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Line Transformers




 
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Author Topic: Line Transformers  (Read 1690 times)
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WA4WAX
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« on: March 22, 2019, 10:06:27 AM »

Maybe this will give someone ideas.

https://www.youtube.com/watch?v=-xrXjw7nt-g
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WBear2GCR
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« Reply #1 on: March 22, 2019, 10:59:48 AM »

No news there...

These are NOT going to work as a SE output transformer, they won't handle DC offset.
You might pull the lams and gap it, but then you might not have enough inductance for the low end.
One would need to test.

Most, not all, of these transformers are wound for low cost distributed sound systems, and do not have
good frequency response at all. Likely more than sufficient for most ham use though. Not hifi.

I'm using one right now to replace the blown 600ohm main output transformer on an R-390A.
Gets me an 8ohm match. And yes, I am living with the DC across the winding and did not gap
the laminations... but I'm running about 1 watt through a 5 watt transformer, which helps.
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W1RKW
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« Reply #2 on: March 22, 2019, 05:10:22 PM »

then there's insulation resistance.
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KK4YY
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« Reply #3 on: March 22, 2019, 09:28:30 PM »

I love that guys accent. "Lion transformers", heh.
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ka1tdq
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« Reply #4 on: March 23, 2019, 08:36:28 PM »

Timmy has a nice write-up on the subject:

http://www.amwindow.org/tech/htm/tvtomod.htm

Personally though, I get ideas visiting gentlemen's clubs.

(the ones with the girly-girls. I mean, naked girls)

Jon
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Patrick J. / KD5OEI
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« Reply #5 on: March 24, 2019, 01:09:21 AM »

I've used a 5W/70V one for a 600 Ohm match (690 Ohms) by putting a 4 Ohm speaker on the 8 Ohm tap. Transformer rating at best would be 1.25W for the same frequency response. There are a bunch of rules of thumb on these things but always best to check actual performance before making it permanent.

Off-label use of these transformers is something I've had a little hands-on experience with. The video shows the right way to use them as a matching transformer sans DC and the math looks OK but it should be clarified on that 5W unit he's showing, for which the diagram was drawn, that if the 2.5W tap is selected as the CT then the start is the common tap and the finish is the 0.62W tap @ 8000 Ohms (8K CT). If the start is the common and the CT is the 1.25W tap then the finish is the 0.31W tap at 16K Ohms (16K CT). These statements are only correct when the 'speaker' side has the stated load of 4, 8, or 16 Ohms, properly attached.

I totally agree about 'no DC at all' on these things, but would temper that by stating 'no unbalanced DC at all'.

So, the following is just opinion, but it's suported with a lot of experimentation and actual use. Those little transformers and their large cousins are very good step-up from speaker-ish impedances to grids, as long as DC through any winding is negligible. They can give excellent performance close to 'hi-fi for communication work' if the unit has a wattage 4-8 times higher than the power level present. This is because they have small cores that don't handle bass well, but this mostly applies to older units which are usually specified for around 70Hz low frequency.

It is super-easy to drive AB2 6146 grids with a 7W/70VCT unit, and the frequency response was much better than it would have been at the 7W rating. 6146's take only 200mW to drive so they sounded beautiful. Anyway, it was tube power amp in a vehicle and didn't cost like a Milbert.

In the case of grid drive, it is only important to provide the low-Z winding with the voltage needed to be transformed, by the turns ratio, to the secondary voltage wanted for the grid to grid drive. The low Z source could have any reasonabl;e internal impedance as long as the voltage demanded by the turns ratio put across the low-z side and has good regulation and does not exceed the voltage for the transformer tap impedance corresponsing to the transformer's wattage rating.

There are much higher quality and higher power ones available from Edcor than the usual 50 year old ones in our junkboxes. Their respose goes to 40Hz at full power according to Edcor. I use a 400W one to drive 3-500Z grids. Only a 100W one is needed for 20Hz audio response (25W peak drive) but ya know.. I didn't want the driver transformer to be the frequency-limiting factor at all. The 400W one puts any transformer problems far outside the audio bandwidth limits in use. But again this is all just my opinion.

