Transformer power rating VA - Elmers Forum at eHam.net

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Transformer power rating VA	Reply
by N4ZOU on October 21, 2002	Mail this to a friend!
I found a HV transformer rated at 510 VA with 220 volts input and 1005-0-1005 output. This would give me 2010 vac output to a bridge rectifer and then a capacitor filter bank. What would be the power rating of the transformer? I want to use it for a single 3-500Z amplifer. Will this transformer handle it?

RE: Transformer power rating VA	Reply
by WB2WIK on October 21, 2002	Mail this to a friend!
It's marginal, but give it a try. 510VA is 510VA regardless of what you do with it. At 510VA rating, its secondary current rating is 510/2010, or 253mA. A full-wave bridge driven by the entire secondary will yield about 2.8kVdc, which isn't a bad anode potential for a 3-500Z (3kV is just about perfect). If you use sufficient filtering, e.g., 56uF provided by a string of eight 450uF, 500WVDC capacitors, equalized and bled by eight 25K, 10W resistors, you'll be dissipating 39W continuously in the bleeder string, leaving about 471W available for the 3-500Z. Using this transformer, you may or may not be able to run a 3-500Z at "full" power, which is normally about 1kW PEP output power -- the transformer impedance is likely too high to achieve that. However, by "VA ratings" alone, it's difficult to tell. I'd build it, and give it a try. If the voltage drop under load is excessive for key-down operation, it may still be good enough for SSB or CW (low duty cycle) work. The 3-500Z's a pretty forgiving tube and will deliver 600-700W PEP output on a fairly lousy power supply, even one that sags down below 2500V. WB2WIK/6

RE: Transformer power rating VA	Reply
by CURMUDGEON on October 22, 2002	Mail this to a friend!
Consistent with what you already know about the transformer, if it doesn't weigh 25-30 lbs or more, forget it.

RE: Transformer power rating VA	Reply
by N4ZOU on October 22, 2002	Mail this to a friend!
The transformer does weight 30 lbs but it's in a steel case and filed with some type of sealing material. I might look into using a diffrent tube than the 3-500Z. I could try using 2 or 3 811A tubes. I plan on using it only on 160 meters and 500 to 600 watts would be plenty.

RE: Transformer power rating VA	Reply
by WB2WIK on October 22, 2002	Mail this to a friend!
The 3-500Z is a far better tube than even four 811A's are, in terms of nearly every characteristic. I'd stick with the 3-500Z if possible. However, if you do decide to use 3-4 811A's instead, you'll have to use a full-wave center-tapped power supply and not a full-wave bridge, or you'll exceed the plate voltage rating of the 811A's. WB2WIK/6

RE: Transformer power rating VA	Reply
by M0BOL on October 22, 2002	Mail this to a friend!
It's worth noting that many commercial amplifiers intended for amateur ssb or cw service use rather small HT transformers, for perfectly acceptable economy reasons! typical 1500W pep amplifiers use transformers of no more than about 800VA rating. For SSB or cw service, your transformer will run a single 3-500 at very nearly all the linear output it is capable of, which for good linearity should be limited to about 800W pep anyway. Transformer ratings are generally dictated by heat limitations, and therefore you are interested in average power consumed from the transformer over quite long periods of time. The average power taken from the HT by a single 3-500, even handling heavily processed ssb, at about 800W pep, is suprisingly low! I would be more concerned with the physical condition of the transformer in question, is it in good condition? primarily, is the insulation in good order? if so, go ahead and use it, it will run a 3-500 in ssb service forever! 73 Rob M0BOL

RE: Transformer power rating VA	Reply
by WA9SVD on October 23, 2002	Mail this to a friend!
Hi, Only one comment on safety factor. The bleeder resistors! Resistors have maximum voltage ratings, and you don't want to exceed that rating. Check that the voltage rating is not exceeded, even if it means using two resistors (in series) across each capacitor.

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