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Author Topic: Why use a modulation reactor?  (Read 35495 times)
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KE6DF
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« on: June 04, 2012, 12:05:20 PM »

The argument for using a modified Heizing circuit with a modulation reactor is that it takes the DC off the modulation transformer secondary and improves frequency response -- especially at the low end -- by preventing the modulation transformer from saturating on peaks.

That makes sense.

But,

Thinking about it more, it seems to me a modifies Heizing design just transfers the saturation problem to the modulation reactor.

The modulation reactor now has all the DC current, plus the audio frequency current also, on it.

Of course, modulation reactors use a gapped core to reduce saturation.

But then, why not dispense with the modulation reactor, use a bigger heavier modulation transformer with a gapped core?

Don't you come out the same?

In fact, many modulation transformers like the UTG CVMs and Thordarson CHTs have a gapped core.

I wonder if the modified Heizing is inherently any better. Couldn't you get to the same point by just making the modulation transformer larger and using a gapped core?

Or, in the case of ham radio HB construction, use a gapped modulation transformer well below it's ratings -- like using a 500 watt CHT unit in a 400 Watt input TX?

Dave
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WA1GFZ
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« Reply #1 on: June 04, 2012, 12:50:02 PM »

If the transformer is rated to carry DC you can improve the response by taking the DC off the secondary and let a choke handle the job. Then you can slam more low frequency audio through the transformer
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AB2EZ
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« Reply #2 on: June 04, 2012, 01:30:26 PM »

When we were in an era where a large variety of modulation transformers and reactors were being produced... and also could be custom ordered, in quantities, from transformer manufacturers for the production of multiple transmitters over a period of time... then the tradeoff in cost, performance, physical size, and weight was the same for either configuration (modified Heising vs non-Heising). Back then, there was no advantage to using modified Heising.  

But now we have to "make do" with whatever used/surplus stuff we can find at a reasonable price; and sometimes we have to mix and match (using modified Heising) to get the desired performance.

Stu
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KM1H
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« Reply #3 on: June 04, 2012, 03:48:30 PM »

With the ARRL's push in the late 40's to limit AM bandwidth and the inherent cheapness of most hams the manufacturers used the minimum iron possible to survive the warranty period.

The Economy Grade (translated "ham") iron was also available thru catalogs while the BC industry bought the high end products.

After the demise of the HT-9 (BC quality as built) I cant thing of one ham rig that was capable of 100% low distortion modulation and all had space shuttle audio.
The Viking I was designed with full range audio but the ARRL put the screws to them and a redesign was required which delayed availability.

Carl
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« Reply #4 on: June 04, 2012, 03:54:16 PM »

"Advantages of the PI filter [xfmr - coupling cap - reactor] are a reduction in the two reactances to 1.41 times the terminating load resistance" ...

Reuben lee, in his book Electronic Transformers, discusses it here:

http://www.vias.org/eltransformers/lee_electronic_transformers_07_09.html

Chris

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« Reply #5 on: June 04, 2012, 04:00:00 PM »

You can pour more turns on a choke per given size than a transformer so you can get more inductance and thus a better low end. A transformer that does not have to handle unbalanced DC through its' secondary will also have more inductance and thus a better low end per given size. What I suspect is that to make a mod transformer that can handle unbalanced DC and have equivalent performance to the modified heising iron combo, the gapped mod transformer would be much bigger and heavier than the modified heising separate choke and transformer approach. Another way to view it is even if it was say the same weight as the modified heising choke and transformer combo you would be better off with the modified heising combo from the point of view of reliability and packaging convenience especially if the iron was being used in an AM broadcast transmitter.
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k4kyv
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Don
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« Reply #6 on: June 04, 2012, 04:18:06 PM »

I have a couple of RCA modulation transformers rated for their 250w BC transmitter, one designed to go with a reactor, and one designed to carry the DC.  The one designed to carry the DC is much larger and probably weighs more than the combined weights of the other transformer and its reactor combined.  Most broadcast iron was designed for use with a reactor.  From what I  have  read on the subject, it is more difficult to design a transformer to carry the DC and  still have good frequency response and immunity from saturation, than to design the transformer without the core gap, and use a separate reactor. 

