Drake L7 ps upgrade

Discussion in 'Amateur Radio Amplifiers' started by W1CJF, Mar 15, 2018.

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  1. W1CJF

    W1CJF Ham Member QRZ Page

    I just purchased a nice drake L7 with emiacs tubes. It has the harbach upgraded board in it. I am looking into getting the Peter Dahl upgraded transformer for the power supply and wondering if anyone ran the upgraded one. I'll be doing a lot of AM so that was one reason and the stock one is good I would just like to have a little overhead for am. I'll have to get it built but seems like a good investment for these amps. Thanks

  2. WB1E

    WB1E Ham Member QRZ Page


    2400 volts @ 800 mAmps is what Drake states in the manual as full voltage & current. Dahl does make custom builds, don't they have something already?

    Mine is coming along, but yes, the HV Xmf may be temporary. The MFJ AL572 replacement transformer ships for less than $200 and comes with 2800 @ 800mA. I used one for my Cliperton L. Also has a 12V & 6V (6V with CT) windings. Has some buck-boost primaries even. http://www.ameritron.com/Product.php?productid=406-1572

    The CJ plug is full up however.

  3. W1CJF

    W1CJF Ham Member QRZ Page

    This is the one from dahl... aka hammond. I didn't know mfj also made one. 1170/870 vac. I just want 3k+ on the plates to make them tubes come alive.

    Attached Files:

  4. WB1E

    WB1E Ham Member QRZ Page

    I read this list today, looks like PD makes a dozen different transformers for the L4B! You'll find it down the list here http://www.pwdahl.com/PWD_HamCatalog.html



    PR) 117/234 VAC 60 HZ WITH HI-LO TAPS

    S1) 1700/2300 VAC @ 1.0A CCS

    DM) HT = 8.375 WT = 8.075 DT = 7.688 MD = 3.937 MW = 5.500
    PRICE) CALL or e-mail Plus S&H FOB Buffalo, NY WEIGHT) 46 LBS

    Good luck !
  5. W4DRA

    W4DRA Subscriber QRZ Page

    I just rebuilt my PS for the Drake L7. After the bigger transformer from Heybour in Mi.
    It now supplies a full 3200v in CW and 4000v in SSB. This thing is a 1.5 amp. I also rebuilt
    the Harbach board with 600v 220mF caps with 5 watt resistors along with 6 amp diodes. I brought the new
    3-500zg's to life. I get full legal limit on CW position with 65 watt drive.
    HOWEVER I get a pretty big voltage drop on the Plate still. It drops about 400-500 volts just keying
    the amp with no RF. Anyone have any idea why this is so much?
    My supply line voltage stays at 235v with no drop. The AC voltage out on the transformer drops about
    25-30 volts before going through the caps and diode rectifiers.

    I have a big metal cabinet for this but wanted to ask before I mounted the transformer @ 57lbs. so as not to
    have to pull it out again. This really has me stumped and don't know where to go from here.

    Any help will be greatly appreciated.
  6. W1QJ

    W1QJ Ham Member QRZ Page

    400-500 volt drop with just idle current is awful regulation. I would expect a 500 volt drop at full power output, but not at idle current. If your line voltage is not sagging you have a problem someplace. a bad filter cap perhaps? That needs to be looked into. A big heavy duty transformer like that should not sag that much. Also, due to the odd configuration of the Drake transformer primaries it is not a "walk in the park" to retrofit an Ameritron transformer unless you completely understand what you are doing. Drake does not had a typical primary configuration.
    W4DRA likes this.
  7. W9JSW

    W9JSW Ham Member QRZ Page

  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    At 1500W output and about 50% efficiency (a little better at low frequencies), expect to pull close to 3000W on peaks. Heathkit and others got by with a smaller transformer because the intended use was SSB (about 25% duty cycle) and Morse (about 33% D/C). They relied on sufficient C in the filter to supply peak current. It works but, forget about running any high D/C modes.

    Bryan WA7PRC
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    According to the catalog, it can source up to 2570 Vac @ 0.8A (2056 VA).

    I looked at that but, when I realized I had a much better transformer (about twice the VA) that needed only rewinding, I spent about 1/4 the cost of that Ameritron unit. It was fun, too. :)

    Bryan WA7PRC

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