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Author Topic: ferrite rod material for GG filament choke  (Read 53324 times)
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kf4qkr
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« Reply #50 on: July 12, 2011, 04:24:56 PM »

You guys will probably think im crazy but hey thats nothing new so here we go. I have not used ferrite for winding bifiller chokes in years.The choke in my main homebrew 3-813 AM LEENYAR amp is wound around an old broken ratchet with the head sawed off.I save all the made in China broken ratchets now days.Ferrite is too expensive. I have also wound chokes on rebarb,I even used a motercycle axal one time with success. I have never had one of these contraptions to not work.I have never worked 160 so I dont know if one of these chokes would work on that band but they work fine 10 through 80 meters.You guys probably have a good supply of ferrite and dont want to try this but if you ever get in a situation where you need it and dont have it dont be afraid to try other materials.They work !                                              73s Mike
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Mike
W4AMV
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« Reply #51 on: July 12, 2011, 05:02:42 PM »

Cool!  Cheesy If I get a hold of a broken wrench I'll put one together and give it a try. Can you imagine sitting in front of a 80K$ 40 GHz VNA, measuring a choke wound on a broken ratchet tool at 7 MHz. That would be quite FUN. Seriously, if the permeability of your broken tool is not unreasonable, than the inductive reactance present may be more than sufficient and it simply works. 
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W0BTU
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« Reply #52 on: July 12, 2011, 08:31:08 PM »

I've heard everything now.

I think I'll go wind a toroid using a stack of big steel washers as the core. Wink
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73 Mike 
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KM1H
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« Reply #53 on: July 12, 2011, 09:54:50 PM »

I wonder how much #6 I can get on a crowbar. Then I can try an old axle shaft.
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W0BTU
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« Reply #54 on: July 12, 2011, 09:58:57 PM »

You have to use multiple crowbars or axles to break up the eddy currents, Carl. ;-)
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73 Mike 
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Ed/KB1HYS
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« Reply #55 on: July 12, 2011, 10:01:23 PM »

Ferrite rods are cheap, find a 2nd hand BC Band radio and remove the ferrite antenna rod.  I used one in my GG linear and no problems at all. I think you can get them for a buck or two at yard sales or flea markets (not the Ham ones, the other kind) or some times junk radios can be had for Free just carry them off.  Strip some 12 or 14 gauge romex scrap (save the bare copper piece, and wrap as many bifilar turns as will fit. tape with crappy black electrical tape. Done. It'll carry a load of current too.  Works FB.
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73 de Ed/KB1HYS
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W4AMV
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« Reply #56 on: July 13, 2011, 09:29:20 PM »

The toroid choke measured and posted in prior posts was re measured via a GDO. There were no anti-resonant frequencies found. So indeed, the resonance in the early plot was due to the measurement itself.

The measurement was done in the same manner that plate chokes are measured, the total choke shorted start-to-finish winding ends and parallel resonance searched.
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KM1H
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« Reply #57 on: July 14, 2011, 10:43:47 AM »

Its very hard to get a good reading on a toroid with a GDO, even a high level tube job such as the Millen and Measurements 59 can have problems especially with ferrites.

An SS version is mostly useless.
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K5UJ
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« Reply #58 on: July 14, 2011, 11:13:24 AM »

Ferrite rods are cheap, find a 2nd hand BC Band radio and remove the ferrite antenna rod.  I used one in my GG linear and no problems at all. I think you can get them for a buck or two at yard sales or flea markets (not the Ham ones, the other kind) or some times junk radios can be had for Free just carry them off.  Strip some 12 or 14 gauge romex scrap (save the bare copper piece, and wrap as many bifilar turns as will fit. tape with crappy black electrical tape. Done. It'll carry a load of current too.  Works FB.

Before you buy, make sure there's a loop-stick antenna in there of sufficient length.  The older AA5 receivers up to around 1955 with the big octal tubes usually had a flat coil of wire glued to the inside back panel for an antenna--no ferrite at all.
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"Not taking crap or giving it is a pretty good lifestyle."--Frank
W4AMV
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« Reply #59 on: July 14, 2011, 11:44:05 AM »

Its very hard to get a good reading on a toroid with a GDO, even a high level tube job such as the Millen and Measurements 59 can have problems especially with ferrites.

An SS version is mostly useless.

Very well. I thought about that, so what I will try is to deliberatly resonant a ferrite structure, I will add C across the winding. Since the torus is self shielding, I coupled the GDO coil well within the structure. And again I will try. As a dress rehearsel of the effort, I took some plate chokes that I had built, they looked great physically, but awful electrically. Again the GDO was used here. But they are solenoid, not toroid.
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KM1H
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« Reply #60 on: July 14, 2011, 04:01:11 PM »

Plate chokes should be dipped in circuit, it doesnt take much of an enviroment change to move an inband resonace well away and vice versa.

