120/240 vs. 120/208

Discussion in 'Amateur Radio Amplifiers' started by W2RWJ, Sep 4, 2011.

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  1. W2RWJ

    W2RWJ Ham Member QRZ Page

    Power supply three-phase (North America):
    INPUT POWER: 230 V between phases
    FUSING: Three 30 A normal acting fuses on the rear panel
    I wish our club could afford it - But it looks like we would be right back where we are now......
  2. K0BG

    K0BG Ham Member QRZ Page

    Well, it was a tongue in cheek response. I happen to know the DX-4 will putout almost 6 kW using the 3 phase power supply.
  3. W9GB

    W9GB Ham Member QRZ Page

    Purchasing custom transformers for HF amplifiers used at this Wall, NJ locaton is a FOOLISH (and Expensive) solution, WHEN a $1500 retail solution at 7.5 kVA is being sold
    as surplus for $300 .. AND it will solve ALL current and future problems for a NEMA standard 120/240 VAC, 20 to 30 amp circuit at the operating position.

    GE Transformer 9T21B1029G02 208 VAC to 120/240 VAC at 7.5 KVA
    eBay Auction # 290524196977
    eBay seller: ccub243
    Price: $300 USD
  4. NM7G

    NM7G Ham Member QRZ Page

    I didn't catch it if someone said it, but you clearly do not want your QRO amp filament voltages below spec.. That would cost the club a bundle of cash for PA tubes, and continue to do so.
  5. WA7KKP

    WA7KKP Ham Member QRZ Page

    120/208 is defined as a 3 phase source -- 120/240 is a single phase source, common in homes only. If you need 240 volts and only 208 is available, you can wire a 120 primary / 32v secondary, and wire the secondary as series aiding, which will give you 240 volts. This technique is called using a buck/boost transformer. 28 volt transformers at the required current are quite common, and very small considering the job they do.

    Gary WA7KKP
  6. KM1H

    KM1H Ham Member QRZ Page

    A drop from a 240 to 208 will absolutely kill the tubes early.

    Since the RF power output will only be slightly affected why not leave it alone and just use a boost transformer for the filament? This can be wired into any amp using a transformer with all voltages or a seperate filament transformer. A simple 24V @ enough secondary amperage/wattage for the tube is all that is needed to get the tubes into spec. Wire across one of the transformers 120V primaries or between one leg of the 208 and ground since its a 4 wire circuit.

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