Using Filter Chokes for Mod Reactors?

[W9GT]

I'm thinking about using a string of large filter chokes for a mod reactor in a 4-400s x 4-400s rig.  Total inductance will be about 50 HY.  Anyone have any experience doing something like this?  I'm not sure about all the differences between filter chokes and mod reactors.
I'm wondering about the pros and cons.  Any serious drawbacks?  I have heard of one fellow using an old pole pig secondary as a mod reactor and other similar experiences.  I have a nice BC xmtr mod iron, but no mod reactor.  The mod iron requires use of a mod reactor (no DC on secondary).  Thanks in advance for inputs, info, and suggestions.

73,  Jack, W9GT

[k4kyv]

The main problem is that most filter  reactors run at 10, 15 or maybe 20 henries.  This is too low for mod reactor use.  The rule of thumb for mod  reactors is at least 8 Henries for every 1000 ohms of modulating impedance.  Thus, a rig running 2000 volts at 500 MA, would be running 4000 ohms modulating impedance, so the minimum recommended inductance would be 32 Henries.

You could series up several power supply chokes to reach the 50 Henry mark, but this would take up much more space and weigh more than a single reactor.  But avoid using a "swinging choke" for this purpose.

One problem with power supply chokes is that they might not pass the higher audio frequencies, since they were designed for 120~.  The laminations may be very thick, and the coil may be wound so that there is more internal capacitance than would be desired in a mod reactor.  But there is a simple solution to that problem.  The drop-off in high frequency response would not likely occur below a couple of kilohertz, and a small reactor to take care of the highs, in series with  the rest of the reactors would take care of the problem.  It would take less than 1 Henry of inductance for frequencies above 1 kHz.  You could even wind one yourself.  It could even be air-core.  This small reactor would go at the top of the reactor string.  Think of it as a "tweeter" reactor, while the heavy iron supplies the "woofer" reactance.

Now if you can find two identical power supply reactors constructed with E and I laminations, each with about 12 Henries or more, you can combine them to form a reactor of about 30 Henries or more.  Take them apart, discard the I laminations, and butt the E laminations together, separating them with the same kind of core gap material that each reactor used between the E and I laminations.  You might need some angle iron and bolts to clamp this "figure 8" core together at the proper gap spacing.  Now connect the two windings in series, and wire them in the proper phase so that the inductance of the two coils is additive, not cancelling.  Run the rated plate current through the choke, and adjust the gap in the core for maximum inductance.

Since both coils now share a common core, theoretically, the inductance should be 4 times that of one of the original chokes.  But since you discarded the I laminations and thus some of the core material, the actual inductance will be more like three times the original.

John Coleman, WA5BXO and I constructed one of these back in the late 70's for his transmitter when I was living in Houston.  I think he is still using it, even after it had been submerged in a flood.

[flintstone mop]

Another problem with the series setup is that the DC resistance in the chokes will add up and reduce the HV to the final.
Fred

[WA1HZK]

Works just fine. String several PS chokes & the radio will play just fine. I use about 32 H with two PS chokes to modulate my 833's.
Keith
WA1HZK

[W2VW]

Works great as mentioned. Swinging chokes work fine too. Average DC current to final doesn't move around too much.

[k4kyv]

The problem with swinging chokes is that their inductance is at a minimum with full plate current because the core is saturated with DC.  They are designed to work that way.  That's a lot of DC resistance in the coil for a  few Henries of inductance.  Plus, the saturated core may exhibit non-linearity over the audio cycle.

Now, if you found a large swinging choke rated at 5-40 Henries at 5.0-0.5 amps, it might make a good mod reactor at 40 Henries for a rig running 500 mills on the plate, with plenty of headroom for saturation.

I have found that many broadcast transmitter mod reactors have relatively high DC resistance.  The UTC LS-103 in my rig, 50 Hy @ 500 m.a., has over 100 ohms of resistance.  IIRC, it is about 200 ohms.

This isn't a problem, since the final pulls steady plate current, the result is simply a slight reduction in DC plate voltage to the final.  Since the modulator tubes are still getting their full plate voltage, this could result in slightly higher positive peak modulation capability.

