What about QRO Plate Chokes??

[Opcom]

The target equipment is that old 3CX3000 Jaws II amplifier. I don't know how large broadcast chokes of decent ratings are, but it has to fit in there, and according to the power supply, the amplifier might be able to do 4-6 KW on peaks if not at least 4 CW for a short time depending on the plate iron. I need to take some measurements and see what space is available. I have no intention of operating it there ;-^ but I want things to be sized right.

Sure I would consider a broadcast choke and I can look for resonances using the gear here, but I think I want to wind my own choke. It has been a long time since I made a radio part by hand and I owe it to myself to at least try to make my own. I guess I could try something smaller first haha but where's the fun in that. I wrote to Frontier Engineering and the gentleman basically told me in some detail how to do it and the only hard parts will be drilling and tapping the holes in teflon (or arranging clamps for ceramic) and getting the windings to stay in place neatly. Whether I really can do it remains to be seen. That's why I have asked about forms.

Is there any reason a plate choke for 1.5-30Mhz cannot be 1.5 or even 2" diameter? I may have some ceramic forms if I can find them.

[W3SLK]

Carl said:

Ameritron supplies the choke to RF Parts. And its more than ample for a pair of 8877's on CW/SSB. I use it in all my AM amps and the cost is so low its not worth the time and effort to wind one.

I bought one of those for a Drake L-75 I am working on. The damn thing is about an inch too tall and sticks out. Now I'm trying to figure out what to use to shorten it. Whether I should cut it with a band saw or sand it down with a belt grinder/sander.

[KM1H]

Most if not all BC band chokes will blow apart on at least 1 HF band. Getting to 160 or 80 is probably safe. I tested a big 5mH choke years ago and the series resonance fell right at 40M. It made for a good static drain on the 160/80 inverted vee.

[KM1H]

And the B&W is even longer.

The old Amp Supply is 4.5"; I have those specs.

Check Commander/Palstar, Ten Tec or other current amp outfits.

Do you have a bad Drake choke or are you starting over? I may have the specs here as I converted many to 6M and kept the originals....no they are long gone.

Carl

[WBear2GCR]

Ok... so how about some soft iron in the middle? It will not melt...

Might add enough uH in some situations??

Although that is suspiciously like the construction of an induction heater...Hmmmm...

                  _-_-bear

[W3SLK]

Carl said:

Do you have a bad Drake choke or are you starting over?

When I got the amp, the filter caps were blown and the glitch resistor 'evaporated' along with some of the runs on the filter board. I accuired a new filter board and re-installed everything. I couldn't get it to load properly ~200 watts out on 75, nada on 160. Then I toasted the bypass cap on the choke. When I replaced the cap, I got the proverbial fireworks and the glitch resistor blown again. So I suspect the plate choke being what it is, isn't to great, which is why I purchased the new choke.

[Opcom]

I think I'll try the teflon from the online metal store.  I think I can tap the hole for the mounting screw OK. I would like to try doing the small windings with 1/4" between them as suggested. Assuming the wire is attached to a lug at each end of the choke, it shouldn't come undone and I can see how a straight solenoid would be done.

How is the wire supposed to alternate closewound sections with 1/4" gaps and stay in place? Do I drill a small tangental hole and pass the wire through, then start the next closewound section? (that is a lot of wire to pass through a hole, several holes). Teflon as far as I know can't be glued to. The wire will probably be #20 or #22 on this.

[N2ZD]

Carl said:

Ameritron supplies the choke to RF Parts. And its more than ample for a pair of 8877's on CW/SSB. I use it in all my AM amps and the cost is so low its not worth the time and effort to wind one.

I bought one of those for a Drake L-75 I am working on. The damn thing is about an inch too tall and sticks out. Now I'm trying to figure out what to use to shorten it. Whether I should cut it with a band saw or sand it down with a belt grinder/sander.

  I did just that, I took the RFC-3 and tried to mount it in an L-4B.. About 3/4 too tall. I taped up the ends and then trimmed it down on a Tile wet saw, cleaned it up and then mounted it in the amp. Make sure you leave equal space on each side and that the mica insulators are in good shape. The screws will still go in without any problem and some nail polish will keep the windings in place.. Been working great over the past year and no problem with resonances.. If you need a hand, let me know.. Richy N2ZD

[Opcom]

One last thing: Normally we see enameled wire. What is wrong with using either cloth covered wire (salvaged from a large pie-would plate choke) and/or plastic insulated 'hookup' wire?

