"Best" Filament Choke Design for 2 X 572B GG Amplifier

Discussion in 'Homebrew and Kit Projects' started by SWL37632, Jul 2, 2021.

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  1. SWL37632

    SWL37632 QRZ Member

    In my continuing iterative design, testing and 'learning' of my homebrew parallel 2 X 572B grounded grid amplifier, I have come to a crossroads on the next iteration of the filament choke design.

    I have already consulted the Handbooks, Orr, QRZ.com and a variety of other web sources including W8JI's.

    The amp design is 160 thru 10 meters.

    My research of the 1950's + Handbooks / Orr, show that bifilar (parallel) winding on ferrite rods (R75 or R61's) were used although some designs also show individual chokes on each leg of the filament leads.

    More recent designs and comments here (QRZ.com Feb 2021) show that bifilar (parallel) windings on ferrite toroids is somewhat preferred on FT-240-K or FT-240-43's.

    My current and un-optimised 'kluge' configuration is a single FT-150-K (8 turns of 10 gauge stranded/insulated house wiring) choke on each filament leg. Measured inductance is: 10 uH.
    A quick low pass Spice analysis shows that the 'break' frequency is in the KHz range.

    Choke lead connection is directly to the tube socket and is minimal.

    However, applying about 35 watts at 40 m from my old Globe Scout 65A, (through a .1 uF cap at one of the filament legs on the socket), shows what I consider 'significant' RF blowing past the choke(s) when using a simple and uncalibrated Field Strength Meter...most likely CM.

    Adding the 'usual' snap-on ferrite "Prayer Beads" does reduce the RF......BUT given the varying and differing designs and opinions, surely there is some more definitive test validated and quantifiable choke configuration/design that would avoid having to 'band aid' this situation with the use of snap-on ferrites.

    So the Question: What choke design will maximize the RF choking effect on the filament circuit ?

    Thanks in advance....

    20+ WPM 1970's Extra
  2. K6BSU

    K6BSU Ham Member QRZ Page

    You can spend a lot of time and effort designing your filament choke. The quick way out is simply to order one from Ameritron
    AD5HR and SWL37632 like this.
  3. W1GCI

    W1GCI Ham Member QRZ Page

    I'm thinking that you don't need #10 wire for a pair of 572Bs. That must be really hard to work with and limits the number of turns you can make. I've built a few amps, 811s, 3-500z, 3-1000z, 4-1000, which all have much more filament current then 572Bs and #12 is the largest gauge I've used. If'n I was building that amp, I'd use #12 on a ferrite rod.
    W9JEF and SWL37632 like this.
  4. SWL37632

    SWL37632 QRZ Member

    Yes. 10 AWG was hard to wind.
    Trade-off was a reduction of about 0.25 volt in IR loss at the tube sockets.

    Since I made this original post, I've built up a bifilar rod choke with 14 AWG enamel wire....next step is install and then measure the CM current.

    20+ WPM 1970's Extra
  5. W1GCI

    W1GCI Ham Member QRZ Page

    0.25 volts filament drop should help compensate for the common high line voltage that we see today - also, although not much of an issue for 572Bs, the small bit of resistance will help with inrush current :). good luck with the remainder of the build.
    SWL37632 likes this.
  6. SWL37632

    SWL37632 QRZ Member


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