Another advantage to using a reactor that I have found, is that transformers that carry the DC tend to talk back more than ones that use a  reactor.

I would use a reactor with the transformer if it was designed for one.  Otherwise, it would work OK without the reactor, but even then, expect better performance using a reactor.  My little 10m rig, an Eico 720/730 combination originally used a transformer with the gap.  I had a 40H, 120 ma reactor on hand (yes, a mod reactor, not a filter choke), so I put it in, and disassembled the transformer and  cross-laminated the core (stacking it like a power transformer without the gap).  It now has a flat low frequency response down to about 30~.  Originally, it began to roll off a little above 100~. I once had a UTC VM-5 multi-match transformer.  When I hooked it up with a reactor, the talkback (which that transformer is notorious for) completely went away, even with the gap in the core. I suspect the gap may contribute to talk-back, since this allows some core movement between the E and I laminations, whereas with the cross-laminated core, all the laminations are locked down tight.

Even if you don't plan to modulate with an audio response down to 30~, you will still get better performance higher up in frequency, with less phase shift distortion.  UTC recommends at least one octave of flat response capability, above and below the actual frequency response used.  So, for example, you plan to run an audio response of 200-4000~, for best quality, the flat response of the transformer should be at  least 100-8000~, or better.
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« Reply #7 on: June 04, 2012, 04:46:36 PM »

The Gates BC-1T mod iron weighs 54# and the matching reactor is #72#. That doesnt sound like much of a total weight savings to me. The transformer is rated at 40-15000cps.

OTOH the 7500W mod transformer holding the floor down is 375# and that includes an enclosure full of oil but I havent been able to find out if its suitable for DC at full ratings.  Ive been running it with 2500VDC thru the secondary for several years now and it doesnt make any noise. The ratings plate says 50-10000cps.
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W2NBC
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« Reply #8 on: June 04, 2012, 05:04:45 PM »

Carl you beat me to the BC-1T..

The slow decline of quality iron and true frequency response wasn't just a "ham" thing.. The commercial broadcast manufacturers set the stage with smaller anemic iron as the years went by. Gates is a perfect example from their hallmark BC-1F with stout and heavy modulation transformer and high value reactor.. to the 'weight watcher's' delight BC-1T with a modulation transformer you could pick up like a football and a 39 Hy reactor..  I use that exact combination (BC-1T)  in my BC-610 and both the mod iron and reactor fit in a BC-610 cabinet! The Collins 32-V series had VERY capable mod iron, as well as the Heathkit Apache. The weakness of almost all ham transmitters was in the driver iron itself.. Winding reactors with smaller mod iron was an economical and cost effective way of broadcasters meeting typical accepted standards. There is a member of this group who was able to switch (in almost real time) his Heising configuration with 40 Hy choke, or straight through his mod iron, and the low end honestly remained consistent. It's all a matter of what you've got on hand, and the space you need to mount the reactor (if you can FIND one!)
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k4kyv
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Don
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« Reply #9 on: June 04, 2012, 05:19:19 PM »

I remember Timtron saying that he used a BC1-T mod xfmr without a reactor, and he couldn't tell any difference in low frequency response.  Don't know if he checked it for waveform distortion at the bottom end, though.  But one thing he did say was that it talked back horrendously when he ran the DC through it. I believe he lost that transformer in the fire.

I think the mod xfmr and reactor in the BC1-F each weigh in at about 200 lbs.  One person, in reasonably good physical condition, could walk across the room carrying a BC1-T transformer under one arm and the reactor under the other.  I don't think one could do that with BC1-F or RCA BTA1-R iron.
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KE6DF
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« Reply #10 on: June 05, 2012, 10:57:03 AM »


 I once had a UTC VM-5 multi-match transformer.  When I hooked it up with a reactor, the talkback (which that transformer is notorious for) completely went away, even with the gap in the core. I suspect the gap may contribute to talk-back, since this allows some core movement between the E and I laminations, whereas with the cross-laminated core, all the laminations are locked down tight.

The CVM-5/VM-5 are notorious for talkback.

