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ETO Alpha 374A advice.

Discussion in 'Amateur Radio Amplifiers' started by N8ZI, Jun 11, 2018.

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  1. N8ZI

    N8ZI XML Subscriber QRZ Page

    I'm looking for advice on converting my Alpha 374A from a 2 to 3 hole. I already have more sockets, anode clips, silicone chimneys, six 8874's and a RF Parts RF Choke to remedy the 12M issue. If I'm reading the schematic correctly it appears a .01uf 50V disk capacitor, parasitic suppressor and strap for the supressor to choke is all I need. I have yet to pull the RF deck and cut a 3rd hole as a template would be an asset (if available). While the deck is removed the RF choke will be replaced and any other questionable components deemed necessary.
    If I did not have the six good Eimac 8874/3CX400A7's I would just mod it for the 3CX800A7 tubes. Many Op's just cut the the additional 3 hole and use a block off plate on the middle hole for the pair of 800A7's so either route I choose involves the RF deck removal and cut.
    Any thoughts or advice, before I dive into the deep end, would be greatly appreciated.
    Tom N8ZI

    Attached Files:

  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    It's nice to have the spare tubes, but I wouldn't do this mod.

    The two tubes can run legal limit output power; adding the third tube provides a bit more "headroom" but that's about all.

    For years, I had the original 374, which did have three tubes, and when the 374A came out it was actually a better amp.:)
    N8ZI likes this.
  3. KM1H

    KM1H Ham Member QRZ Page

    The last thing I would do is try and get 1500W out of a pair reliably!

    That is an old FCC rules 1000W INPUT amp on CW and 2K PEP on SSB, be happy with 1000W OUT on CW even if you can get 1200. The FCC had their thumbs where the sun dont shine when they made that SSB rule.

    For 3 tubes look at the Alpha 76PA/CA schematic and photos. I run a PA at 1200W and dont need any more.

    N8ZI likes this.
  4. N8ZI

    N8ZI XML Subscriber QRZ Page

    Primarily I'm a DX chaser with CW as my preferred mode. The 374A passband tuning is a great asset for me as I jump around the bands. Although the P out is less, being able to flip the band switch and start calling is like having an auto-tune amp :)
    The loss of power on passband is only 100-200 watts vs manual tuning which really isn't that much yet it can help in the pile-ups. This is why I was considering the 3 tubes, to get the same power out on passband as I do now with manual tuning.
    The 76PA was a consideration as one just changed hands on eBay for $900 + S&H but I didn't toss in a bid.
    I also have a nice project amp based around the 3CX400A7 or 3CX800A7 triodes i would like for use on 2M EME and even 6M DXing. A pair of extra tubes can be used in that amp leaving my remaining 2 tubes as spares. A KW+ would be fun on 2 & 6 with my pair of 17B2's and the 5 element 6M beam I built.
    Is there a "Ham Help Group" for Op's like me that always want more, more MORE??
    KD2ACO likes this.
  5. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    Sadly, there is no cure.
    N8ZI likes this.
  6. KM1H

    KM1H Ham Member QRZ Page

    Get one of the late production LK-800C or 800TNY. Comes with three 3CPX800A7's and the 3000V+ LK-550 transformer. Some had a tapped transformer for 3-500Z and 3CX800's so you can go to the 3CPx and still switch to "LOW" power.

    Ive built up several for HF and 6M over the decades and 5 out is possible if you dont mind replacing them fairly often as they lose emission.

    N8ZI likes this.
  7. N8ZI

    N8ZI XML Subscriber QRZ Page

    With three 3CX800A7's one can create their own propagation! They are some awesome triodes and the most recent sheet lists very conservative power out specs. The original data sheet (correct) that Eimac published raised some concerns with the FCC so a few months later they issued another one and cut the power figures down. The specs were 1370 watts out with 36w in from a single tube! That far exceeded the FCC rules :) See the original specs Eimac published in the attached picture.
    The GU-74 tetrode are another workhorse with impressive gain figures if one doesn't mind running another tap for the screen.

    Attached Files:

  8. KM1H

    KM1H Ham Member QRZ Page

    Im very much aware of the 3CX800 history and have an original of that "preliminary" spec sheet as I was doing some consulting work for Amp Supply at the time.

    The GU-74B is a decent tube at its original Russian specs but the 4CX800A version is a complete fraud orchestrated by George Baxter W6TC of Svetlana USA and formerly Eimac. It is a 600W tube and loses emission rapidly when run beyond specs.
    I also have copies of the actual engineering tests from Svetlana in St Petersburg showing how poor the IMD really was.

    K4FX and N8ZI like this.
  9. N8ZI

    N8ZI XML Subscriber QRZ Page

    Good deal on the 3CX800A7 and Amp Supply knowledge. Some of my friends run Amp Supply equipment and are quite satisfied with it's operation.
    If I'm going to become more educated in this hobby I prefer to learn from those who know, not those spewing BS hoopla hi hi. I like to meet Amateur Radio Guru's who know what they are talking about. A far cry better than those who merely repeat the same misinformation another wannabe told them. I'm still dumbfounded by the lack of intelligence exposed when Operators claim to run an amp on 240v instead of 120v so the electric bill will be half as much !!
    The parasitic supressor debate is another battlefield some well known Hams have drawn blood over.
    I think you were referring to George Badger W6TC instead of George Baxter. I know a little history on the first but absolutely nothing on the latter.
    73 Tom N8ZI
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  10. KM1H

    KM1H Ham Member QRZ Page

    Year, Badger.

    The best battleground by far was the original AMPS email forum in the 90's back when several real engineering hams and university professors tried to keep Rich and Tom in line as neither knew much and never shut up.
    K4FX and N8ZI like this.

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