Electrical substation transformers do parallel secondary windings.
There are circulating currents due to imbalance.
Normally the transformers are the same design, made at the same factory.
Tap changers set the same, Auto tap changers in manual, etc.
and we check for SYNC before doing the parallel...
Makes a nice hum.
well, not an amp subject but paralleling secondaries is done in industry.
It's up to the owner if you do it.
GL! Robert W5AJ
On Mon, Oct 27, 2025 at 8:50 AM Michael Tope <W4EF@dellroy.com> wrote:
> I was curious about this so I did a real quick simulation in LTspice.
> When you parallel transformer secondaries with a small imbalance in
> secondary voltage, there is an inverse relationship between current
> imbalance and the winding resistance. The secondary voltages have to
> match at the terminals where they are wired in parallel. This will force
> the transformer with the slightly higher secondary voltage to carry more
> current in order force enough voltage drop in its winding resistance to
> make its terminal voltages match its lower voltage counterpart. If there
> were a 40V rms difference in secondary voltage between the two
> transformers, the rms current difference would have to be
> 50V/[R_winding] (assuming here R_winding is the same in both to make the
> math simpler). If R_winding were 40 ohms, then the current difference
> would be 1 amp rms.
>
> My take away from this simple exercise is that you can't generalize as
> to whether or not paralleling secondaries is an acceptable practice. You
> have to do the engineering. If the secondary voltages match closely
> enough and the winding resistance isn't too low, it will definitely
> work. To Steve's point manufacturing tolerance, if one of those 5300V
> transformers in the 12KW CCS amplifier dies at some point, the
> replacement will need to be carefully checked to make sure it matches
> the surviving transformer well enough not to cause excessive current
> imbalance. However, I don't know offhand how much sensitivity there
> would be to core permeability difference between two transformer builds.
> I think core permeability mostly impacts magnetizing current (i.e
> winding inductance). As long as the flux leakage between primary and
> secondary is low (coupling factor, K, close to 1.0), I would think that
> voltage matching would mainly be a matter of getting the turns ratios
> the same. Note, I am choosing my weasel words carefully here since I
> have never actually designed a plate transformer.
>
> Jim, do you have data for the winding resistance of the 5300V transformers?
>
> 73, Mike W4EF....................
>
>
>
>
> On 10/23/2025 9:16 AM, jim.thom jim.thom@telus.net wrote:
> > Date: Wed, 22 Oct 2025 17:58:08 -0700
> > From: Steve Harrison <k0xp@k0xp.com>
> > To: amps@contesting.com
> > Subject: Re: [Amps] Paralleling plate xfmrs.
> >
> >
> >
> >
> >
> >> Actually, since you need the full current available from BOTH
> >> secondaries, the whole issue is moot since you won't get that from the
> >> secondaries connected in series, for your application.
> > #### You can't run 120 vac into the 240 vac winding.( 240 vac primary's
> in
> > series with 240 vac applied). You will starve the input. You can only
> > go so low or high on the primary of a xfmr. With 120 vac applied to the
> 240
> > vac input of a xfmr, the magnetizing current will drop to zero every
> time.
> >
> >> Both Asamoto and also Henry radio paralleled plate xfmr's (both pri and
> >> sec) on their RF generators and also FM broadcast PA's. Some of the
> FM
> >> broadcast PA's used 3 phase supplies, either 208 3 phase, or 360 vac 3
> >> phase. On their single phase versions, 2 x identical plate xfmr's
> were
> >> used....again with pri in parallel..and sec's in parallel.
> > I'm not aware of those.
> >
> > ## where have u been for the last 40 years ? There are thousands of
> henry
> > RF generators that use 2 x identical parallel plate xfmrs.
> > Charlie Asamoto, who used to work for Henry radio, started his own
> > business, called CE.... or creative electronics...... and he too used
> > paralleled xfmrs. Later on, the name changed to the current ASI.
> > amplifier systems inc.
> >
> >
> >
> >> It's extremely poor engineering design to connect transformers in
> >> parallel and expect them to share output current precisely, as there is
> >> always some imbalance between them. Whether or not it matters depends
> >> entirely upon just how much imbalance there is. If it were done
> >> professionally, I would expect to find something in series with each
> >> secondary winding which would absorb any imbalance between them, such as
> >> a large power resistor or even an inductor.
> > ## None issue. And no, you don't want to insert ANY resistor or
> > inductor in series with one / both secondary windings. Folks pay HUGE
> > bucks to have a xfmr wound, with a low secondary DC resistance...esp on
> > high C, B+ supplies. You put any resistance in the sec, and u will
> screw
> > up the regulation real fast, asap. You can test it yourself, or
> > simulate it in software, results are the same.
> >
> >
> >> Perhaps: I've never seen those high-power amps but if I did see that
> >> being done, I would seriously question the competence and aforethought
> >> of their "engineer". What happens when a shorted turn occurs on either
> >> transformer?? (And you know as well as I do, that happens all the
> time!).
> >> It might not be catastrophic initially; but it puts more stress on the
> >> pair, which inevitably builds up.
