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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