[Amps] Paralleling plate xfmrs.

[donroden at hiwaay.net]

I maintained a 5 kw output Elcom-Bauer 707 FM transmitter many years 
ago.  It had a strange configuration for it's single plate transformer.  
It had two separate 2500 vdc supplies ... diodes/capacitors/ inductors 
and they were in series. One supply was grounded and the other was 
floated 2500 volts DC above ground for a 5,000 volt DC supply to the 
final tubes.

   It seemed purposefully overly complicated.

Don W4DNR

On 2025-10-22 7:58 pm, Steve Harrison wrote:

> On 10/22/2025 4:07 PM, jim.thom jim.thom at telus.net wrote:
> 
>> I have been paralleling plate xfmr secondary's since 1977..with zero
>> issues.
>> Back then, it was 80 lb hammond plate xfmrs.   Those came with a 
>> 0-105-110
>> -115-120   primary...and the usual  CT secondary.
>> 
>> We would series the 120 vac primary's to run on 240 vac.   Then 
>> parallel
>> the secondary's.
>> Zero issues.... and a 50-50 split.
> 
> There wouldn't be... because the series connection of the primaries 
> took care of any imbalance in transformer turns ratio or output 
> voltage. With the primaries seriesed (iz dat even a word?? 8-) and 
> secondaries paralleled, the higher-output secondary would provide more 
> current until the output voltage drops to equal the other transformer; 
> then they would tend to share the total output current.
> 
> 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.
> 
>> 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.
> 
>> And no,  nobody is gonna wire plate xfmr secondary's in  SERIES.   Why
>> would you ?
> 
> Because by wiring the PRIMARIES in series, you effectively HALF the 
> available output voltage. So you merely series-wire the secondaries and 
> hey el presto: yer back up to full output voltage. (However, you won't 
> get the output current available from both secondaries in parallel; so 
> the issue is really moot, for your application.)
> 
> 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.
> 
>> As is, these xfmr's have  4300 and also 5300 taps.   Why
>> the hell would u even think of  series 2 x 4300 secondarys....u would 
>> end
>> up with a whopping 8600 vac = 12.16 kvdc.   And 14.9 kvdc  if the 5300 
>> taps
>> were used.
> No, because you're also only applying half the AC input to one 
> transformer.
> 
>> The pair of plate xfmrs  used on the 16 kva  supply  are identical.
> Actually, what should be said is that they are "SIMILAR". They cannot 
> be identical (they don't share the same DNA!! 8-). Their cores are not 
> exactly the same magnetic material, their copper wire is not precisely 
> the same length and thus presents a small resistance difference, the 
> placement of the wire on the forms is very slightly different for every 
> transformer, etc. etc.. You can never fully-guarantee that one 
> transformer is precisely, exactly, "identical" to another; there will 
> ALWAYS be SOME slight difference. That's all that Clark and I are 
> getting at.
> 
>> That
>> combo, using the same xfmrs is already in use in the above mentioned
>> commercial applications......was done all the time.
> 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 alla time!). 
> It might not be catastrophic initially; but it puts more stress on the 
> pair, which inevidtably builds up.
> 
>> I went through all this with Dahl himself.
> That would be really surprising, and distressing, considering his 
> reputation otherwise.
> 
>> 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.
> 
> The only way this is an acceptable engineering practice is to put 
> something in series with each secondary before paralleling them; that 
> "something" then absorbs any imbalance that may exist, either now or in 
> the future.
> 
> I suppose we'll just have to agree to disagree on this issue, Jim; but 
> I would never consider doing this paralleling, myself.
> 
>> Installing  a FWB assy on the output of each sec  will also work..then
>> paralleling the  outputs of each  FWB assy...... but it's a wasted 
>> effort.
> That would have been the far-preferable way to do it.
> 
>> A client wanted a 25 kw pep output 80-10m amp, using a 3x15 tube in 
>> GG.
> 
> We don't need to know who this person may have been (although if 
> he/she's still around, I can guess, based upon whom I hear busting 
> through pileups first time these days  8-).
> 
> Let's agree to disagree on this, Jim. Keep in mind that we are just 
> making suggestions to help avoid future problems that we foresee; we're 
> not really critisizing the design itself (wellllll... we sorta are, but 
> it's supposed to be CONSTRUCTIVE criticism  8-).
> 
> 73,
> 
> Steve K0XP
> 
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