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@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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