MODULATOR ADVICE

[W8UJX]

During my 58 years as a ham I have built many CW rigs from the milliwatt to the kilowatt level.  Mostly tubes. 

I would like to get some experience building modulators.  I have a Globe King 500B that puts out the legal limit on AM.

The stock modulator does ok, but to gain some experience I was thinking of building the modulator portion of K1JJ's quad 813 rig to use with the GK.  I plan on driving the modulator with a Heathkit 20 watt tube amp.  I have all the parts but the modulation reactor.  The schematic shows a 50 hy, 500 mA choke.  I would like a fairly flat response, +/- 3 dB from 100 to 5000 Hz.

Now for the question.  Can I use an ordinary filter choke for the reactor?  I have four large filter chokes.  They are about 4 1/2 inches high, four inches wide and three inches deep.  They each weigh 7 lbs.

I don't know the current; I would guess several hundred mA.  The measured inductance of one choke is 40 hy.  I could wire them in series/parallel to double the current at 40 hy if necessary.

Comments?
Jerry, W8UJX since 1954


 

[ke7trp]

Jerry.  The GK 500s are worth some good money now.  If yours is operational. You should keep it that way or sell it to someone who would apretiate it and use the money to build your dream AM rig.

Its your radio but I hate to see it modified like that. So much will have to be changed it might not be worth it.  The Tank in the RF deck is marginal as it is and running a higher power modulator into it, will surely give trouble with arcing and sparking.  The Power supply is already week and the voltage is low. 

The GK is what it is and its NOT alot of things.  Its a Great ham band 300 watt radio.  It can be made to sound great with minor modulator speach amp modifications. 

It wont ever be a 4x 813 transmitter with a big power supply and hifi modulator.

C

[KA2DZT]

Unlikely your 7Lb chokes will do the job.  They need to be insulated for high voltages.  Most small PS chokes are not rated for the high voltages that are in high level plate modulation xfmrs.

Thordarson made mod reactors.  I use a Thordarson 50hy 200ma audio reactor in my single 813 rig.  I also have three 20hy 300ma Kenyon chokes that I plain to use in a larger xmtr.  The Kenyon chokes are insulated for 15KV, so they should work.

I think you should try to find a reactor made for modulation service.  Although, power supply chokes have been used for mod reactors but maybe not in a legal limit xmtr, voltages are too high.

Fred

[VE3LYX]

Two negative replies would be a go ahead for me. I get nervous when folks agree.
Don VE3LYX

[K3YA]

Most of the modulation transformers commonly sold to the ham radio market don't need a modulation reactor and are designed to have DC through the secondary.  What mod transformer are you planning to use?  You may not "need" a choke.

The choke's that you describe sound small for the current that they will have to carry.  There is quite a lot of voltage across a modulation reactor too, and they sound small from that standpoint also.  You can use filter chokes in series to serve as modulation chokes, but they need to be up to the task.

[kb3ouk]

40hy by itself should be enough as long as it can handle the voltage and current passing through it to the final, something that can handle 2500 volts at about 400 milliamps should work. Don't parallel the chokes, though. It will lower the inductance.

[KM1H]

What do you have for the mod transformer?

Old ones already have questionable insulation so a reactor would be good insurance.

Id look around for a suitable toroid or try for some quotes as they are generally a lot less than the old EI type. Since ther is very little to be gained at the very low frequencies something in the 25-30H range would be fine. My Gates is 30H if I remember.

Carl

[WD8BIL]

Can I use an ordinary filter choke for the reactor?

Yes!

Mounting the choke(s) on insulators of some kind will keep the core way above ground and limit the chance of hipot breakdowns.

[W8UJX]

Thanks for the replies guys.   

First of all, Clark, I did not mean I would permanently modify the GK.  Never, ever do that!  I had one of these rigs when I was a kid in the 50's.  It is a great old rig and I get a lot of enjoyment using it.  Yes, it has its faults but I agree it is a great 300 watt ham band radio.

I am doing this because I love to tinker.  If something works well, I have learned something.  If it goes up in smoke, I have learned something else.  I have already learned, several times over, to be very careful when working with 2KV power supplies.  Have the scars to prove it.       

