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Alpha 78 owners

Discussion in 'Amateur Radio Amplifiers' started by W8NIC, Nov 16, 2019.

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  1. W8NIC

    W8NIC Ham Member QRZ Page

    I just acquired this nice amp and need help. Now this question might sound stupid to most so please humor me.

    What is the safe plate and grid limit for this amp? so far I drive it with 60w and puts our 650 carrier on the meter. It did a 1kw with 100w drive. I don't mind lower output so long as I don't harm the tubes by exceeding safe limits on the grid and plate. Does not have the low drive upgrade.

    Thanks in advance.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Hmmm, I didn't know the "low drive" was an upgrade; it's more like a simple modification, and I believe you don't need to any any parts to do it. Just some time with a soldering iron.

    The manual details safe max grid current; plate current, provided the blower is working okay and there isn't a dust build-up in the blower or tube fins, doesn't matter very much. I hope you have the manual!

    Max grid current is ~40mA, and if when applying drive and tuning for maximum output power you see the grid current exceeding that, either reduce drive or increase LOADing to make it go down. Plate current at full output can be close to 1 Ampere, but that's not really the thing to watch.
  3. W8NIC

    W8NIC Ham Member QRZ Page

    I downloaded the manual, you're right, it's a modification and not an upgrade. Hopefully something I can do myself.
    Now, with 70w in, I get 800w out with the plate current at .7 amps but the grid is only at 30ma. Is that ok or normal? With these figures, will it hurt the amp/tubes?
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Nope. You're running it pretty conservatively.

    As long as it's tuned up properly (keeping an eye on grid current) you can drive it with 100W and achieve 1kW or slightly more output power -- safely. Keep the grid current down to 40mA or less by LOADing it properly.
  5. W8NIC

    W8NIC Ham Member QRZ Page

    ok, with a 100w drive it does 1kw, plate current at 0.82 amperes and grid is at 35mA. are these numbers safe? based on your last input, this should be fine:) right?
    Thanks in advance.
    Last edited: Nov 17, 2019
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page


    Of course, if you have no spare 8874s you may want to use it at somewhat lower power to make them last longer. AFAIK nobody makes them anymore.

    But the readings you're getting were typical for the 78 when it was new.
  7. W8NIC

    W8NIC Ham Member QRZ Page

    yes, I drive it with 70w to get 800w, plate at 0.7 amps and grid at 30mA. I'm content with that.
    Thank you for your input and time, I really appreciate it.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    No sweat. It's a good amp. If not overdriven to excessive grid current the tubes seemingly last forever.

    I had an Alpha 374 (1970s, predecessor to the 78) with three 8874s, used it for ten years, sold it, and the new owner has been using it ever since and it still has the original tubes, and they still work fine.

    Don't ever overdrive it and it may outlive all of us.:p
  9. W8NIC

    W8NIC Ham Member QRZ Page

    ok good. 40 to 60w drive is all I'll need.:) thanks again.

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