[W3SLK]

Richy said:

I did just that, I took the RFC-3 and tried to mount it in an L-4B.. About 3/4 too tall. I taped up the ends and then trimmed it down on a Tile wet saw, cleaned it up and then mounted it in the amp. Make sure you leave equal space on each side and that the mica insulators are in good shape. The screws will still go in without any problem and some nail polish will keep the windings in place.. Been working great over the past year and no problem with resonances.. If you need a hand, let me know.. Richy N2ZD

Thanks Richy, I appreciate that. I have a guy that really wants the L-75 since it is a nice table-top amp that does 160M. I need to get back to that project.

[K1JJ]

One last thing: Normally we see enameled wire. What is wrong with using either cloth covered wire (salvaged from a large pie-would plate choke) and/or plastic insulated 'hookup' wire?

If you look into any of my homebrew linear amplifiers, (and class C rigs) you will see plate chokes made from white 1" PVC forms and #26 insulated plastic wire. I stand them up on L brackets. I have never had heating problems with PVC, despite what some guys warn, even on 6M.

I don't even bother to check them for series resonance either. If they blow up, they blow up. It is actually difficult to randomly wind a choke that happens to fall into a ham band with a resonance like that. If it smokes, just wind it longer or shorter next time. I can remember only one choke that imploded on me. It was a pair of 750TL's and the wired ripped itself together in a rat's next. Other than that, I have not been "lucky enough" to be able to randomly wind a series resonance into a ham band. Just like gambling with 20:1 odds, in my opinion, so why worry.


BTW, series resonance must be checked with the choke IN-CIRCUIT to allow for stray and tube capacitance to make a real environment.  Checking it on the bench is false security and may swing into a ham band once in-circuit.

Using staggered turns, groups of windings, starting with small groups at the top and getting bigger as we go down the choke, is a good idea. Just wind it big enuff for the lowest band and try it out.

If the tank tuning is broad, sluggish and shows poor output power on 160M (lowest band) then the choke probably needs more turns.

T

[KM1H]

Insulation reduces the L requiring a longer form and it also adds interwinding C which does nasty things. If you never go over 40M its mighty hard to get a series resonance unless you use some ridiculous amount of L such as in a BC rig.

[K1JJ]

Insulation reduces the L requiring a longer form and it also adds interwinding C which does nasty things. If you never go over 40M its mighty hard to get a series resonance unless you use some ridiculous amount of L such as in a BC rig.

Hmmmm... Granted it would take a bigger form and more turns, but I would think that by using an insulated wire, the copper turns are farther apart, thus less inter-winding C, not more???  Or do you feel the plastic is acting more like a dielectric?

The technique of using string between turns is an example that appears to improve inter-C, no?

I've always had better luck with insulated wire with almost no blow-ups - whereas when using enameled wire with close-wound turns I have more than a few lose their guts.


T

[KM1H]

Insulation reduces the L requiring a longer form and it also adds interwinding C which does nasty things. If you never go over 40M its mighty hard to get a series resonance unless you use some ridiculous amount of L such as in a BC rig.

Hmmmm... Granted it would take a bigger form and more turns, but I would think that by using an insulated wire, the copper turns are farther apart, thus less inter-winding C, not more???  Or do you feel the plastic is acting more like a dielectric?

Yes


The technique of using string between turns is an example that appears to improve inter-C, no?

Dunno, never tried it. B&W did with the 801 and it blew up on 12 or 17M. Ive always used a close space solenoid wound and let it rip after it passes the in circuit GDO test.



I've always had better luck with insulated wire with almost no blow-ups - whereas when using enameled wire with close-wound turns I have more than a few lose their guts.

I thought you said earlier only one choke blew up

T

[K1JJ]

The string spacing idea comes from the Handbook, IIRC. A number of project amps used it. 

Unless I hear more evidence against it, I'm still  cornvinced that using plastic or Teflon insulated wire that inherently auto-spaces turns is a good idea.


-----------
JJ Said:
I've always had better luck with insulated wire with almost no blow-ups - whereas when using enameled wire with close-wound turns I have more than a few lose their guts.

KM1H said:
I thought you said earlier only one choke blew up

-----------

I should clarify and say in the early experimental days (1970's) I blew up many chokes. These were commercial BC types, those smaller Millen? types and ones I close-wound with enameled wire. Once this test period ended, I went with all #26 insulated wire homemade types and since lost only one in actual service.  The bad choke amp was a pair of 750TL's and had neutralization problems anyway, so was hard to tame.


T

[KL7OF]

In a TMC GPT-10K transmitter....the plate choke is 4 1/2 inches of close wound INSULATED #22 wire on a 3/4 inch diameter glass form....this TX goes from 2-30 Mhz @10kw pep....

