4CX300A ceramic tetrode

Discussion in 'Amateur Radio Amplifiers' started by K0NXM, Oct 30, 2018.

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  1. K0NXM

    K0NXM Ham Member QRZ Page

    Hi all,
    I've just discovered that 4CX300A tetrodes are dirt-cheap and plentiful, compared to other ceramic tetrodes, and it seems like 2 would more than handle enough power for a homebrew amp. Is there something I don't know that makes this tube a bad choice? Thanks in advance,
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page


    They should not be operated grounded-grid, they're intended for grounded-cathode, grid-driven. The sockets may be expensive, since they're breech-lock type and I think only Eimac made them.

    They have enormous gain, so a 1kW output amp using two of them only requires about 5-10W drive (max); you may need a way to attenuate the drive power if you have a 100W rig.

    My "first" 2m amplifier which I built back in 1967 used a pair of these, and I got the sockets for free from a local "Elmer" who was an engineer for Western Electric and had a 100% homebrew station. He just gave them to me (thanks, Neal W2KME). It ran 1kW output power (CW) on 144 MHz with 6W drive.
    K0NXM likes this.
  3. K0NXM

    K0NXM Ham Member QRZ Page

    I see now that SK700 sockets are available for $95 used, from one source. Not too horrible, but for HF use, I'll bet a homebrew clamp setup would be more than sufficient. What makes a tube like this unsuitable for GG operation?
  4. K0NXM

    K0NXM Ham Member QRZ Page

    Even better yet - further down on the very same page are listed SK740 sockets for this tube, at $24. They don't have the internal bypass caps.
  5. AI3V

    AI3V Ham Member QRZ Page

    July 1967 qst has an article by Bill Orr about using a 4cx300 in a "super cathode drive" configuration (some negative feedback)

    He got a amplifier with modest gain, about 5 times the power (75 watts in 375 watts out) but with a third order imd of about -46 db.

    K0NXM likes this.
  6. K0NXM

    K0NXM Ham Member QRZ Page

  7. N6UH

    N6UH Ham Member QRZ Page

    I used a similar setup in the 1980s to build a 4cx1000 grid driven amp. It was easy enough to fab some spacing and add discrete caps in the gap. Great find at that price, worth snagging a couple for stock!

    K0NXM likes this.
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    That operation allows cathode drive but since you still need grid and screen bias supplies, it doesn't save any effort in amplifier construction.

    The 4CX300A is quite a good tube when operated as designed, if you have a surefire way to limit drive.

    Even though I had a low-power exciter at the time (~15W from a 6360), I always had a 6 dB resistive pad between the exciter and the amplifier, so it was virtually impossible to overdrive the amp; ran 800W output with grid current indication of "nothing.":)
    K0NXM likes this.
  9. WA9SVD

    WA9SVD Ham Member QRZ Page

    Bill Orr wrote an application note for EIMAC on the subject. (Do a "Google" serrch on EIMAC.) The 4CX series (tetrodes,) by design, are high gain tubes, with very low amount of grid dissipation. Running them in GG will burn them out in VERY short order. (Seconds.) Many years QST offered a design with two 4CX250's in GG, and even the author cautioned about long periods of operation. Of course, the tubes, (mil surplus) were only about $10.00 each, so burn-out wasn't a huge problem. NOW, with 4CX250's running $100.oo + each, they are no longer "toss them and replace" on a frequent basis.

    I'd be curious where you found 4CX300's cheap and plentiful. I have two (with sockets) that I'm saving for either 6M or 2M.
  10. WB5WPA

    WB5WPA Ham Member QRZ Page

    47 bucks for 1 here -

    A set of 4 tubes -

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