First, I will remove the stock GK modulator and put into warm dry storage.

Then I will build another chassis/front panel to fit in the King and build the new modulator there.

If the new modulator works better that the stock mod I may leave it there for awhile.  This gives me a platform to experiment with various audio circuits, and maybe learn something.

The modulation transformer I plan to use is a Thordarson T21M64.  It is rated at 300 watts.

If I do try to use the filter chokes I will definitely mount them on insulators.  I forgot to mention the DC resistance of the choke is 78 ohms.

I will test the frequency response and distortion of the modulator, with and without the choke to see if there is any difference and go with the best design.  When I am done tinkering with the modulator I will restore the GK and build the 813 amp that goes with this modulator.

Jerry, W8UJX since 1954

[Jim, W5JO]

Some years back Don/K4KYV wrote an article about how to remove the couplates in a GK 500C.  That article applies to the B&C and can be made to work in the A model.  It utilized the stock modulation transformer and subs some parts in the audio chain to enhance the operation. 

It works quite well and will match what you currently have.  If not mistaken, it was published in the AM Press Exchange.  Someone should have a copy of that ariticle and it might be good to get the infomation to keep should you want to do something with the supplied modulator deck.  The best part is the mod looks like factory.  In the A model it will increase the overall gain enough that you can eliminate one of the 6C5 stages and it resembles the B&C model.

[ke7trp]

I have never seen that information.  That would be neat to check out a schematic. I used W0VMCs schematic on mine Just like you said. Minimal mods, Looks factory.  The only thing I did do is remove the clipper. I might put that back next time I have the deck out.

C

[W8UJX]

I would like to see a copy of Don's article.

I also believe I have a schematic to replace the 6L6 driver with a 6DZ7.  That is like two 6l6 tubes in one envelope and will plug into the same socket as the 6L6. 

Since one half of the 811A driver transformer primary is not used, it would be easy to wire in the 6DZ7 and make a PP 6L6 circuit to drive the 811A's.

This would be a fun mod to play with and one that would be easy to remove to restore the King to its original condition.

Jerry, W8UJX since 1954

[Jim, W5JO]

My 500A has the original 4-250 final and gives about 325 watts without protest.  I took the diagram that Don suggested and tuned it to my voice and see ~ 1200 or so watts PEP with the LP 100A meter and on the scope.  You need not replace the 6L6 with the 6DZ7 unless you choose.  The mod iron is stock but one from Peter Dahl.  The orginal transformer arced while the original owner had it.  He kind of kludged a multimatch in and I wanted a better fit so I ordered the transformer.

I don't know about yours but in its original form I had a bit of negative carrier shift which was cured by using solid state rectifiers in the HV supplies of the final and modulator. 

I would be happy to share the ariticle with you provided Don does not object.  He would have to approve it before I would send.  I also made changes to the final tank circuit so it would match 50 ohms on 160 meters which is where I really like to use it.

[k4kyv]

No objection to anyone sharing the information.  I'd have to dig that article out of the archives myself, something that probably would take a couple of hours at least.  Basically, just take out the couplettes and  replace them with conventional R-C coupled interstage components.  I think I took the values recommended in the 1943 ARRL handbook, which were supposed to go down to something like 100 kc/s, and multiplied the coupling capacitors by a factor of 4 or 5.  The idea is to get the flat response of the coupled stage well over an octave below what the actual lowest modulation frequency will be.  That keeps the phase shift to a minimum.

Regarding the reactor, figure out the modulation impedance.  Zm= final amp plate voltage (volts) / final amp plate current(amps).  For every 1000 ohms of modulating impedance use a minimum of 8 henries of inductance in the modulation reactor.  For example, a final running at 2000 volts on the plate, and 500 ma plate current = 4000 ohms modulating impedance.  So, for 8 hy/1000 ohms, the minimum recommended reactor inductance would be 32 henries.  It would be OK to round that off to 30 hy.  A 50 hy reactor would be better still, since the figure of 32 hy would be the minimum recommended inductance.

[Jim, W5JO]

Jerry, if you and Clark will send an message to my QRZ address so I can get yours, I will scan and send the article to you.  It is about 4-5 pages long.