I've also heard that the VM-4 has talkback issues.

But, I'm wondering if anyone has experience with the CVM-4.

While the VM-4 is of the end-bell with exposed core type -- sort of a mini VM-5 -- the CVM-4 went to the potted 3lb coffee can configuration.

I'm wondering if the coffee can design corrected the talk back problem.

I have a CVM-4, hence my question.
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« Reply #11 on: June 05, 2012, 05:08:48 PM »

Quote
Carl you beat me to the BC-1T..

I think a lot of it had to do with the improved lamination materials and better insulations allowing a more compact product. After all the BC-1T is rated 40 15000cps and thats not the sign of a reduction in quality. Also getting rid of all those ridiculous cast iron end bells saved weight and real estate.

Look at a modern plate transformer. A 46# Dahl will run 1500W out CCS in RTTY service and barely break a sweat. Its easily capable of 5000W PEP.

OTOH a 3500-0-3500 with 3000-0-3000 taps @ 1A CCS behemoth built in 1946 and powering a big amp plus modulator weighs about 150#...maybe more as its been ages since I weighed it.

Carl
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ke7trp
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« Reply #12 on: June 05, 2012, 05:45:34 PM »

Carl. I have and use the bc1T mod transformer in a rig here.  A modern hypersil version. Do you happen to know the ratting of the cap and reactor that was meant to be used with it?

This is an interesting topic about old vs new transformers.  They sure are smaller and lighter.  I have a 1940s power transformer here that is several hundred pounds.  Its rated at 2000 volts /1.4 amps.  A new transformer is about 1/4 that size and weight.

Some have said the old where under rated.  I dont think so.  Those engineers knew what they where doing in the 40s.  After all. They had been producing transformers like that for 60+ years by the 1940s.  In 60 years, You figure something out. 

C
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« Reply #13 on: June 05, 2012, 07:08:22 PM »

Usually it is from 2uF to 4uF, 30 Hy to 60 Hy.

It varies by design, but most all are within that range.

73DG

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« Reply #14 on: June 05, 2012, 07:12:52 PM »

I have a 2 UF and a 60.   What if I change that cap to 4. What is the effect?

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« Reply #15 on: June 05, 2012, 07:15:14 PM »

More linear bottom end below 100 Hz.

I wouldn't bother.

73DG
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« Reply #16 on: June 05, 2012, 07:24:46 PM »

Ok.  The old transformer shorted on the one side. I replaced it with a Hypersil trans for a Gates.  The impedance is perfect for my rig.  It seemed the low end was less then the old trans. I had to turn the bass up about 15 to 20 db on the audio rack to get the low end back.  I wondered why.. I think now I know.   

Thanks man. 

P.S.  I called you a few times and you never pick up.  I was wondering how you guys where doing.

C
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kb3ouk
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« Reply #17 on: June 05, 2012, 08:30:22 PM »

What if the transformer has a high amount of inductance by itself, then can you use a lower inductance reactor?
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« Reply #18 on: June 05, 2012, 09:21:46 PM »

Quote
Carl you beat me to the BC-1T..

I think a lot of it had to do with the improved lamination materials and better insulations allowing a more compact product. After all the BC-1T is rated 40 15000cps and thats not the sign of a reduction in quality. Also getting rid of all those ridiculous cast iron end bells saved weight and real estate.

Look at a modern plate transformer. A 46# Dahl will run 1500W out CCS in RTTY service and barely break a sweat. Its easily capable of 5000W PEP.

OTOH a 3500-0-3500 with 3000-0-3000 taps @ 1A CCS behemoth built in 1946 and powering a big amp plus modulator weighs about 150#...maybe more as its been ages since I weighed it.

Carl

I think I have the same plate transformer here. UTC GC-309? or something. They came in two flavors, one appearing "vertical" and one appearing "horizontal" (which was about 300 lbs and the older type). Didn't they have also a 4800V set of taps?