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W4AMV
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« Reply #61 on: July 15, 2011, 10:16:30 PM »

Its very hard to get a good reading on a toroid with a GDO, even a high level tube job such as the Millen and Measurements 59 can have problems especially with ferrites.

An SS version is mostly useless.

I looked at this issue in a little more detail. I measured some material 73 and 61 with ui in the range of 850-2500. If you look at the impedance of these particular elements as ferrite "chokes" they have a significant real part and therefore the loss is high and the Q is small. Yes, there is no GDO dip. However, in the case of the toroid I posted, the real part was VERY small. The angle of the Z nearly 90 degrees. I added 22 pF which would resonate this unit near 3 MHz, indeed the GDO dips. Have to tune with care, but it is there. So... in many cases if the ferrite is chosen to be used as a lossy element with no resonant condition and a large reactance, NO DIP. After I removed the 22 pF, again a search showed no resonance present. So I guess the up shot of all this, is IT DEPENDS.  Smiley
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KM1H
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« Reply #62 on: July 17, 2011, 12:37:20 PM »

Quote
So I guess the up shot of all this, is IT DEPENDS.

Yup, thats what makes homebrew and restoring boatanchors fun. Sure as hell beats using a black box and a PC...BORING
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G3YRO
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« Reply #63 on: January 13, 2013, 09:54:23 AM »

Sorry to resurrect an old thread . . . but having searched the internet this is the only decent information I have found on this topic!

I have a Yaesu FL2000B Linear, that is basically the same circuit as the Heathkit SB200 (which itself I believe was a copy of a Collins linear?) . . also the FL2100B and KW1000 all use the same circuit, ie two grounded-grid 572B valves and a voltage doubler PSU.

The unit I got had been cannibalised for spares (by the Yaesu dealer I was working for while at University) so I rebuilt it just to cover 160m & 80m, by winding my own PA coil and not bothering with any input matching circuits (with a 5000pf coupling capacitor straight to the filaments it's about the right impedance . . certainly OK for a valve PA to match into.

The only problem I had was there seemed to be insufficient inductance for 160m in the bifilar filament choke feeding the filaments. (presumably the later FL2100Z which covers 160 has a bigger choke?) . . . but I got round this by taping an equal length of ferrite rod to the existing choke - this was a normal ferrite rod from an old portable radio.

This all worked fine . . . 600W output on 160 & 80 for 1200 W input, about what you'd expect.

HOWEVER . . . . and here's my question coming up!  Having used this linear for over 30 years I have always found that if there is heavy use (like in a contest) the power gradually goes down . . . . not just the output power, but the input current too.  If you let it cool down for a couple of minutes, the power comes back up again . . . but once it has started getting this hot, it very quickly starts going down again.

Now I had always presumed this was the valves getting too hot . . . . and maybe there was secondary emission from the anodes?  But to be honest, the anodes can glow red quite quickly ANYWAY, and this problem DOESN'T occur . . . not until the rig has been on for a while and the whole thing is getting hot.

But what ALSO occurs when this is happening is the dial lights glow BRIGHTER in time with speaking or CW transmission.

SO . . . I am now thinking that the problem is the BIFILAR CHOKE starting to not work properly . . . thereby reducing the drive . . and also why the dial lights glow brighter, due to the RF getting into the PSU.

I have also realised that the PA current of the driver rig goes down too - as would happen if you started feeding a lower and lower impedance. (which obviously would be happening if the problem IS the filament choke)

SO . . I figure that the problem must be the FERRITE in the choke . . and also my additional ferrite rod . . DON'T like getting hot! And when they reach a certain critical temperature, they STOP having their normal property of increasing the inductance.

I can find no info about this anywhere . . . but would you agree with my conclusions chaps?

If that IS the problem, I don't know whether the ferrite is getting hot from the RF from the driver rig, or whether it's just the whole Linear Amp case getting hot causing the problem.

Wonder if anyone else has had a similar problem . . . and what you suggest?
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W4AMV
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« Reply #64 on: January 13, 2013, 11:13:54 AM »

If RF plus DC current is present on the Ferrite rod and the rod is constructed with high perm material, its material losses could generate significant heat. The heat gets elevated to the point that you can actually smell it as in ceramic heater disks. If you get the rod hot enough (above the Curie temp), its perm drops and the problem gets worse, analogous to thermal runaway! Not suggesting that is your problem, however, if you believe that is an issue, I suggest you first measure the rod inductance with NO current. Then heat the rod, either pass the IV similar to its operation in the amplifier and even add additional heat, say a hot air blower gun. And measure the L value as the unit heats. You did not say what Ferrite material the rod is using. That would be helpful.   
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W4AMV
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« Reply #65 on: January 13, 2013, 11:19:03 AM »

this was a normal ferrite rod from an old portable radio.