[Bill, KD0HG]

I like the idea of using glass.

It would be easy enough to cut the ends off a Pyrex test tube from a scientific supply store...after two or three broken ones- LOL.

[KM1H]

In a TMC GPT-10K transmitter....the plate choke is 4 1/2 inches of close wound INSULATED #22 wire on a 3/4 inch diameter glass form....this TX goes from 2-30 Mhz @10kw pep....

Anything will work if you throw enough engineers at it. I like hard Teflon myself but Im not trying for 10KW.

[KD6VXI]

In a TMC GPT-10K transmitter....the plate choke is 4 1/2 inches of close wound INSULATED #22 wire on a 3/4 inch diameter glass form....this TX goes from 2-30 Mhz @10kw pep....

Anything will work if you throw enough engineers at it. I like hard Teflon myself but Im not trying for 10KW.

Pshaww...  Drivers!


--Shane
KD6VXI

[Detroit47]

Unless I hear more evidence against it, I'm still  cornvinced that using plastic or Teflon insulated wire that inherently auto-spaces turns is a good idea.

I have seen Teflon wire on Teflon round stock in use in the 50kw range with no problems. I have seen it in use in Dielectric equipment that was used for fusing PVC impregnated cloth and Vinyl. This was part of the process for embossing Vinyl door panels as well as head liners. They would be RF welded to PVC impregnated high loft non woven padding. I was also involved with maintain the seamer's for those inflatable buildings. Same process different resins though.

[Opcom]

The 1" teflon stock and #18 magnet wire showed up.

Leaving 1" space between the top of the choke form and the amp cover, there is 4.5", so make that 4" of windings. The choke is supposed to sit atop a large Sangamo doorknob style RF cap of .00075@6KV.

From what I can tell there is a space for another choke at a 90 degree angle. That one can have 3.75" of windings on it.

So it looks like the double choke system previously mentioned. That second choke is bypassed by a 0.001uF/6KV Sangamo RF cap. So it is

Plate ---|--> 4" choke---|-->3.75" choke---------T--->B+ 4.5KV
           |                     |                               |
           |                     |-->750pF--->GND       |---1000pF--->GND
           |
           |--->4700pF--->PI network

Unless there are objections to this I will soon wind up the chokes and install them. Then I should be able to apply the fire test. I'll take pictures as well.

One thing I still need is a plate clip for the 0.8" diameter cap on the 3CX3000. Does anyone know where I can find this? It might be like a resistor clip but I do not know.

Last question: I was told to use non-ferrous (Brass) screws when the connection is electrical, such as bolting together this plate circuit. Is this a true need?

[Opcom]

There are some chokes on you know where touting 'special ferrite' inside.

"Here is an RF choke on a Delrin form with a ferrite load.  The special ferrite raises the over all inductance without raising the distributed capacitance. The series self resonant frequency is therefore lowered below the 160M band.  This creats a choke usable across the entire HF range including the WARC bands.  This RF cokes measures 4.25" L by about .75"D. The wire is 23 AWG H200 insullated good to 200 Degress C. The ends are pressed fit delrin for sturdy mounting.  Mounting is 1/4-20  with the supplied delrin mounting screw

I have included a picture of one of these chokes installed in a Heathkit SB220 that was converted to cover 160M"

and

"This is a 3A RF Plate Choke good for 160 -10M.  Measures in at 500uH thanks to ferrite load inside the Delrin coil form. 5.5" long x 1.125" D.    1/4-20 bottom mounting with a included delrin screw and 10-24 top delrin screw."


this is just interesting..what sort of special ferrite?

[KM1H]

Notice in that first pic that he is also using a pair of Measures nichrome voodo science suppressors as well as nichrome to cary all the RF to the tank ciruit of that SB-220.

Obviously this clown knows very little about RF design. This is even better:

The series self resonant frequency is therefore lowered below the 160M band.

What a complete doofus

Carl

[Opcom]

Well that's ebay sellers for ya. It's better to have any nonsense debunked so fewer mistakes are made.

OK here's a picture of what I am about to do for the plate choke(s) next weekend. If there is any idiocy involved, please point it out!

In the picture there is not a supressor in the strap from the anode to the blocking cap but I'll steal one from one of those 3-1000 RF generators if that will do. IIRC they are a carborundum resistor with a 3-4 turn strap around it and seemed much larger than necessary.

[Opcom]

Speaking of QRO plate chokes, does anyone recognize this one? It's not in the greatest shape but probably could be repaired with a little glue and tape. It has "9308" stamped on it.