SB220 transformer mod...

Discussion in 'Amateur Radio Amplifiers' started by KD2DYY, May 25, 2020.

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  1. KD2DYY

    KD2DYY Ham Member QRZ Page

    Hi Folks... I was reading on the HV modification done to the SB220 by "Lee" (ZL2AL) and would like to try this project this winter....Has anyone else tried this mod and is there any schematic write ups on the section pertinent only to the HV section including what Lee did with bias voltage...
    I would need the exact toroid transformer model number.(hopefully still in production)
    I visited Lee's page and was saddened to see he is now S-K...
    Any help with this proposed mod. would certainly be appreciated... Perhaps by someone(s) that have tried this..

    Mike (KD2DYY)
  2. W1QJ

    W1QJ Ham Member QRZ Page

    I think you are referring to the Avetek(sp) toroidal transformer replacement. Yes it’s been done by several and the results are favorable. If makes sense and the price is reasonable, there is a bit of modification to make it fit but it’s minor. The bias mod is quite simple it’s just a matter of moving the resistor location on the relay and disconnecting the 120vdc wire. Takes a couple of minutes to do. Someone will probably come along with the transformer part number from the manufacturer.
    W1BR likes this.
  3. W1BR

    W1BR Premium Subscriber QRZ Page
    AC0OB likes this.
  4. KD2DYY

    KD2DYY Ham Member QRZ Page

    Thank you Lewis.... I did visit your page.... Very impressive... I think you might be familiar with the SB220... Ha!
    Very good to hear that those who have tried this have had good results....I have installed a vacuum variable cap for tuning and that worked flawlessly..
    Now to see If I can get just a little more output...
    For Peter.... I am a little confused .. Lee(ZL2AL) specified an 800va transformer..(Not 1500va).. I checked out that list and the 8T650 would be more than adequate..
    Seems I would be pushing the class B push pull by some voltage but the tubes will take PEP ok ???? Yes , that is a question... What were you referring to in the 1500va
    selections??? I was going to try and keep the voltage doubler circuit in PS..
    Thank for the great help so far.... Again, this is the info gathering stage and I have some time for the cold wx to start back up in the NE....(Maybe)

    Mike (KD2DYY)
  5. W1BR

    W1BR Premium Subscriber QRZ Page


    This one, with the two 950 volt secondaries paralleled, then the voltage doubler.
    The smaller one below is the 800 VAC dual secondary version, a bit light for a SB-220.
    The larger one is a for a four tube 813 amp, the other is for a smaller dual 813 amp
    under construction.

    Last edited: May 26, 2020
    AC0OB likes this.
  6. KD2DYY

    KD2DYY Ham Member QRZ Page

    Peter..... I'm missing something .... If I install the transformer you recommend it will result in ......950vac doubled to 1900vac then filtered X 1.41 equals
    2679vdc.....What I have now for output on the original power supply is 2300vdc low (cw/tune) and high 2800vdc(SSB)..... Seems like I will be LOOSING
    peak high voltage.... Is there something I am not realizing???My original intent was to try and increase RF output... Lee's approach(ZL2AL) was using the Antek
    AS-8T650 and that would give me 3600+ dc.... IF I serialed the secondary HV windings...
    I very much appreciate your suggestions but think you are talking about just replacing the original HV transformer.(That is running fine).... I have two sb220's.... By the way my call out of
    final dc voltage was done with a Triplet 630 and both SB220's agree voltage wise...
    Please get back to me with you thoughts Peter......

    Mike (KD2DYY)
    W1BR likes this.
  7. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    AS-15T950 Parallel Winding: For a FW voltage doubler - 950VacX2.83 = 2,689 volts DC; Loaded output voltage is ~ = 950VacX2.3 = 2,185V DC@1.4A.

    AS-8T650 Series Winding: For a FW voltage doubler - 650VacX2X2.83 = 3680 volts DC; Loaded output voltage is ~ = 650VacX2X2.3 = ~ 2,990V DC@0.66A.

    If you want >, = 0.8A capability the second transformer won't cut it.

    Last edited: May 27, 2020
  8. W1BR

    W1BR Premium Subscriber QRZ Page

  9. W1BR

    W1BR Premium Subscriber QRZ Page

    they're not many options for a replacement SB-22o HV transformer. I'd forgo the CW
    option and concentrate on airflow and a transformer that has some reserve.
  10. AB2NI

    AB2NI Ham Member QRZ Page

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