Modulation reactor circuit

[W8ACR]

I know this subject has been discussed before, and I have gone through some of the old threads, but I'd like to bring this subject up again to make sure that I am thinking clearly about this. I have never built a plate modulator using a modulation reactor before. I am nearing completion of my homebrew 160M AM transmitter, and am trying to figure out how to arrange the modulation circuitry. I have a 50H Dahl modulation reactor that I would like to use. The mod transformer is a UTC S-22 250W unit. The RF final is a pair of 254W's in parallel using a pi network tank which will operate at 2000V@250mA designed for an operating Q of about 15. The modulator tubes are 811A's zero bias with 1250V plate voltage.

Figure 1 is a standard modulator with B+ current flowing in the mod transformer secondary.

Figure 2 shows the audio return directly to ground through a blocking capacitor.

Figure 3 shows the audio return back to the B+ line.

Figure 4 shows the audio return directly to ground but with the blocking capacitor on the "hot" side of the mod              transformer secondary.

I know that the arrangement of Figs. 2 and 3 are usable since I have seen these circuits described in textbooks. I do not know if the circuit of fig 4 is valid, I just threw it in. I have a choice of two oil caps to use as blockers, one is 4uF@2000V and one is 3uF@3000V. In case it makes a difference, The 2000V power supply is a standard choke input using 866A rectifiers (!) and 4uF@4000V capacitors. The bleeder is 50K@225W.

I'd appreciate comments on how to best do this. I'll post some photos of the completed transmitter when it's all done.

Thanks, Ron W8ACR

[KE6DF]

A couple of thoughts.

Either 2 or 3 would work as you say.

In each case, I think you need blocking caps with a higher voltage rating than 2KV. Even 3KV is cutting it close. I'd rather see at least 4KV or better yet 5KV for the Heising blocking caps.

In circuit 4 you are returning the audio to the B+ line rather than to ground.

That would also work, but only if  you used a large capacitor as the output cap on your power supply.

If the output cap on the power supply is only 4uf, I think you would have a problem with the audio effecting the dynamic output of the supply. You would have big trouble getting 100% modulation with only 4uF.

If your output cap on your PS was 40 uF or something in that range,  then circuit 4 would work. It has the advantage that the DC voltage isn't across the Heising cap.

[W8ACR]

OK Dave,

Thanks for the reply. I think you may have gotten the pics wrong. For some stupid reason, I put fig 4 in the upper right hand box rather than the lower right hand box. But I think I understood your point. I can do 12uF in the output of the 2000V power supply. Would that be enough?

Ron

[KA2DZT]

Circuit #4 is valid and will work.  The only thing you have to consider is the insulation between the two windings on the mod xfmr since the secondary is grounded.  Placing the cap from the low end of mod secondary to ground is probably better as it keeps the high voltage on the secondary.  This circuit results in less of a voltage difference between primary and secondary.

I would use the 3uf 3KV cap in any of the circuits, a 2KV cap probably isn't high enough.

Fred

[W2VW]

Put both caps in series.

Voltage rating too low on either and you don't want too much C.

[K1JJ]

They will all work, however:


I don't like #2 and #4 because if  C1 shorts, you have a direct HV short to ground through the mod xfmr secondary and Heising reactor... This COULD open up (melt) your mod transformer secondary and/or Heising reactor if not well protected by sensitive fuses, breakers or electronic shutdown.  And, in my rigs, I've had C1 short several times over the years.  As already said, C1's voltage rating at 3KV is too small and should be more like 6KV or so to be safe. 1 to 2ufd is a good value for C1.

#3 is the best all-around circuit and C1 can be placed in either the neg or pos lead. Neagtive lead is probably best as already mentioned. If C1 shorts, not much will happen except for normal final amplifier tube DC going through the mod secondary. (The equivalent of how #1 functions now)

(Be sure to have a ~25 ohm power resistor on the HV output for current limiting  and use at least 40 ufd in you HV supply for good measure)


T

[KE6DF]

OK Dave,

Thanks for the reply. I think you may have gotten the pics wrong. For some stupid reason, I put fig 4 in the upper right hand box rather than the lower right hand box. But I think I understood your point. I can do 12uF in the output of the 2000V power supply. Would that be enough?

Ron

12uF sounds too low to me -- especially if you are returning the audio through to the B+ terminal.

Better to scrounge a 40vF output cap for the PS.

