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Inexpensive HV transformers for your HB Linear Amplifier

Discussion in 'Amateur Radio Amplifiers' started by N8FVJ, Dec 14, 2019.

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  1. N8FVJ

    N8FVJ Ham Member QRZ Page

    A HV linear amp transformer is very expensive. Peter Dahl transformers start around $350. Even a used Heathkit SB-220 that is only a 1KW input transformer sells for around $200 used. Transformers that are more reasonable in cost are Antek Transformers in NJ.

    The Antek AS-AT800 is 850 volts output and costs $105. It is rated at 960va on 60Hz power. Using a voltage doubler it produces about 2100-2200 volts under load and is excellent for a pair of 572B tubes. Placing the two windings in series makes about same voltage using a bridge rectifier. Transformer includes two 6.3 volt windings with enough current to operate a pair of 572B tube filaments.

    Another transformer is the AS-AT950 that is about 2000va rated on 60Hz power. It will produce about 2500-2600 volts using a voltage doubler or bridge rectifier with the two windings in series and will support a 1500 watts PEP output amplifier with a pair of 3-500Z or high power ceramic tube such as a 3CX1500. Transformer includes two 6.3 volt windings to produce 12 volts DC for accessories or can operate a 3cx1500 filament with a voltage dropping resistor from 6.3 volts to 5 volts.. It costs $195.

    Unless you can find a surplus transformer or a bargain from another Hams parts box, this is your best deal.

    Note- the transformers are rated on 115 or 230 volts input thus the output voltage is a little higher on the standard 120 or 240 volts input. The transformer are hi-pot tested above 3.5KV.
    Last edited: Dec 14, 2019
    K0UO likes this.
  2. K1LKP

    K1LKP Premium Subscriber QRZ Page

  3. KH6AQ

    KH6AQ Ham Member QRZ Page

    The AS-8T800 is rated 800 VA. In the usual capacitor input power supply it might be suitable for a 500 watt (output) ICAS amp. The 6.3 VAC winding is rated for 5 amps, which is enough for one 811A tube.

    Ameritron is a source for reasonably priced amplifier power supplies; simply copy their power supply. The AL-811H transformer is $106 and the AL-80B transformer is $180.
    Last edited: Dec 14, 2019
  4. N8FVJ

    N8FVJ Ham Member QRZ Page

    Wrong- The AS-8T800 is rated at 960 VA at 60Hz, not 800 VA at 50Hz as you posted. The USA uses 60Hz. It has two 6.3 volt @ 5 amp windings, not one 6.3 volt winding thus my post stating operating a pair of 572B filaments is correct. A 900 VA transformer will operate at 800 watts PEP out at 50% duty cycle. Your estimate at 500 watts is continuous carrier, not the more common PEP or CW output and the Antek transformer would operate 500 watts continuous carrier continuously. The AL-811H transformer is unfortunately only 1500 volts under load and who would want an 811A tube based amplifier. The AL-80B transformer is not as powerful as the higher voltage Antek I posted that will support a 1500 watt PEP output amp. The AL-80B transformer will not.
    Last edited: Dec 14, 2019
  5. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    I'm still scrounging for that Pole Pig
    AD5HR likes this.
  6. KH6AQ

    KH6AQ Ham Member QRZ Page

    Good points and it looks great for two 572Bs.

    Using the 20% increased 60 Hz VA rating the transformer is rated for 600 mA RMS. Let's see what a SPICE power supply model shows. The load is adjusted to maintain an average winding resistive loss of 46 watts.

    Running on-off keyed CW, alternating 5 minutes alternating on and 5 minutes off, the transformer appears to be good to run an amp at 1280 watts DC input. Given 65% DC-RF efficiency the amp can output 830 watts.

    Spice model
    240 VAC input, 1.4 ohms winding resistance
    1600 VAC output, 74 ohms winding resistance
    AC line impedance, 0 ohms
    Rectifier, fullwave bridge
    Capacitance, 30 uF

    100% duty cycle, brick on key operation
    Secondary winding current, 600 mA RMS
    DC load voltage, 2030 V
    DC load current, 300 mA
    DC load power, 610 W
    Winding loss, 46 W average

    50% duty cycle, on-off keyed CW operated continuously
    Secondary winding current, 848 mA RMS
    DC load voltage, 1970 V
    DC load current, 453 mA
    DC load power, 890 W
    Winding loss, 46 W average

    50% duty cycle, 5 minutes on alternating 5 minutes off
    Secondary winding current, 1200 mA RMS
    DC load voltage, 1880 V
    DC load current, 682 mA
    DC load power, 1280 W
    Winding loss, 46 W averaged over 10 minutes
    Last edited: Dec 14, 2019
  7. N8FVJ

    N8FVJ Ham Member QRZ Page

    My hat is off to you- you are very talented sir. How about a spice model on the larger AT-950? At 60Hz it is rated 1800 VA, but at 20% more weight vs other Antek 1500 VA units, assume it is 2 kVA.
  8. N8FVJ

    N8FVJ Ham Member QRZ Page

    The Ameritron AL-82 has a 43 lb Hypersil transformer that produces 3300 volts under load for $402. It is cheaper than a Peter Dahl 40+ lb hypersil. It would likely support an amp producing 2.5 KW PEP out.
  9. KD8DEY

    KD8DEY Ham Member QRZ Page

    But, The big question is:
    How does this translate into Dosey Watts?
  10. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    One point five to one.

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