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Alpha 374A or 76 series power switch

Discussion in 'Amateur Radio Amplifiers' started by AD0AC, Sep 8, 2019.

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  1. AD0AC

    AD0AC Ham Member QRZ Page

    Yeah. I think before any money is spent on this amp, especially on HV power supply capacitors, I'm going to have the tubes tested. Granted, a 3CX800A7 conversion could be fun, it's still a 40+ year old amp. Seeing the condition of the inside of the amp makes me NOT want to test the tubes in our good 374, although I may be overly cautious. When I repaired the Henry 2k, the two 3-500ZG tubes were only $350 shipped, and I think I had about $50 in relays and other discrete components aside from my time.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I agree, unless you have spare "known good" 8874s or some remarkably cheap source for them (you can still buy "new ones" from Richardson/Eimac -- for like $1000 each!), I wouldn't sink much into repairing an old 374A before I determined if the existing tubes still work.

    They did have protection circuits which helped, but you can fry grids really fast by mistuning (insufficient loading) and it happens faster than you can say "Oh crap.":p
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    BTW, I should note with that particular amplifier I wouldn't bother "tuning up" by looking at plate current, at all. Those tubes have very healthy cathodes and very large and super healthy anodes which can take way more current than the amplifier's power supply can provide. The fragile element is the grid. I had my 8874s last "forever" (I sold the amp, but the second owner is still using the original tubes, 40 years later) by tuning up only observing output power and grid current. When you tune through resonance, grid current exhibits a large peak (same a plate current exhibits a dip, and at the same tune point) which is easier to see than a plate current dip -- it's more noticeable, especially when the amp is fully loaded.

    The grid current peak will occur at the same point as maximum output power; then, continue "loading" for full output, re-peaking the PLATE TUNE for a grid current peak but keeping the Ig very low. If Ig climbs up, add more LOADing.:)

    The 374 also had "bandpass tuning," where you click to that position on a band and you didn't have to tune anything -- but that only worked with a nearly perfect 50 Ohm load, and if you overdrove the amp the grid current could still be excessive -- so it pays to always watch that.

    The Ip meter can run up to the "pin" and it hardly matters with those tubes. They don't fail due to that, especially in the 374A where the tubes are cooled by a powerful blower -- this was a better solution than the original 374 which used only an axial fan and ductwork to do that job.

    Tetrode 8122s are very similar, although they have a screen grid; with those, only grid current and screen current were important, and proper tuning involved looking at both. Plate current didn't matter much with those, either.:p

    With the Alpha, I could plot drive power vs. output power on a piece of graph paper and it would be a straight line up to about 1300W output power, then bending slightly to the right (towards compression) above that. I'd keep it in the "straight line" range, which was usually very, very low grid current.
    AD0AC likes this.
  4. AD0AC

    AD0AC Ham Member QRZ Page

    Good info! Thanks!

    I just tested the relays again. It didn’t feel right that they all failed. So I used a power supply with a bit more oomph at 20v (drill battery ) and all three clicked on and off. I think there’s still a supply issue, but at least the relays are good.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Check the low voltage power supply.

    Might be a $1 repair.
    W9GB likes this.
  6. W1QJ

    W1QJ Ham Member QRZ Page

    Bad high UF cap
  7. W9GB

    W9GB Ham Member QRZ Page


    The CONTROL BOARD for the Alpha 76A and 374A amplifiers sits above the Capacitors.

    The 24 Volt wiring is at the far edge of that Control board (photo above).
    The 28/24 volt rail is used for lighting, relays and bias circuit for the 8874 triode tubes.

    Alpha had a Service Note in early 1980s about three (3) resistors (R115, R116,R117)?
    This was a 1980 problem for an Alpha 76A ... that was missed by many techs (including Alpha).
    There was a hairline fracture of a 1/2-watt resistor, on the control board.
    Changing that resistor to a 1 watt resistor resolved heating issue.

    The 1/2-watt Resistors should be checked and one of these should be increased to 1 watt.

    Alpha RF Systems 374A Manual
    Last edited: Sep 24, 2019
  8. AD0AC

    AD0AC Ham Member QRZ Page

    Thank you! I'm going to try to get this reassembled in the next couple of days. My son has been ill this week and I haven't wanted to work on it late at night when I'm tired. I'll check out those resistors and replace the 470uf cap on the LV supply. I think I have all of those parts in my collection, definitely the capacitor. If not, we have a well-stocked supply house nearby for everything else.
  9. AD0AC

    AD0AC Ham Member QRZ Page

    Well, I wound up checking all of the resistors last night and replaced the cap in the LV supply. Reassembled everything and still dead. K1 does not react when the ON button is pushed, and it's directly connected to the mains through the interlock on one 120V leg. The interlock tests OK, the fuses are OK, so something else is going on.

    I'm going to remove the transformer again and test for line voltage at the fuses, then at the interlock. I checked the line cord for continuity and it was fine, but maybe there's something going on with the mains voltage connection. The cord IS a little different than the one on the Henry. The Henry uses a 6-15P plug, and the Alpha has a 6-20P. My outlet is a 6-20R, so it should support both plugs.
  10. W9GB

    W9GB Ham Member QRZ Page

    Alpha RF Systems 374A Manual
    There are Seven (7) Relays in the Alpha 374A.

    Sounds like your Power-ON Relay (K1, GUARDIAN 1390-2C-120A) is NOT engaging.
    CHECK Resistors R3 (750 ohm, 2 watt, 10% carbon comp.), and R4 (10K ohm, 12 watt, wire wound)
    It is buried at bottom along right side front panel, LEFT of the Capacitor Bank.

    As long as your 240 VAC outlet is a “T-Style” NEMA 6-20R,
    it will accept the NEMA 6-15P and 6-20P appliance plugs.

    NEMA 6-20P ; NEMA 6-20R (T-Style).
    DOUBLE CHECK that the Previous Owner Did Not Change Factory Wiring.
    Some radio amateurs keep trying to operate 1 kW Plus HF Amplifies with 120 VAC outlets.

    8874 Grid Protection Circuit
    by K6XX
    Last edited: Oct 3, 2019

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