You could use two of the 4 uF 4000v PS filter caps in series as the Heising blocking cap (this assumes they are oil based and not polarized).

Dave

[W2PFY]

Yup, figure three is how I use the cap in me Westinghouse. BC rigs like to put the cap to ground and that may be ok for rigs that are always on but for a rapid fire break-in operation, I think the cap to ground puts a strain on the mod transformer as that cap charges up.

[W2XR]

They will all work, however:


I don't like #2 and #4 because if  C1 shorts, you have a direct HV short to ground through the mod xfmr secondary and Heising reactor... This COULD open up (melt) your mod transformer secondary and/or Heising reactor if not well protected by sensitive fuses, breakers or electronic shutdown.  And, in my rigs, I've had C1 short several times over the years.  As already said, C1's voltage rating at 3KV is too small and should be more like 6KV or so to be safe. 1 to 2ufd is a good value for C1.

#3 is the best all-around circuit and C1 can be placed in either the neg or pos lead. Neagtive lead is probably best as already mentioned. If C1 shorts, not much will happen except for normal final amplifier tube DC going through the mod secondary. (The equivalent of how #1 functions now)

(Be sure to have a ~25 ohm power resistor on the HV output for current limiting  and use at least 40 ufd in you HV supply for good measure)


T

Hi Tom,

Thanks for the usual insight, and more importantly, your input based upon personal experience; the "been there, done that" factor.

Figure 2 is the design approach I use in my rig. It works well, and has been pointed out by others, is the classical circuit used in most plate modulated broadcast transmitters.

Figure 3 is intriguing to me, for the reason you pointed out; in the event the DC blocking cap should short, it removes the threat of zorching the mod xfmr and mod reactor. And as Terry mentioned, it reduces the turn-on surge through the modulator magnetics as the capacitor charges with the application of B+.

I suspect, however, that the effects of figure 3 may be more complicated than is obvious. Although both figures 2 and 3 represent fairly complex impedances and resonant circuits at audio frequencies, especially when the power supply and RF PA are included within the circuit, I am wondering if the resonance effects of figure 3 are even more complex, and if there is some kind of hidden detrimental issue here than perhaps we are missing. My thought is this; if the benefits of figure 3 are so obvious, how come the BC transmitter manufacturers, who employed some pretty talented engineers, did not broadly adopt this circuit?

I'd like to see a technical analysis of both circuits, and then make a final determination as to which of the two may be more desirable overall, from a pro and con basis.

73,

Bruce

[W2PFY]

The RF final is a pair of 254W's in parallel using a pi network tank which will operate at 2000V@250mA designed for an operating Q of about 15. The modulator tubes are 811A's zero bias with 1250V plate voltage.

Ron, will you be posting any pictures of the rig? I have been collecting the 254W/PL-254W tube for a long time. I think I have the worlds largest supply now so I look forward to design ideas.

[W7TFO]

A simple fuse between the RF B+ and the modulator secondary proper might be prudent if one is worried about component failure in choices 2, 3, and 4. 

A 2A slo-blow would probably handle it fine, and it could be nothing more than a 3AG in a holder stuck on top of a porcelain insulator.  At which point it would be only a choice of topology.

73DG

[k4kyv]

If you have good overload protection or the proper size fuse in the primary, failure of the cap in fig. 2 shouldn't damage the mod xfmr or reactor.  A similar situation exists when the final tank cap arcs over or there is a short in the HV final circuit.  I have  had that to  happen on several occasions and the only thing that happened was that the overload or fuse kicked out. I don't think I have ever heard of someone losing a mod transformer or reactor because something shorted out in the final. Some BC transmitters are designed with a safety interlock that shorts out the HV after it has passed through the reactor. A similar surge is carried through the reactor whenever a shorting stick is used to discharge the filter cap for safety. In any case, the power supply choke would suffer the same abuse with any kind of shorted load or a shorted HV filter cap. 

The greatest disadvantage with fig. 2 is the voltage difference between pri and sec of the modulation transformer that exists for a brief instant when the HV is first tuned on, until the capacitor charges up. 

I wouldn't run fig. 4.  There is no advantage to maintaining the unnecessary HV stress between windings of the mod transformer.  Remember, in many of our rigs, we are using decades old transformers whose insulation, often nothing more than plain old paper impregnated with varnish, may have deteriorated with age.  For the same reason, I always float the core and frame of modulation transformers, modulation reactors and HV filter chokes by mounting them on insulators and not grounding them.

