home brew amp filament choke placement

Discussion in 'Amateur Radio Amplifiers' started by KC0VVB, Jun 24, 2020.

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  1. KC0VVB

    KC0VVB QRZ Member

    I am just finishing up a home brew amp project and I am still getting some stray current into the transmitter from the amplifier on transmit. It continues to melt the coupling capacitor and tank choke on the ranger.

    I have tested and found a few worn out items; defective roller inductor, in the tank circuit and a worn coax relay but nothing that really screams, high curent in the wrong place.

    The issue seemed to start when i re installed the original tuned input back in the amplifier. I converted it to grounded grid and there is now a trifillar fillament choke near the tube socket that was not there originally.
    I think it may be too close to the tuned input coils. I can see now that they are actually touching,

    Would this induce 11 volts and 14 amps into the tuned input circuit? Would it help if i moved the filament choke farther away from or parallel to the input coil?

    Erik KC0VVB
  2. KC0VVB

    KC0VVB QRZ Member

  3. W9WQA

    W9WQA Ham Member QRZ Page

    as an outsider, no help here. cant tell anything in that crowded area...
  4. AG5CK

    AG5CK Ham Member QRZ Page

    10lbs of stuff in a 5lb box. My opinion is that the filament choke should be as close to the tubes as possible.

    Are your coax shields bonded to the chassis on each end? They don't look like it in the picture.

    Rollers in tank circuits are more trouble than they're worth.
    AC0OB likes this.
  5. K6BSU

    K6BSU Ham Member QRZ Page

    Your input tuned circuit feeds RF to the filaments. The top end of the filament choke is also connected to the filaments. Therefore, the input inductors and the filament choke are at the same RF potential. There is no "feedback" from the filament choke back to the driver transmitter.
    (there is no high power RF in the filament choke)

    Could be your driver transmitter suffers from a high SWR due to your amp input circuit being way off tune. Try an SWR meter between driver and amplifier.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Wiring abomination.:)
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    I really think you need to check the plate blocking capacitor on the Ranger. There is NO way an amplifier should cause the problems you are seeing, unless the final B plus from the Ranger is reaching the SO-239 and being shorted by the input network on the amplifier. I always add a safety RFC choke on Ranger outputs, just to prevent having DC appear on the antenna in case the plate is leaky or shorts.

    BTW, I have seen a lot worse homebrew amplifier efforts. I can't find much fault with yours.

    Other than that, make sure there is no RF short between the input and PA cathodes. Running the Ranger in a short could cause extremely high circulating currents that might melt the PA tank in the Ranger.

    What is the PA tube? That small fan adequate for cooling all of the seals?
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Seals cool off by going back into the ocean.:)

    That amp appears to be a butchered Johnson Thunderbolt, which had two fans originally. It normally used a pair of 4-400A's in grid-driven config (AB1) and with only 2200V B+ it was a "one kilowatt DC input" amp with tubes that can run far more than that, so it never needed much cooling for the tubes (other than the filament pins). But with an internal power supply that included regulated screen and grid bias supplies using gas tube regulators, and a choke-input HV supply with a lot of bleeder current, the power supply section definitely needed cooling!

    What's shown in the photos has such wiring abominations I'd want to strip it down to bare metal, de-solder and clean all terminals, and start from scratch. Maybe that's just me.

    The "T/R relay to tube cathodes" wiring appears to be RG-8/U or RG-213/U with the shield not connected at either end, so it's not coax, it's just hookup wire and very overkill for the power it handles, as well as taking a much longer path than it needs to. It appears to also "loop" next to another coaxial cable which may be handling output power, and since the "drive" cable doesn't have its outer conductors connected, it may be forming a transformer by coupling to the other cable. Who knows?

    Anyway, that whole thing is in need of serious re-wiring.:p
  9. K6BSU

    K6BSU Ham Member QRZ Page

    Ugly" is only a relative term.

    Sometime, check the bottom of a Hallicrafters radio if you want to see a rat's nest.

    (underline is not my idea. computer does strange things)
  10. N6UH

    N6UH Ham Member QRZ Page

    CTRL + 'u'

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