meters on the left test okay, but the one on the right (F.S. = 1 mA) is an open circuit, but may be repairable.
    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     
  2. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    After searching the QST archive (took me a while to master the navigation) finally found the article, entitled “A 1500 W Centenial Amplifier for the 80 - 6 Meter Bands” by Ralph J. Crumrine, @N0KC in the December, 2014 edition, beginning on page 30. Ralph's masterpiece of homebrew ingenuity features components and technology of the past 100 years, such as the 8877, an external anode triode of the 1970s. For anyone contemplating building an MLP vacuum tube linear amplifier, I highly recommend this article.

    I was wrong: Turns out, it's not a dedicated cooling fan. Instead air from below is forced through the hollow core of the plate choke, which is wound on a 1920s vintage ceramic house wiring tube. In order for the choke's series resonances to be well outside the bands of interest, it's lower impedance results in heating losses on the 80 meter band. Had the B+ been grounded, with plate current flowing through the tank coil, and choke through the low impedance output to ground; not only would the efficiency and heating problem on 80 meters go away, but, as a bonus, the amplifier could include 160 meters.

    https://edition.pagesuite-professional.co.uk/html5/reader/production/default.aspx?
    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     
    Last edited: Sep 6, 2021
  3. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    A huge advantage of grounded B+ amp design is safety: no need for lethal voltages in the space where “cut and try” experimentation takes place.

    Although “Switch to Safety” is the main mantra, the ARRL can supply no statistics on the number of hams who've been electrocuted while working on vacuum tube RF power amplifiers. In my 66 years of hamming, I've heard (mostly VIA on-air talk) of several such deaths.

    Can anyone direct us to a credible source of info on electrocutions while working on in-shack amateur radio equipment?

    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     
  4. W9JEF

    W9JEF Platinum Subscriber Platinum Subscriber QRZ Page

    [​IMG]
    Current, Comprehensive, and Complete*

    *except for...
    Shows only these 3 ways to feed B+ to an RF power amplifier:

    [​IMG] [​IMG]
    (A) shows the conventional circuit, but it's noted that since the choke does not present a sufficiently high impedance on the wide range of frequency bands, the heat could burn it out.

    (B) shows the choke at the low impedance end of the tank coil through which plate current flows, but this doubles the peak voltage on both the Tune and Load capacitors.
    [​IMG] [​IMG]

    (C) has the blocking cap taking the voltage off the Load capacitor, but not the Tune capacitor.

    (D) is the omitted way, grounding the B+, eliminating the problems of (A), (B), and (C).

    Maybe next year???

    73,
    Jim
    EARTH: LOVE IT OR LEAVE IT
     

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