Ah, I missed this. The perm used in these AM radios can be quite HIGH, say 1500-2000. Their magnetization loss could be significant and aggravated at elevated temperature. However, in the end, this is all speculation and a measurement is in order.
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G3YRO
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« Reply #66 on: January 13, 2013, 11:33:00 AM »

Sadly I have no test gear to measure the inductance !

But given what you have said about Ferrite not liking getting hot, I would have thought my diagnosis (from the various symptoms) makes sense?

If this is what is happening, it's not just my ADDITIONAL ferrite rod that is failing to increase the inductance . . . . as before I added it (ie with just the original ferrite wound choke on its own) I could still drive the amp up to 800W input on 160M (ie 400W out)

When this starts happening, the input and output drops right down . . ending up with less than 100W out!

By the way, I failed to mention that this has happened over the years with different sets of valves (I've replaced them every 6/7 years) . . . I originally had spare Cetron ones, but recently fitted some Chinese 572Bs, which are SUPERB copies of the originals ! (behave just like them)
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W4AMV
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« Reply #67 on: January 13, 2013, 05:27:30 PM »

No problem. In the 1973 ARRL Handbook, the classic B&W like Filament choke is noted on page 155. It is a 30-A choke using No. 10 AWG, 28 turns on a 1/2 inch diameter rod, 7 inches long. Similar is sold and advertized at 7.5 inches in length. In any case the same is stated at 40 uH of inductance and at that value of XL is 250 ohms of reactance at 1 MHz. That value seems reasonable for GG although perhaps off by a factor of 2 for the balun connection. In any case, NO where does anyone state the perm value. So, for grins, lets reverse engineer this thing and see what perm results. Stay tuned.
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W4AMV
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« Reply #68 on: January 13, 2013, 07:57:50 PM »

I calculated backwards the permeability required to obtain the inductance per the ARRL Handbook and the dimensions used in the 73 edition, Fig. 6-22. I find that a perm of 125  gives good agreement with 10 gauge wire, 28 T on the Ferrite form size they suggest for an Inductance of 40-50 uH. This would suggest Material -61 would be a good choice. If the choke temp is kept under 300 degrees C  Undecided, you should be fine. Welcome any comments. If anyone knows the exact material type used in these Filament chokes in the past, that would be great.

And if you look at all the prior posts as I should have, indeed -61 was indicated by several folks in prior notes.

Hope this helps,
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John K5PRO
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« Reply #69 on: January 14, 2013, 12:41:40 AM »

Finally reading this thread, I imagine that an additional reason rods are typically used is the isolation
that they afford, having the RF voltage at one end only. There is very low capacitance from that end to the cold end of the rod. In a toroidal winding, one has to take care that the hot and cold ends of the bifilar winding are not adjacent or overlapping, as others have said. Otherwise, the effectiveness, esp at higher end of the HF frequencies, diminishes. We are trying to bring the filament AC power into the RF circuit, at the cathode of a GG tube, without loading the RF circuit, and without allowing RF to get back into the AC circuit. Toroids can certainly be used, but in high Z circuits, strays from the end of windings can limit effectiveness.
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G3YRO
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« Reply #70 on: January 14, 2013, 06:19:39 AM »

I appreciate your replies chaps

Well looking at the original bifilar choke in my FL2000B, it looks like it's wound on half inch ferrite rod with very thick enameled wire . . . . but it's only about 4 inches long, so only has about 20 turns!

That's only 10 turns per winding, so I guess from the ARRL design that means it's only about 15 uH . . . which on 160 would only mean a reactance of about 150 ohms. (On 80m I guess 300 ohms, which is obviously enough)

The FL2100 and FL2100B are internally the same . . . but the later FL2100Z (which covers 160m) PRESUMABLY has a bigger choke . . . . but the only online photo I have found of the valve base compartment shows NO choke there!  Certainly there is no space for a bigger choke there . . . but I can't see it under the chassis either! (I really can't see where they have put a bigger choke, so if anyone knows, please tell me!)

Anyway, as previously stated, just taping an equal length of ferrite rod to my original choke made it work OK on 160m, so presumably doubled its inductance . . . as per my original post my issue is I think the overheating. Presumably that's because it's relying on a LOT of ferrite to up the inductance? (what do you think?)

