Looking For Tube Transmitter Design Suggestions

Discussion in 'Homebrew and Kit Projects' started by WA9WFA, Nov 15, 2021.

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  1. KE4OH

    KE4OH Ham Member QRZ Page

    Hard to beat a 6AG7 oscillator driving an 807, as others have mentioned. Any number of designs out there from QST plus commercial Knight T-50, Johnson Adventurer, and others.

    You could do worse than putting together a Globe Scout without the AM modulator. 6V6 or 6AG7 oscillator and 6146 final. Or a Globe Chief 6AG7 oscillator and 2x 807 final. Simple, effective, no neutralization needed.
     
    W7UUU likes this.
  2. W7UUU

    W7UUU Principal Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    One to avoid like the plague: The W1TS "Boosted Pierce" - that schematic is all over the web and hams get suckered into building it all the time (I'm one of them).

    Sure there are lots of "fixes" for it on the web but why bother? Go with a better design to start with.

    Dave
    W7UUU
     
  3. W0RIO

    W0RIO Ham Member QRZ Page

  4. W9BRD

    W9BRD Ham Member QRZ Page

    Do not key the common connection of the 5763 grid-return R along with the tube cathode; doing so puts the 5763's full B+ on the keying line in KEY UP. Connect the non-grid end of that 470-k R to common; then you're just doing cathode keying of that stage, and its peak open-cathode circuit will be no higher than the tube cutoff voltage (which is affected by the stage's screen voltage and the absence or presence of drive in KEY UP).

    Despite the shield, the 6L6GC in the above will likely oscillate if keyed without drive with its grid and plate tuned to the same frequency. The unneutralized internal grid <--> plate C of the 6L6 is the reason. (I see the 4.7 k is used as the 6L6 grid R, likely to avoid this problem -- but then it's hard for the 5763 to drive the 6L6 into efficient class C operation. Use the 6L6's specified grid R for class C (18 k) and neutralize the stage instead.)

    Good work on including the negative-R stopper in the 6L6 screen. I'd also include one in the 5763 screen, as it's prone to the same class of parasitic oscillations ( http://dpnwritings.nfshost.com/ej/beam_power_tube_parasitics/ ).

    In such lower-half-of-the-HF-range circuits, we can pretty much do away with the possibility of VHF parasitics in the 5763 and 6L6 stages by connecting a high-voltage-rated 10-pF ceramic cap from the plate of each tube to common, right at the socket pin. Then that classical few-turns-of-wire-on-a-resistor parasitic stopper in the 6L6 plate is unnecessary, as we've rolled off the VHF gain of the stage with the added C.

    Experimenting with C (a few hundred pF to 0.001 pF) from the power-MOSFET drain to gate will reveal some keying-waveshaping options there.
     
  5. W9BRD

    W9BRD Ham Member QRZ Page

    Oops, 0.001 uF. Drain-to-gate C turns the MOSFET keying transistor into an integrator, with the C providing negative feedback that slows current buildup through the device channel. Similar output-to-input C has been used for decades with bipolar keying transistors as a means of slowing keying rise and fall times; much lower C is needed with a power MOSFET than with BJTs for a given rise/fall time.
     

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