The AM Forum

Ok, so what about QRO plate chokes?

Simply "solenoid" - straight winding - wound on a ceramic form, all you need??

If so, why buy one if you have wire, varnish, a ceramic form and some time??

What's the sophisticated RF savvy ham's inside info??
What's the considerations to check for or design to prevent resonances in band, or in general??


          _-_-WBear2GCR

A strapping and cheap way to go is to cut off a 1" diameter, 14" long piece of PVC and wind #20 - #22 insulated wire on. Put three 'L' brackets on the bottom.

As far as resonances, it should be checked in circuit becuz of additional tube and component influences. ie, At higher freqs, you may see a good choke, series resonant out of the ham bands, on the bench... but in circuit, it could be pulled into trouble areas.

I just wind it and then test it on all bands in a hot rig. Out of about 25 chokes I've made, I can remember maybe two that blew up cuz of resonances right off the bat.

My point is it's actually difficult to wind a choke that happens to fall into the ham bands simply cuz the bands are such a small spot in the spectrum. You'd have to be unlucky to do it.  But if it blows up, just rewind it a little differently next time.  ;D Quickest way.

Here's the plate choke I wound for the new 4-1000a X 833A's.  I tested it on 160-20M and runs FB at maximum power out. Like the pretty stripped blue color?

http://home.comcast.net/~k1jj/wsb/html/view.cgi-photo.html--SiteID-3018584.html

Same with the 813 rig - this choke is black cuz I taped it over:
http://home.comcast.net/~k1jj/wsb/html/view.cgi-photo.html--SiteID-3018598.html

73,
T

B&W 800 I've tested at 5 KW peak designed for 1 KW AM 80 through 10 meters. it does have a bad spot around 25 MHz. The B&W801 works good on 160.

I've used RFParts RFC-3 ($25ish) on 160 thru 10 with good results.  It's relatively small (1" x 6") if space is an issue.

Bill N2BC

http://www.rfparts.com/choke.html

GFZ,

can you post a pix of that B&W choke?

Tom,

14" long!! Dats a big un!

          _-_-bear

The RF Parts and the Ameritron Plate RFC's look the same to me,  and not too expensive,  as noted ... ceramic form,  tapped at the base and has lugs for wire -- covers 160 - 10 M.  Ameritron runs about 3650 VDC on plates of largest amps,  but for Plate Modulated rigs,  the V rating of this item may not be enuf ... prob OK,  but U never know.

VU san  has wound many more RFC's than I,  so would use him as a reference.

Richie Measures has some strong beliefs about plate RFC's.  Some feel that he issa bit too compulsive,  but he has some good reading in his articles ... somewhere in there is one about Plate RFC's ...             

 http://www.somis.org/

Good AM  Vic  K6IC

I made the plate choke for the Collins BC transmitter by close winding #24 enameled on a 3/4 inch PVC pipe form.  There are 6 inches of winding.  The choke showed no resonances on 40 75 or 160 with the dip meter and has worked in the circuit on 75 without heating or any other problems for 3 years...wrap it tight and use several coats of cheap fingernail polish on the end windings to hold everything together. Good for full scrote .......

Yep, Art Collins would be proud... ;)   EZ enuff to wind them.

BTW, To hold the end wires, I drill a tiny hole through the PVC on both ends and slip the wire through.

T

I read somewhere about a guy that drills out the center of a 1" teflon form and stuffs a ferrite rod down in there, then seals it back up with hot glue, of something. I guess it's supposed to make a short coil think it's a big one. (eg 90uH will act like a 1000uH.)

I'll see if I can find the article and send it up here.

SK

I once had a big rig that wud not quite cover 160M, just 75M cuz the plate choke did not have enuff inductance. So I tried adding a short stick of ferrite rod down the center of the choke.   On 160M the ferrite got so hot it snapped into two pieces. I burned my fingers taking it out.

Probably not a good idea doing it with high permability ferrite... :-)   (But it works really FB for low impedance fil bifilar chokes)

I have seen this done for tank coils in the Alpha linears... they will use a donut core for 160M if I remember right...  Probably low perm material. Used to save space in the rig vs: a big coil.

T

Yah mon!

Kewl beans!

I shall wind me own, matey!

(found the B&W pix on the web... no prob)

Have some delrin on hand... PVC probably, and the guts of a commercial Wilcox "exciter deck" to check to see if there is one already made in it... ceramic would be nice ( it does have a big arse neutralizing cap fwiw...) .

Will check Rich Measure's site... may use his nichrome parasitic concept... seems to make sense.

Also take a look at both of my copies of Orr's hdbk...

Anyone ever put two in series where the realestate is at a premium??  ::)

           _-_-WBear2GCR

Yes, agreed Steve. I tried it myself and found the same. In addition, I've put some serious power through various linears on many HF bands and then felt the PVC choke form to be sure. Except for some radiant heat from the tube, they run cold on HF. (as you'd expect)

If the choke heats up, it's probably a series resonace. As you know, there are "degrees" of series resonance... it can be right in the band of close by...

BTW, for the archives,  I've found if an amplifer tunes real broad and sloppy, especially on 160M, there is a good chance that the plate choke needs more inductance. By JSing in another choke in series as a test usually cures it. The original choke may even show some heat if too small. This can confuse someone thinking it might be a series resonance instead. It's always amazed me how much choke inductance is needed for 160M. That's why you see 14" long, 1" diameter chokes in my 160M rigs... overkill. (using #24 insulated wire can require them bigger than normal)

73,
T

Quick note regarding the RFParts choke.