> > ## Buy a quality xfmr, and you won't get shorted turns. There is
> magnet
> > wire..and there is magnet wire. Magnet wire, round, square rectangular,
> > will have both a V rating, and a temp rating. (square and rectangular
> wire
> > have the corners rounded off). I have polyimide magnet wire in 8 ga,
> > rated for 15 kv. That's 15 kv to metal, and 30 kv between adjacent
> > turns. Mine is 200 C rated. Polyester Polyamide is rated for 225 C.
> > Dahl did not go cheap on the pri and sec wire. Dahl rates his plate
> > xfm's for a 55 deg C temp rise over a 40C ambient. That's 95C..... then
> > add another 10 C...cuz deep inside the core, it gets hotter.
> >
> > ## Ok, now it's up to 105 C max. Class A insulation is rated for
> 105C. U
> > can also get Class B-C etc. Dahl used very high temp rated wire.
> >
> > ## BUT if u are just looking at magnet wire on ebay, chinese vendors,
> > 'brand X', etc, V rating will be as low as 300 V.... and temp ratings
> > as low as 60 C. You don't see quality xfmrs with issues. What
> > happens with low temp ratings is....... the insulation eventually breaks
> > down over time....and then the V rating goes down with it.
> >
> >
> >> I went through all this with Dahl himself.
> > That would be really surprising, and distressing, considering his
> > reputation otherwise.
> >
> > ## I spent a LOT of time with Dahl on this subject. At the time, I was
> > specing xfmrs for other folks, so it had to be done right. If dealing
> > with any xfmr maker, and wanting a custom xfmr wound....folks will spec a
> > pri V, various pri taps, then a sec V..and also sec taps..and a 'current
> > rating'. Unless u spec ambient temp, allowable temp rise over ambient,
> > insulation u want, V rating of wire u want, etc, etc...... u have no clue
> > what u will get.
> >
> >
> >> Xfmrs are made to an exact winding spec. XX turns on the pri...and YY
> >> turns on the sec. Why u think there is gonna be a V difference between
> > em
> >> is beyond me.
> > So be it. K9YC has carefully documented differences in ferrite
> > transformer cores that were supposedly manufactured at the same time,
> > from the same batch of raw materials. I fail to see why there would not
> > be similar differences found in low-frequency magnetic material.
> >
> > ### I know K9YC very well..and for a long time. Here's where your
> > analogy falls apart. Fair Rite moved it's plant to CHINA, then it all
> > went to hell in a handbasket....and fast. N3RR bought 700 (seven
> > hundred) of those FT-240, type 31 cores (2.4" OD) that K9YC was
> using
> > for his original CM choke cook book. And they all came from the same
> lot
> > # and batch number.
> >
> > ## what N3RR found was the tolerance was all over the map ! He ended
> up
> > using a simple 1 turn link winding, and measured the uh of each of the
> 700
> > x cores....and numbered em all from 1-700. And it's not like most of
> em
> > were in the middle, with a few flyers off to each side. They were
> evenly
> > spread from -20% to +20% ! How the hell are you supposed to make a CM
> > choke, or in his case....several, with Z results from A-Z. By
> grading
> > em 1st..at least he could enter em into a master spreadsheet, then play
> > 'matchup'. IE: take equal numbers of high and low tolerance cores to
> > arrive at a center point, that was acceptable. If he had not done that,
> > it woulda been a mess.
> >
> > ## Hypersil low loss silicon tape is a Westinghouse trademark. There
> are
> > 4 other USA makers of the same material. All of em use materials
> designed
> > for commercial applications..and high tolerances. Pole pig and dry type
> > xfm's require tight tolerances.
> >
> > ## we also put the flike IR on the casings and all over the xfmrs. to
> > measure surface temps. I'm not seeing differences in temps at all.
> >
> >
> >
> >> I suppose we'll just have to agree to disagree on this issue, Jim; but I
> >> would never consider doing this paralleling, myself.
> > ## in many cases, it's done because of space constraints. In many cases
> it
> > is done simply cuz then only 1 type of xfmr is needed. IE: 1 xfmr for
> low
> > power...and 2 for a higher powered PA.
> >
> > ## check this out. This a CE-10,000 PA using a pair of 3CX-3000A7
> > tubes. Uses parallel xfmrs.
> > https://www.amprepairguy.com/ce-10000-repair.html
> >
> > Here is Asamoto's ASI site. He has a few that use paralleled xfmr.
> Drool
> > over the pix.
> > http://www.ampsystems.com/asiprod-old.htm
> >
> > Told scott this AM abt the fuss abt paralleling xfmrs. He just laughed.
> > He said tell em he has been paralleling all sorts of identical xfmr's for
> > 20 years now....and as many as 5 of em.
> >
> > He's doing a budget build on this current amp..... with stuff on
> > hand...which included the pair of 5.3 kv xfmrs.
> >
> > Jim VE7RF
> >
> >
> > 73,
> >
> > Steve K0XP
> > _______________________________________________
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>
>
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