[K1JJ]

Don,

Yes, if the Heising cap never shorts out there is no problem. But here's a couple of reasons why I worry about the cap shorting out:

1) Many homebrewers underestimate the voltage rating on this cap and it goes. For a long time I figured rating it a little above the plate voltage was OK - but I rate it at X3 these days.

2) I lost a mod transformer secondary as a result of this cap shorting to ground. It is the only mod transformer I ever popped. It was a FB  500W BC unit in my 813's X 813's rig - I got the xfmr from the Tron. Heartbreaker, indeed.  The reason it popped is cuz I used a power supply that was fused for a big linear. I wanted to use common supplies to simplify, but this was a mistake.  If the fusing fails and the power supply is a strapper, we can put a valuable mod xfmr at risk. (Risk overkill, I realize)


Fuses in the primary won't help if the supply capacitors are huge, like 200ufd.   I use a LONG fine wire after the PS filter caps and on the diode bridge for additional protection these days.

But as you mentioned, if the Heising cap is overrated and never shorts AND the supply is fused properly to match the rig, there should be no problems using the cap to ground.

To eliminate all the worry, I simply use circuit #3...


Bruce:  Interesting on analyzing the various configs. We talked about this once before and figured they were complex circuits. As you said, a plate modulated rig using mod and Heising iron is a very complex at audio frequencies indeed.  

T

[Bill, KD0HG]

A simple fuse between the RF B+ and the modulator secondary proper might be prudent if one is worried about component failure in choices 2, 3, and 4.  

A 2A slo-blow would probably handle it fine, and it could be nothing more than a 3AG in a holder stuck on top of a porcelain insulator.  At which point it would be only a choice of topology.

73DG

A 3AG fuse is going to arc right across if it blows with high voltage on it, providing little, if any circuit protection. And you get the fun of dealing with a flame and small explosion. You can find suitable HV fuses online, at Newark, or at least use an industrial flameproof fuse with a 600V rating.

[KE6DF]

These fuses might do the trick.

http://cgi.ebay.com/Microwave-Oven-High-Voltage-Fuse-5KV-0-9A-/260680933127?pt=AU_MicrowaveOvens&hash=item3cb1cb5f07

[k4kyv]

I wouldn't use a fuse in the line between the mod transformer secondary and rf B+.  If it blows for some reason while the modulator is still functioning, you are left with the modulation transformer running full output with no load, a sure-fire way to crap out any mod transformer.

In my HF-300 rig I use separate supplies for modulator and final.  The blocking cap is wired as in Fig. 2.  It is an ancient paper cap (perhaps oil-impregnated but not oil filled; the can has open seams at the bottom and sides and would not hold a liquid.  The nameplate says "paper capacitor".  It is large for its size and ratings, 4 mfd @ 3000 volts.  I run 2 kv on the final.

In almost 40 years that cap has never shorted, and it was probably manufactured pre-WW2.  But it could go any day and if it did, it most likely would simply trip the overload relay. Just a month or so ago my O/L relay kept tripping.  Then I realised I had forgot to re-set the link after I had changed the coil in the PA.  The link was shoved all the way out and was touching the frame of the plate tank capacitor which is at full high voltage, resulting in a dead short of the +HV to ground. The full current of that short was carried by the modulation reactor. That certainly wasn't the first time the HV has been shorted to ground  through the mod reactor and power supply filter choke, and so far they have both survived. The stock blocking cap in the Gates BC1-T is similarly anaemic in its voltage rating. In both rigs, the O/L relay is set to trip whenever the plate  current exceeds the nominal current by about 20%.

The mod xfmr secondary and mod reactor each have a DC resistance of a few hundred ohms at most. A dead short should pull enough current through the winding to trip the O/L relay and/or blow the primary fuse long before the winding has  had time to heat up enough to burn out the transformer. That is, if the power supply is properly protected with a fuse and there is some form of DC overload protection.

The advantage in Fig. 2 is that the audio is returned directly to ground, not through the power supply output filter capacitor. This allows the modulation reactor to serve as an additional ripple filter choke and reduces hum on the carrier. This can especially be a problem if one is not using a large amount of filter cap.