I've used 3 of them:

1)  2x4-1000 amp, 4KV on the anodes, class C and AB1
2)  2x4-400 AM rig, modulated by 2x4-400s (rebuild of an RCA BTA-1R1)  3.2KV on the anodes in 'illegal mode'
3)  5CX1500 AB1 linear /w 4.8KV on the anode

1 & 2 are 160 thru 10 w/o WARC, 3 is 160 thru 10 including WARC.  The chokes have worked fine.

While a near operating freq resonance will kill a plate choke in short order, equally important to the choke doing it's job is the use of a good quality, high current rated bypass cap. 

Regarding the RF Parts RF plate choke, any idea as to the max plate voltage rating?

Joe (aka PW Fallon)

That's one of the things I  like about series-fed link-coupled tank circuits.  The rf choke is at a low rf-voltage point in the tank circuit, the midpoint of a balanced tank, and the bottom end of  at unbalanced tank.  The rf choke is less critical.  Almost any rf choke will do, if it has enough current carrying capacity.  Choke resonance is not a factor.

Using the link noted above for the RFParts choke :

"1.8-30 MHz Amateur Bands including WARC.

Ideal for amplifiers using 811A, 572B, 813, 833, 3-500Z, 8877, 3CX800A7, etc.

Used extensively in the Ameritron and similar Linear Amps.

1.5A, 225 uH, 4 KV max

Size: 5 5/8" L x 1" diameter Ceramic coil form

Part Number: RFC-3     •     $24.95 ea."   ...

I plan to try one in my 'ol  CCA BC rig ... 4-400's X 4-400's @ abt  3100 VDC Plate V.

Look to the west for the flash.

73  Thanks for all the info.   Vic   K6IC

Talk about an old topic, but might as well resurrent that on that has the best title.

Of the choke materials, which do you prefer and which do you dislike and why?

ceramic

teflon

delrin (acetal resin)

PVC

I'm going to try winding my own. 4500V, 2.1A peak on a 3CX3000. I would not like it to melt or otherwise explode the first time. A 12" long 1" diameter teflon rod is $20. Not sure about ceramic except it is not easy to drill. I been warned about PVC burning at the higher end of HF.

I use the microwave test if it gets hot in a microwave don't use it, so that leaves PVC out. I have also seen ceramic fail at high power due to impurities. Teflon is by far the best choice.
John N8QPC

You can get Teflon round online for about $13:

http://www.onlinemetals.com/merchant.cfm?pid=6517&step=4&showunits=inches&id=214&top_cat=181

I order from them a lot  ;D

I was also wondering about Delrin though. Machines very nice, and is a bit stiffer.

But be careful with teflon as well. If you do do something that smokes it out
(arcing and burning) it puts out some seriously noxious and poisonus smoke.
It is not likely to happen easily, but it can happen. It takes a LOT of heat to do it.

I had the same experience as JJ using a ferrite bar inside a hollow plate choke. Got so hot it shattered pieces everywhere.

You can help inhibit resonances by close winding the choke for an inch or two, then leave 1/4" gaps between the succeeding close-wound sections.

If used, VHF parasitic suppressors should be air wound out of lossy wire like nichrome, which will also help stabilize an amp. I use pieces of #16 galvanized steel fence wire.

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. Its high inductance and 160-80M reactance make it ideal for keeping RF out of the PS and not overheating the bypass cap as do some amps that actually use the choke as part of the tank circuit.

The 802 is the currently available 160-10M B&W choke and it includes the WARC bands where the 801 fried. Its a bolt in upgrade in many commercial amps going back to the 60's including the Alpha 77DX and SX.

The DC voltage is virtually the same at both ends of the choke and isnt an issue. However RF voltages from an improper HB version with in-band series resonances can get extremely high and destroy a choke in under a second. Large wire has no benefit and can create problems with distributed C forming extra resonances. For 1500W PEP AM #26 is sufficient if you dont use some easily melted form such as PVC.  Ceramic or hard Teflon are the choices commercially.

Carl

Curiosity speaking here.....
How about a plate choke from an old broadcast transmitter? I have several of what appear to be old broadcast chokes, in that they are much larger than what you find in most amatuer equipment.

If they have enuff inductance to work down there, they should be OK-Fine up on the short wave bands. Just check em for series resonances.

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.

Carl said:
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.

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.

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

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

Carl said:
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.

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.

  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

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?

Richy said:
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.

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

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

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

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....

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.

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

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.

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?

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?

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:
What a complete doofus ::)

Carl

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.

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.

Dont be surprised if you wind up using a vacuum relay to bypass L2 on the higher bands.

Impressive looking, but I've tried a few of those in the early days for HF QRO and every one went off like a howlizer.. ;D ;D

I think the distributed capacitance or potential for multiple series resonace situations of those pie-wound rat nests is way too high for HF use. It's more suited as a BC/VLF choke device.

T

  Old thread revival, but here is some good info, cut the glass under water (carefully!!)

Glass would be the most elegant, but the Teflon is here. I do have to cut out L2 due to a resonance at 22 MHz of all places.

About glass, I was told to cut it with a hot wire, but that might not work well on Pyrex. Why is it better to cut glass under water?

.

Ive seen curved untempered auto glass cut with high pressure water and by sandblasting so anything is possible I guess.

Carl

This choke looks like the same type as the one used in the Harris AM-7224.

http://cgi.ebay.com/HARRIS-RF110A-High-power-RF-plate-choke-Assy-/150581415140?pt=LH_DefaultDomain_0&hash=item230f5a0ce4

It has a ferrite core which increases the inductance at low frequencies. The ferrite mix is doesn't do much at the higher frequencies. Less coil needed. Supposedly works from 1.6 to 30 megs without self resonances.