1.25m linear amplifiers?

Discussion in 'Amateur Radio Amplifiers' started by KK6VQK, Nov 27, 2019.

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  1. KK6VQK

    KK6VQK Ham Member QRZ Page

    i'm looking to build something to put out 50w-100w on 1.25m. Right now I'm using a TS-430S drving the transverters-store 1.25m transverter and it's putting around 5w out. I'd like it to be linear so I can do SSB as well as FM/CW.

    This seems .. underserved. w6pql does the big amplifiers but I don't need /that/ much power. Honest. :p

    Does anyone know of something I can steal/modify? I'm eyeing the MRF-101's to potentially make something to push 50W out but I don't yet know enough to design it from scratch.


  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    If you don't mind, an MRF151 TV broadcast pallet will give you 300W & they are surplus on Ebay.
    50VDC supplies are everywhere, now.

    K0UO, W2VW and KD2ACO like this.
  3. KK6VQK

    KK6VQK Ham Member QRZ Page

    hi! Thanks! I was hoping to try and learn how to make one rather than repurpose something that exists. I've been reading the ARRL experimental methods in RF design book and .. well, there's a lot. Heh.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd build a simple 4X150A amp: 5W in, 250W out pretty easily without a huge power supply: 1500-2000V at 200-250mA, screen voltage derived from a big resistor and some voltage regulators in series, bias derived from almost anything (-55V operating at only a couple of mA), and 6V heater requirement can come from a 6.3V filament winding with a small resistor in series.

    Not a big project, and the tube's cheap.

    My own 222 MHz amp, which was actually designed for 220 MHz back in the late 70s when that's where all the action was at the time, is a pair of 4CX250Bs that run about 700W out with ~10W drive. Built that in one weekend and used it mostly in VHF-UHF contests. Pics of that amp are in CQ magazine in a couple of my VHF columns from the 80s.

    I like tooobs. But now that large scale power FETs for VHF are popular and not expensive, for a new project I'd probably use those instead; they also have very low drive requirements and lots of gain, and only a 50V power supply (non-lethal :)) is required.

    But it's hard to beat the gain and power of a golfball-sized 4X150A.
    K0UO likes this.
  5. KK6VQK

    KK6VQK Ham Member QRZ Page

    If you have a schematic or pictures then I'd love to see it. I'm not afraid of building tube amplifiers; I've been zapped enough in my early teenagerhood to have a healthy safety margin around HV lines.

    Also it'd be fun to build something tube-y as well as something solid state-y. ;-)

  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'll try doing this on Monday 12/02 if I remember...all those files are in my "work" computer and I'm remote and don't share that one, even with myself!

    But I didn't invent anything...the 4X150A 220 MHz 1/2 kW amp was a project detailed in old ARRL VHF-UHF manuals in the 60s.
    K0UO likes this.
  7. KK6VQK

    KK6VQK Ham Member QRZ Page

    Thanks, I really appreciate it!
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I looked a few mins and didn't find the single-tube 222 MHz amp, but did find a dual-tube one for either 4X150As or 4CX250Bs (either will work the same way with no circuit adjustments). Schematic and parts list for RF section below.
    220 MHz amp P-P 4X150A or 4CX250B.PNG

    The power supply circuit is below.
    Dual 4X150A power supply.PNG

    It's pretty simple and 10W drive produces about 600W output. Top view of PA plate compartment, which has almost nothing in it:
    220 MHz P-P amp plate compartment.PNG

    The underchassis view of the grid tuning compartment is below; it's more "busy" since it also contains all the wiring for the tubes as well as some shielding, all made from Home Depot pierced metal stock (galvanized steel, cheap).
    220 MHz P-P amp grid compartment.PNG

    I'll look some more to see if I can find my original single-tube amp version. But other than punching and drilling metal, these amps are pretty easy.
    KK6VQK likes this.
  9. KK6VQK

    KK6VQK Ham Member QRZ Page

  10. KH6AQ

    KH6AQ Ham Member QRZ Page

    The 4CX250B amp looks good and I can use the HV from my AL-811 amp.

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