As far as DC and mod iron, the only one I have DC through is from a late 1940's RCA 1KW BC transmitter and is supposed to be used with a reactor, which I do not have. The transformer apparently does not care about the DC because it is about 3x bigger than it needs to be.
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« Reply #19 on: June 05, 2012, 10:26:19 PM »

My Gates choke has a decal stating 32H min at 5000V RMS and 60H @ 600ma. Hi-pot is 10000V Just right for a 4-1000A!

The cap is 2uF @ 6000V.

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« Reply #20 on: June 05, 2012, 11:06:52 PM »

My Gates choke has a decal stating 32H min at 5000V RMS and 60H @ 600ma.

I can't figure that out.  I get the 60H @ 600 ma, but the 32H min at 5000V RMS baffles me.  The voltage across the choke doesn't significantly affect the inductance, but the DC flowing through it does.  Seems to me you would have to force a lot more than 600 ma through it to drop the inductance from 60H to 32H.  As I  recall, the BC1-T reactor is 32H at 600 ma. It might read 60H with no direct current through it.

A rule of thumb for the inductance of the reactor is to use a minimum of 8H for every 1000 ohms of modulating impedance.  So, for example, you run 2000v @ 500 ma DC to the final, that makes 4000 ohms modulating impedance.  So the minimum recommended reactance would be 32H.  A 30H reactor would be close enough.
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« Reply #21 on: June 05, 2012, 11:11:46 PM »

Thanks carl.  Thats what I got.  60h, and 2uf.  I guess this is normal. The old tranformer was a 1.12 to 1 ratio.  This one is 1.3 to 1. So maybe that had an effect.  I am going to leave everything alone.

C
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« Reply #22 on: June 06, 2012, 12:28:58 PM »

Sounds good to me Clark.

As far as the 60H @ 600ma, etc Im just reading what Gates claims since its their choke and label.

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« Reply #23 on: June 06, 2012, 02:06:13 PM »

John/WA5BXO came up with another idea, if you are using a reactor, and a 1:1 modulation transformer and/or a common B+ supply.

This setup uses the entire secondary as an 'autoformer' and will certainly add a lot more 'kick' to the audio.
It will over-modulate quickly but should add a healthy current kick in the actual modulator.

In my case, the modulation transformer has a primary Z of 20,300 ohms, and a secondar of 9,100 ohms.  In order for me to get decent audio (no modulator flat-topping on voice peaks) I had to run 2x the B+ on the 250TH's in the modulator as I did in the final.  1500v @ 200mA is 300w DC Input and 7500 ohms in the final.  I find that I can get more audio onto the B+ line if I pull the link out, and guess what?  1500v @ .150mA is 10k Ohms.

Just for fun (and since I'm using a 60Hy @ 400mA choke and 4uF @ 5kVDC coupling capacitor and a common supply) I'm gonna tack in my RCA 1:1 5,500 ohm mod transformer and see what happens.


* pptube-w-reactor2.JPG (26.43 KB, 766x451 - viewed 683 times.)

* IMG_20120529_143357.jpg (1704.36 KB, 2592x1944 - viewed 576 times.)
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k4kyv
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« Reply #24 on: June 06, 2012, 02:29:10 PM »

My BC1-T came with a smallish modulation reactor that was hardly any bigger than a large TV power transformer.  I put it in the 8005/805 rig, and replaced the one in the Gates with a Kuhlman 50H 600ma potted reactor, about a cubic foot in size.  I managed to shoe-horn it into the bottom of the transmitter after I moved the power supply choke and eliminated the 8008 filament transformer, using SS replacements for the rectifiers. The big reactor is totally quiet.  The smaller one talked back so badly I could hardly crack the mic gain before I got feedback.  I have a spec sheet for Gates iron, and they claim the reactor for the BC1- series is 32H.  The older version, BC1-F used much  larger and heavier iron.  Some used Chicago iron and others, Thordarson Tru-Fidelity.  I have seen the reactor in that rig specified at between 60H and 90H, but the Gates replacement was identical to the smaller reactor in the BC1-T. I suspect it depends on which manufacturer's iron they used in a given transmitter, and in later years they offered the s ame smaller one for replacement in any BC1- series rig.

Carl, have you been able to determine whether the actual inductance of your reactor is 32H, or 60H?
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