It also eliminates the possibility of a mysterious form of distortion that may result when a common power supply is used for modulator and final, particularly if the filter cap is not large.  The class-B modulator draws plate current in pulses.  Take the example of a pure sine wave.  First one modulator tube draws current on one  half cycle, then the other tube draws current on the other half cycle.  The current waveform would look somewhat like the waveform out of a full wave rectifier before the filter, with two half-sinewave pulses per audio cycle.  This pulsating load on the power supply will induce a distortion component heavy in second harmonics of  the audio, onto the DC output of the supply if the output capacitor in the supply is not sufficient to completely smooth out any ripple caused by this pulsating load presented by the modulator tubes. This form of distortion was quite common in WW2 era and before, when it was very difficult and expensive to obtain more than about 4 mfd in a high voltage filter capacitor.  This distortion shows up in the BC-610, and partly  responsible  to the distinctive sound of that rig.

When using a common power supply, for best protection it is advisable to use individual tripping circuits or separate O/L relays to respond to over-current pulled by the modulator and over-current pulled by the final. Don't use just one big O/L relay that is tripped by the total current (modulator current + PA plate current) pulled from the supply.

[WD5JKO]

I am envious of you that have such deep rooted experience on this topic. I only did the reactor thing once with an AM KW rig (1000W in 850 out) back in the 1980's. In my case I had an undersized mod transformer (Freed 120W), and by using this technique I got ~ 600 watts of audio through it with a quartet of 808's with 2250v B+.

I chose circuit C and used a 4 uf cap from a BC-610. I'm sure other variations would have worked as well, with the pros & cons as already discussed.

I wonder though about circuit E (not shown). Here we simply parallel the reactor with the mod transformer secondary (no cap). If the DC resistances of the two windings are the same, then the mod transformer secondary current is cut in half. It could be cut more than half if the reactor has even lower DC resistance. I think circuit E might have an application where the mod transformer is already gapped, and the user wants either more power, or the same power at a lower audio frequency. Taking the mod tranny secondary current to zero is not always necessary.

Jim
WD5JKO

[K1JJ]

Don,

Good point about a fuse in the mod xfmr secondary opening and creating no load for the mod primary. This is certainly a no-no.

Also, using a separate overload for each the modulator and final separately - will help make the protection system more sensitive to shorts.  I'm thinking that the way to do this with a common HV supply is to add a ~1 ohm resistor in series with each filament CT lead (before the TR relay contacts) to sample some overload voltage for processing.


It's always a good idea to discuss the advantages and disadvantages of these circuits and make everyone aware - so we don't make dumb mistakes with such rare and valuable iron. I've been there...

T

[W2PFY]

In my Westinghouse the fuse is located in series with the center tap on the mod transformer. when it blows, the mod section shuts down.

HV fuses are often seen on eBay, the fuse is filled with powered glass. When I rebuild mine, I dump that out and I have had no problems doing so. The fuses look just like a 480 volt fuse but they are made of glass and are all rated at 6KV. The fuse block is porcelain. Another thing mentioned in another thread is a statement about going too large with a coupling cap. 2 Uf is plenty and if you go larger you have overshoot.

[W1AEX]

Maybe there's a reason not to do this, but using Terry's picture, why not fuse it this way to shut down the supply source through the reactor and the modulator primary in the event of a catastrophic fault? I realize this doesn't help the original poster since he's using separate supplies.

[W2PFY]

why not fuse it this way to shut down the supply source through the reactor and the modulator primary in the event of a catastrophic fault?

The Westinghouse actually fuses both, the mod fuse is one amp and the mod reactor and final are fused again with one three amp fuse. I found that the 1 amp fuse in the modulator would not stand up to atomic yellloooowww'ssss and had to increase it to two amps.

In Ron's transmitter he could fuse the mod deck with a 3/4 amp fuse and the RF deck with a 1 amp fuse.  I also noted that on eBay you can purchase many different size in amperage fuses along with holders intended for microwave ovens as noted by an earlier post. I think I'll use some of these in my next project.

http://shop.ebay.com/?_from=R40&_trksid=m570&_nkw=Microwave+Oven+High+Voltage+Fuse+ 

end of hi jack!

[W8ACR]

Thanks for all the great comments. I have decided to use circuit #2. With the parts I have on hand, I can put three capacitors in series to get 1uF@7000V to use as the blocking cap. Also gonna rework the RF deck to add 75 meter capability. Hope to be back on the air soon. I'll post some pics later.

Ron