Cascode triodes in T-R switch

Discussion in '"Boat Anchor" & Classic Equipment' started by K1APJ, Jun 3, 2021.

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  1. K1APJ

    K1APJ Premium Subscriber QRZ Page

    There was a spirited discussion some time ago about the cascode circuit, I forget the context.

    Anyway, I came across this schematic of the Johnson 250-39 TR switch and thought it was interesting. Cascode triodes, and the grid bias developed during cutoff is also coupled to the second triode. DC-wise, the two triodes are in parallel, ac-wise, it is a cascode.

    Note the choice of tubes, the 6BL7 is a vertical oscillator/output tube with a combined plate dissipation rating of 12 watts. The heater power alone for that tube is nearly 10 watts! Pretty sturdy for basically an RF preamplifier.

    Last edited: Jun 3, 2021
  2. WB2GCR

    WB2GCR Ham Member QRZ Page

    The Dow Key Electronic TR switch is very similar, but a different circuit... I think nothing special about the tube.
    The schematic is online... thought I had it in jpeg form, but no.
    Maybe they could get them dirt cheap at the time?

    Attached Files:

  3. K6BSU

    K6BSU Ham Member QRZ Page

    Only problem putting vacuum tubes betwixt antenna and receiver is the NF will increase.
    K4KYV likes this.
  4. KE0ZU

    KE0ZU Ham Member QRZ Page

    Possibly, but I think the OA noise figure is pretty much set by that of the first stage in the string.

    I suspect there were some limitations in selection since the T/R switch is rated at 4KW PEP.

    Last edited: Jun 14, 2021
    N2EY likes this.
  5. SM0AOM

    SM0AOM Ham Member QRZ Page

    What is dimensioning in this design is the maximum allowable grid-cathode voltage.

    The circuit works by grid-rectification of the incoming RF waveform to create a cut-off bias for both triode sections.

    At a PEP of 4 kW, it translates into a peak voltage of over 600 V, which takes a tube with a sturdy grid structure and quite substantial internal insulation properties.

    N2EY and PY2RAF like this.
  6. K1APJ

    K1APJ Premium Subscriber QRZ Page

    You know, I never worked out the math on this design; I had always seen similar circuits that use a miniature tube (B&W 380, ARRL Handbook designs, etc.) and the use of an octal dual triode seemed unusual. But, now I note the 4 kW peak rating and it makes a bit more sense.

    Thank you again for your insightful comments.
  7. AF5XF

    AF5XF Ham Member QRZ Page

    Last edited: Jun 19, 2021
    W1BR likes this.
  8. N2EY

    N2EY Ham Member QRZ Page

    The 4 kW PEP rating makes sense when you consider when that thing was designed.

    Before the early 1980s, the US legal limit was 1000 watts DC input, which could mean up to 750 watts CW output with the techniques in common use. But in plate-modulated AM mode, the peak power could be four times the carrier power, or even more, so the 4000 watt PEP rating made sense.
  9. KE0ZU

    KE0ZU Ham Member QRZ Page

    All you need are a couple of diodes back to back, across the Rx out Jack. Been using that for probably 20+ years without any problems.
    WB2GCR likes this.
  10. AF5XF

    AF5XF Ham Member QRZ Page

    a pair of silicon diodes across the receiver jack will clip the signal enough to protect it?

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