[W9GT]

Thanks to all for the comments!  I don't think the DC resistance or smoothing vs swinging chokes will be an issue.  All the chokes that I am planning to use are high current-rated, smoothing-type chokes.  I have a couple of 15 HY, a 20 HY, and a 10 HY.  I think all are 400-500 MA rated.  I could also add a small choke in series as Don suggested.  Anyway, I was just looking for a little encouragement from someone who had actually tried it.  Looks like it should work OK.  Space is not a major concern, as it is going into a full-rack cabinet, with plenty of space avalable in the bottom for power supply and modulator components.  Now I just need to find a nice 2 Mfd @ 10 KV oil cap to use as the audio coupling/DC blocking cap.  Any out there?  I have a couple of old power line power factor correcting caps, but they are huge....a little too big for the space available. 

73,  Jack, W9GT

[WBear2GCR]

Don,

One thing I'm puzzled about - big reactors do not pass HF, by definition.
A choke in the X Henries range looks like a very high Z to audio... which is the idea, pass DC, not AC... so I am not sure about your comment in this regard??

So are you suggesting that the Z of a too small reactor starts to look closer to the load Z (that the secondary of the mod iron sees) at some audio HF and so makes it start to look like a voltage divider (Load Z : Reactor Z), rolling the highs? So, now the small choke sitting on the top, will it not saturate first, and look like mostly like a fixed, lowish Z? What am I missing  in this picture??

[k4kyv]

The little reactor sitting on top needs to have the full current rating the same as  the larger ones.  For example, I have an old "splatter choke" removed from a Wilcox aircraft base station modulator, rated at .9 Henry @ 750 mills.  It would be excellent for that application.

If the choke is wound for power supply use, little attention was likely paid to the internal capacitance of the winding, which is of insignificant importance at 120~.  The winding pattern can affect capacitance.  That's one reason that many cheap audio transformers don't have good high frequency response.  With high internal capacitance, it's the same as placing a capacitor of, say .001 mfd, across in parallel with the choke.  It by-passes the higher frequencies to ground.  Consider that this capacitance is in parallel with the rf by-pass capacitors in the final, which may be anywhere from .001 to .005 mfd, and you may have a pretty good low-pass audio filter in the modulation iron.  In fact, there was a popular technique used in the 50's, called "building out" the modulation transformer, which took advantage of the leakage reactance of the modulation transformer to make it double as a splatter filter.  It consisted of adding a capacitor across the primary of the transformer, and using a certain value of rf bypass capacitance, to make the mod transformer frequency response cut off very sharply above 3000~ to eliminate high frequency response, which was considered "splatter" regardless of whether it was high frequency audio components or spurious distortion products. 

Another cause of high frequency loss can be the iron itself.  Quality audio transformers use a steel alloy called "audio C" for the laminations.  I'm not sure how it differs from the steel to make power transformer laminations.  Audio laminations are usually much thinner than power supply laminations.  With the power supply iron, at 120~, hysteresis and eddy current losses are insignificant, but may become significant at upper audio frequencies.  This would be equivalent to placing a resistor in parallel with the reactor at higher frequencies, and consume audio power.  The lost energy would be converted to heat that warms up the laminations.  The result would again be loss in high frequency response. 

Combine the core losses with shunting capacitance due to sloppy winding, and a power supply reactor may block a considerable portion of the highs.

The small reactor at the top of the stack eliminates the problem (assuming it has good high frequency response) by acting as the primary modulation reactor at upper frequencies, so they are unaffected by losses in the heavy iron.

The amount of inductance required for the mod reactor is inversely proportional to the lowest audio frequency.  For example, if 50 Henries is needed for flat response down to 50~, only 5 henries is needed if the lows begin to roll off at 500~, not uncommon with "communications quality" audio.  If the lowest frequency is 5000~, then only 0.5 Henries is needed.  In that transmitter, a 1-Henry mod reactor would allow flat response down to 2500~.  That's about where power supply reactors often begin to falter as mod reactors, so a 1-Henry reactor, with low winding capacitance and low core losses at frequencies between 2500~ and 10K~ would allow a flat frequency response from 50-10,000 Hz, even if the bulk of the modulation reactor's inductance is from a couple of power supply chokes in series and those power supply chokes are ineffective above 2500 Hz.

[WBear2GCR]

 

Thanks Don!

A very clear and excellent explanation of how that works.

          _-_-WBear2GCR

[W2JBL]

    i use a pair of 20H 450 MA chokes in series and a 4MFD oil cap in the heising on my trusty ART-13, which runs at 1650 volts, and about 225 watts out. the stock mod transformer just didn't hack it at that power. after i put the heising setup in line (outside the rig obviuosly) the audio cleaned up a ton, and i got a bit more low end out of it too. so i guess that's OKfine for me. after all this is AM, not rocket science.