3 4-125a for a 2 meter amp

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

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  1. KM1H

    KM1H Ham Member QRZ Page

    I doubt if you would even get two in parallel to work well on 2M so I didnt even speak to it. That Eimac circuit is real dated so I didnt go to that either, even my amp has a regular LC circuit for the input.

    You wont be getting 500W out in AB1, be satisfied with 300-350. I can get 450 on FM but backed it down to 400, I typed 300 yesterday by mistake
  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    I did a full search on the ARRL & found the original 4-125A 2 meter amp & a couple better renditions. I guess I will just follow what is on the latest & be happy with it. I know I can just get another Larcan RF deck & do limit, but have never done tubes & want to learn. There is a hamfest at Bryce Canyon this year & I will try to gather up what I don't have. Mostly power supply parts. I should have the RF deck ready by then. After I get it working, I will probably do a pair of 4-400s. While I could do ceramics, I like the looks of the glass envelopes.

    Thank you all,
    AI6KX likes this.
  3. WB3BEL

    WB3BEL Ham Member QRZ Page

    Maybe back in the 1950's - 1960's starting with the tube and building amp from there made some sense. Not sure about that even but possibly true...

    Today for most builders, if you have some parts on hand, you should start with the power supply and go from there. Not some cheap tubes. The power supply is the most expensive part for most tube amps now. If you have to buy power supply new you may want to look for some surplus parts. No sense building an amp around some on hand <$100 tube when the power supply transformer will cost $300 and up new.

    If you don't have any of the parts then you should wait to decide which way to go until you have the tubes and power system figured out together.
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    I agree on that.

    I am sure someone has some tubes to donate if needed.

    There is really nothing better than Homebrew.

    No one to blame but yourself, When you make that cool colored smoke. The smell is cool also.

    Have Fun. Use Fuse(s). Some of that old stuff used fuse wire, If you remember that, You may be a old Fart.
    Last edited: Jun 5, 2015
  5. W1VT

    W1VT Ham Member QRZ Page

    K2RIW (half wave) and W2GN (quarter wave) devised very popular two tube stripline designs for 4cx250bs. One might study those designs and see if they could be adapted somehow to glass envelope tubes. I think I even saw mention of a W2GN three tube design somewhere in DUBUS.


    Zack W1VT
  6. K9STH

    K9STH Ham Member QRZ Page

    In the late 1940s, 1950s, and into the 1960s, Motorola and General Electric used a pair of the 4-125 and the equivalent 6155 in their 250-watt output low band (30 MHz to 50 MHz) and high band (150.8 MHz to 174 MHz) FM commercial transmitters. However, the tubes were not in parallel but in push-pull. At 250-watts output, the tubes were loafing and lasted a long time. A number of amateur radio operators modified these amplifiers to change them from Class "C" so that they could be used as linear amplifiers for SSB.

    Glen, K9STH
  7. W1BR

    W1BR Premium Subscriber QRZ Page

    I agree with the above... you'd want to run two tubes, in push-pull, using parallel tuned plate lines; with variable link coupling for the output. No way would those work well in another configuration, unless single ended.

  8. N2EY

    N2EY XML Subscriber QRZ Page

    There's a reason you can't find any plans: It's not a workable design.

    The only way to use three tubes in such an amp would be to parallel them. Doing so would result in such high input and output capacitances that 2 meters would be unworkable. (This is why, when you look for 2 meter amps with tubes, there are so many that are push-pull rather than parallel.)
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    You could use 3 tubes.

    Just only connect the filament on the third one. Just for looks.

    Then tweak your Watt meter to read more RF out. Just for show.

    I run my SWR meter in Cal, and adjust the power out to what ever I want.:eek:

    I think the 4cx250b would be cheap tubes to play with.
  10. WA4SIX

    WA4SIX Ham Member QRZ Page

    I do truly appreciate all of your suggestions. The power supply will be capable of legal limit or more, but lowered to these tubes. That way, if I blow them, I have minimal outlay for the RF deck. Seems that I have pretty much all of the parts for the RF deck. It could later be easily upped to 4-250s or 4-400s.
    The power supply will probably end up being switchable for a few amps connected when needed. As I stated, I have HF & 6 meters covered with bipolar & Mosfets, respectively. (16 SRF-2072s for HF & a 6-MRF151G Larcan RF deck for 6 meters) I am just finishing up the Larcan deck this weekend.
    The tube amps are for learning & the pure beauty of them. As with my SS amps, I will include all possible modern safety curcuits.

    THank you,

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