If that's the case, I guess the best fix would be to make a proper longer choke with twice as many turns on a SINGLE ferrite rod. But the problem is I can't see WHERE I could fit a longer choke in the Linear!

The only other solution I guess would be to drill holes in the back panel opposite the choke and fit a fan on it (I already place a mains operated fan on top of the Linear to improve cooling if I'm operating for any length of time)
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K5UJ
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« Reply #71 on: January 14, 2013, 06:35:25 AM »

Something's heating up but what, that's the question.  What band or bands does this happen on?  

The usual suspects are your caps in the pi tank circuit on the output--the fixed value load caps.  Other possibilities are the blocking cap and plate choke bypass cap.  

Because of the dial light thing I think your problem is in the power supply, especially if you have one of these deals with a single transformer with with windings for all your voltages.  I'd first try getting the amp out of the cabinet, firing it up and running it in the condition that causes the problem and put a six inch fan that moves a lot of air, 150 to 200 CFM right on the power supply iron and see if the problem goes away.   If you have an IR thermometer measure the temp. rise on the transformer(s).

If the problem is only on some band like 160 then the load fixed caps and fil. choke maybe but these don't explain the higher voltage on the lamps to me.  You can try putting some snap on material 31 beads on the leads from the fil. choke to the sockets and see if that changes anything.
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G3YRO
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« Reply #72 on: January 14, 2013, 06:51:06 AM »

I really only actually use it on 160 . . . the new (bigger) PA coil I fitted is only tapped for 80, and as there are no longer any input switching/tuning circuits I figured it might be unstable on the higher bands. I used to use it on 80 occasionally . . I never noticed this effect, but that MAY have been due to not operating for over an hour.

When I first noticed this happening (YEARS ago!) I measured the PSU voltages and they were fine . . and they still are. As I say, I originally just put it down to the valves overheating, but now realise it ISN'T.

I also wondered about the output capacitors, but if any of them were failing, the plate current would surely go UP (as the pi tank isn't fed or going off tune) . . but the current goes DOWN, as if the DRIVE is going down. And even when this effect is happening, the pi-tank still tunes in the same place.

More recently I've noticed that my FT101E's PA current ALSO goes down when this happens, which would happen if it was feeding a very low impedance - such as the bifilar choke having no reactance! (there's nothing wrong with that rig, if you by-pass the linear it's immediately fine)

If the bifilar choke is ending up with very little reactance when it's hot, surely the RF will be trying to get back in the PSU? (despite the decoupling at the bottom of the choke) . . . that's why I figure the dial lights start to glow slightly brighter as I key. (this only happens when this overheating effect occurs)
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W4AMV
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« Reply #73 on: January 14, 2013, 09:49:23 AM »

Finally reading this thread, I imagine that an additional reason rods are typically used is the isolation
that they afford, having the RF voltage at one end only. There is very low capacitance from that end to the cold end of the rod. In a toroidal winding, one has to take care that the hot and cold ends of the bifilar winding are not adjacent or overlapping, as others have said. Otherwise, the effectiveness, esp at higher end of the HF frequencies, diminishes. We are trying to bring the filament AC power into the RF circuit, at the cathode of a GG tube, without loading the RF circuit, and without allowing RF to get back into the AC circuit. Toroids can certainly be used, but in high Z circuits, strays from the end of windings can limit effectiveness.

Thanks for the feedback John. If the stray C coupling is an issue it would show up as a reduction in the SRF and the same case occurs in the rod (solenoid). Allowing the start-finish ends of the torus to come to close is NOT desirable and it certainly would lower the SRF. I have used the toroid in Filament choke, no issue, as have others. I think the real issue of ROD-vs-TORUS is PRACTICAL. Have you tried winding #10 onto a toroid? Unless you have a huge one, its tough to get the bending radius. However, the 1/2 inch rod is quite straightforward! My motivation for looking at the material property was to address G3YRO query as he indicated broadcast band type rod material was used. The material I have used in those applications had quite larger perm values than desired for a fil. choke. Anyway, it looks like he is on track.
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G3YRO
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« Reply #74 on: January 14, 2013, 10:05:58 AM »

Just found the FL2100Z choke - it's twice as long as in the non-160m versions, and mounted horizontally across the top of the valve base compartment.
Quote
You can try putting some snap on material 31 beads on the leads from the fil. choke to the sockets

Yes I meant to comment on that suggestion . . . . Do you really get enough inductance that way?  How long would the leads need to be?

Maybe the answer in my case would to REMOVE the additional ferrite rod I strapped on, so it's just the original Bifilar Choke . . but then add ferrite beads on the wires to the filaments to increase the inductance sufficiently for 160M ?
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