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3 4-125a for a 2 meter amp

Discussion in 'Amateur Radio Amplifiers' started by WA4SIX, Jun 4, 2015.

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  1. WA4SIX

    WA4SIX Ham Member QRZ Page

    I have worked with several SS amps, either building or converting. I will have HF & 6 meters covered with bi-polars on HF & MOSFETS on 6 meters.
    I got 3 4-125a NOS tubes. I would like to build a 2 meter amp with the 3 tubes. I am finding it difficult to find any plans for this. It will be strictly for SSB & possibly AM (If I can find anyone up there).
    This will be my first time building a tube amp. I have been reading about them for a few decades.

    Yes, I know that I would probably be better off with a single bigger tube, but these should look so nice warmed up and I have them with sockets.

    Thank you,
  2. W1VT

    W1VT Ham Member QRZ Page

    • Nov 1955 - QST (Pg. 11)
      High-Powered Tetrode Rig for 144 Mc., A

      Author: Tilton, Edward P., W1HDQ
      Article: QST Archive [PDF]
      Keywords: 2 METER TRANSMITTER TUBE 6360 6524 4-125A

      Note the keywords field--you can search for tube type in the ARRL Periodical Archive and Search if you are a registered member. You do need the exact tube type, it won't return results for the 4-125 if you enter in 4-125A.

    This article has useful hints on what the author did to get around issues that were discovered.

    Zack W1VT
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    thank you very much. I have noticed that searching the ARRL database is very finnicky.

  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Not sure I would build a amp just because I have tubes. Unless you bought them for your project.

    The 4CX250B may be a better choice.

    Not sure the power you need, but the handbook had a nice amp that used the 4CX250B.

    Enjoy your project. Don't forget the Fuse(S).
  5. WA4SIX

    WA4SIX Ham Member QRZ Page

    Well, this is one of those "Yeah, I have the parts & it will look nice" introductory tube amp projects. It may not get used much, but it will be fun. The tubes & sockets were free, not worth selling on Ebay, as they are cheap. This would be a starter amp for me in the tube catagory. Once done, I will move up in power & technology. I probably have 90% of the parts, outside of the power supply. For the supply, I will build one capable of high power & derate it for this amp. In the future, it would be usable for the larger units.

    I do understand steering me away from this project, as it may not seem economical to most. I just cleaned out an RF engineer's basement & have a 2X250 & an X578 (Experimental 4cx300a equivelant) for future projects.

    THank you all,
  6. KM1H

    KM1H Ham Member QRZ Page

    A pair of grid driven 4-125A's in push pull was a popular 2M amp in the 50's and they worked well on Class C AM and CW. The ARRL, CQ, W6SAI VHF and other handbooks had several circuits.

    I have a converted 130 mHz FAA AM (50W carrier as a linear) amp on 2M FM that has been running around 300W for decades. Id expect similar PEP output as a SSB linear. On AM you better blow a little air on them.

  7. WA4SIX

    WA4SIX Ham Member QRZ Page

    I am not looking for class C/FM. WHere I live in Utah, there is no real need for it. I would prefer class AB. SSB & if I happen to glean an AM contact would be the goals. I also have no real access to any books here, but will search Fleabay for them.

    They will be well cooled. Both of my solid state amps blow outside in the summer & inside (Through filters) in the winter for heat. I tend to overcool things.

    Well, I have been googling for hours. I haven't found anything for 3 tetrodes at VHF, so I guess it'll be a pair with a spare sitting on the shelf.

    Thank you,
  8. KA9JLM

    KA9JLM Ham Member QRZ Page

    In no way did I want you to steer away from your project, Sounds like you have a good plan.

    If building was economical no one would do it. :)

    Have fun with your project.
  9. KM1H

    KM1H Ham Member QRZ Page

    The only real difference there is the bias and maybe the screen voltage. the RF part hasnt changed.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I don't think you'd get three in parallel to work at 144 MHz.

    The Eimac 4-125A data sheet has a circuit for a P-P RF amp at 175 MHz that could be a good baseline for a dual-tube amp, with the input/output lines adjusted for 144 MHz. I'd think a pair would yield 500W output power or so.

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