I want to share my experiences with Edcor line matching transformers for off-label uses because it has been really good. These are newer and better audio quality that those shown in the video and I think the best available for a reasonable cost. I suppose that modern audio distribution systems are subject to better expectations, so Edcor supplies appropriate parts.

The Edcor WA series is for low-z drive to a high Z load. They can be ordered with a 140/70V (140VCT) winding and choice of a 4 or 8 Ohn mwinding, so there is the high voltage center tapped side and the low impedance side.

Edcor WA series https://www.edcorusa.com/ampoutput#/specFilters=122m!#-!824

The attachment is my cheat sheet for doing the experiments with several Edcor models. I have used the 30, 120, 240, and 400W ones.

* line matching transformer calculations 20190323.pdf (122.6 KB - downloaded 53 times.)
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WBear2GCR
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« Reply #6 on: March 24, 2019, 07:45:03 PM »

Really 400 watts to drive the grids??
Seems to me that you only will draw probably less than 10 watts??
Seems like excess core...?
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KK4YY
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« Reply #7 on: March 24, 2019, 09:14:05 PM »

Bear,

I recall a "rule of thumb" stating that the low end response of a transformer drops by an octave every time the applied power is reduced by 6dB. Looking at Patrick's' line transformer cheat-sheet, this appears to be true.

I'm going by that same theory in using a UTC S-22, 250W mod transformer in my DX-100. However, I think Patrick has me beat in the overkill department. Grin


Don
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« Reply #8 on: March 25, 2019, 09:09:53 PM »

400W core is way overkill, done only to move the transformer's failings, if any, away from the low end of the speech bandwidth.
The 100W unit for $46 works fine, and even the 15W unit for $17 should work great if bass is cut out below 200Hz but we get into ART-13 modulator fidelity/performance at that point!

One physical fit point is that the 40W UTC potted can driver transformer replaced (because of gross impedance mismatch to grids) is overall a little bigger than the 400W matching transformer for $136.

I bought several sizes and tried them because this was at first an experiemnt, no one at Edcor could answet to any of thiese kind of 'driver' use questions. In the end, just left the biggest one in.
 
Peak grid on 3-500z is 25W, but quite less is needed for modulating a 400W RF carrier.. It also depends on the modulator plate voltage.
Further cuts could be made to save a few dollars but why do that? The 400W unit was not that costly compared to the 60W or 120W ones. Consider the cost of the entire transmitter. Drop in the bucket.

Menno Vanderveen wrote in an article that when the low frequency is halved, the power handling is quartered, which is exactly what Don wrote.

"The output transformer must be able to
handle the output power without major losses
and distortions, at the intended frequency
range. This is a rather difficult topic because
not all manufacturers deliver all the
information you need to judge whether the
OPT is applicable or not. What you need is
the following: “at which lowest frequency the
transformer can handle its nominal power”?
I give an example: suppose you select an
OPT which can handle 50 Watts at 30 Hz.
Then this transformer can handle 50/2 = 25
Watt at 30/1.414 = 21.2 Hz. Or the
transformer can handle 50*2 = 100 Watt
at 30*1.414 = 42.4 Hz.
The rule behind all this is: “the power
capability doubles when the frequency is a
factor 1.414 larger. The power capability
halves when the lowest frequency is divided
by 1.414 (square root 2)”."

Of course these transformers and their rules have limits based on winding inductance, so this rule of thumb interacts with another regarding the rated voltage and current of any winding, on the transformer in question.

This issue is why I sold the wonderful but partly stripped Philips 2KVA amp running a pair of QB-750s that I had originally wanted to use as a subwoofer amp. While the transformer was not cited as the reason, that high cost item almost certainly was.
Amp spec was good to 50Hz at 2KW. For 25Hz that's 500W. I have 900W solid state units rated 5Hz with native load impedance of 0.9 to 1 Ohm, made by Ling. So sub bass will be debauched by solid state but my pocket cash is not unlimited.

* menno Vanderveen secrets of output transformers.pdf (115.28 KB - downloaded 56 times.)
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« Reply #9 on: March 26, 2019, 03:01:51 AM »

Here is the excel file for any random line transformers

* line matching transformers basic 20190323.xls (18.5 KB - downloaded 45 times.)
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