Ferrites advice please

Densil wrote: ... harry is often a bit vague with ferrite info and he writes many times to just give it a try with what you have. its ok for him who has a feeling for broadband stuff and coils but i sometimes need more specific data. i think harry just has a lot of unmarked ferrites that he uses.

Hi Densil, yes, I am indeed back from Spain. I arrived yesterday but it took me some time to sort out the internet.

If you are to use ferrites for the inductor in a tuned circuit then yes, you need to be specific. But in this project you need to have a low impedance for DC and have a high impedance to RF.

If you had used just 5-turns then it would have worked fine, depending on the grade of ferrite. Possibly a problem at 1MHz.
If you used 200 turns then the DC resistance would still be low, but the impedance to RF would be even higher Possibly  problem over 50MHz.

The only problem with too many turns is that of inter-turn capacitance, so too many turns in an RF choke could be a problem, so 1000-turns would probably be too much, and difficult to wind. Anything from 5-turns to 200-turns would be perfect on almost any ferrite ring (a small stone or a 200kg rock will both crack an egg with equal efficiency).

The reason for using a ferrite ring is that the magnetic field is contained in a ring and there is a huge reduction in magnetic radiation that could cause instability and cross-coupling with other parts of the circuit.

Densil wrote: Question for harry. you wrote that you are coming back on sunday the 24th. are you back yet?
... you said that the third ferrite was needed for safety reasons. please can you explain more. with key-up i do not have any dc or rf on the antenna without the third ferrite so it looks safe enough to me. my antenna is a long wire from the window to a carpet beating frame in my own back garden so nobody should not be touching it anyway. i also have a good real ground- a 1m copper pipe deep in soil.
 /D

When you build a radio transmitter, or any other homebrew equipment, the most important point is safety. No equipment can be allowed that is unsafe or a danger to anyone. This is why the Radio Amateurs Exam includes so much circuit design information (or it did, when I took my written examinations).

Priority #1 - don't kill anyone
Priority #2 - don't cause interference to other services
Priority #3 - don't transmit out-of-band

In the 2-tube transmitter there is about 310V-DC on the anode of the final tube (valve). This is coupled to the antenna through a coil. The tuning capacitor(s) have no effect at DC. The ONLY component that prevents 310V DC from being coupled to the antenna is the single 1$ capacitor that couples the RF, but blocks the lethal DC component.

Capacitors have a voltage rating, and this is often degraded with time, temperature and humidity. Capacitors do leak DC. Normally the leakage is infinitesimally small, so you can ignore it. The two most common (and frequent) faults with capacitors are:

1 - Certain types dry out and the capacitance falls (eg. tantalum and heat)
2 - DC leakage current rises, especially with ceramic, and other old (reused) caps

If the capacitor feeding the oscillator to the grid of the second tube goes leaky, then the second tube will draw a LOT of current (common fault with many old tube HiFi amplifiers). If the 1$ capacitor feeding the antenna becomes leaky then raw DC will be coupled to the antenna. If the capacitor were to have a leakage resistance of just 25,000 Ohms then at 300V input it can allow 10mA to pass to the antenna, if there is no DC short at the antenna to clamp the DC.

50V DC, or 35V AC, (at 10mA) is the internationally recognised voltage that can kill humans. The human skin resistance can be as low as 5K Ohms. I learned this in 1968 when I worked as an electrophysiological technician (recording EEG and ECG), applying electrodes to patients heads and chests.

If you do not provide a DC short-circuit at the output of any valve (tube) transmitter then there is always a risk of lethal voltages being coupled to the antenna in a fault situation. This DC short is a legal requirement dating back to the 1930s.

You pointed out that you use a carpet-beating frame in your garden to anchor your antenna. We have one in the communal apartment area, here in Märsta. These are not more than about 1.4m high, well within the reach of people. They are also well Earthed as they are set in concrete. Local children sometimes use them as a climbing frame. Perhaps the antenna could be blown down in a gale? Perhaps you wife can hang wet clothes on it?

Please do not trust a 1$ capacitor to prevent anyone getting a lethal electric shock. Accidents do happen.

Put a big RF choke across your antenna output connector, inside the transmitter chassis, using wire that will take 10x the PSU maximum output current. Even if you should suffer a fault situation, you can demonstrate that your equipment is safe.

Incidentally, I have two Yaesu FT-101ZD HF transcievers, and one of the common problems is that the high-voltge mica capacitor from the 6146 output tubes (900V DC) can become (relatively) short circuit. Intead of killing anyone, it normally burns the PSU transformer out. In mine it just blew the mechanical fuse that I fitted.

The cost of safety is very small: an inductor, a fuse, a little thought. If you are worried about it then two series-coupled capacitors can be good, especially today when a lot of 50-year old components (NOS) are used.

Have I answered your question clearly?
If you have any questions or comments then please respond. All home constructors must be aware of safety. It only takes one isolated incident to hit the headlines and give a bad reputation to everyone else.

To quote Rowan Atkinson (the Thin Tlue Line): "It's your ass on the line and you don't want a cockup".

Very best regards from Harry - SM0VPO

PS - This was a very good question and should be a part of basic constructor knowledge when dealing with tubes. In recent years, with solid-state, transistors and ICs, it has become less relevant. May I use your question (re-formated) on my homepages in a new article? I will not mention your name (unless you want me to).

Question for harry. you wrote that you are coming back on sunday the 24th. are you back yet?
you said that the third ferrite was needed for safety reasons. please can you explain more. with key-up i do not have any dc or rf on the antenna without the third ferrite so it looks safe enough to me. my antenna is a long wire from the window to a carpet beating frame in my own back garden so nobody should not be touching it anyway. i also have a good real ground- a 1m copper pipe deep in soil.
/D

Stan AK0B wrote:Yes ferrite will work.  I just went through the calculations for a 2 tube 50 watt xmit.
for 2.5 mH use size FT-114 material #43 and 64 turns, I would use 20 ga or 18 ga
For 1.0 mh use size FT-114 material  #43 and 41 turns again I would use 20 or 18 ga
Kits and Parts have this size - 5 pcs at $6.00  ($1.25 per core)  A 114 core is 1.14 inchs in dia. 

Stan AK0B

Hi stan
thanks for the the info. harry is often a bit vague with ferrite info and he writes many times to just give it a try with what you have. its ok for him who has a feeling for broadband stuff and coils but i sometimes need more specific data. i think harry just has a lot of unmarked ferrites that he uses.

Harry. stan gave the source info in his post when he wrote Kits and Parts have this size - 5 pcs at $6.00  ($1.25 per core)  A 114 core is 1.14 inchs in dia. kits and parts is a web address; http://www.kitsandparts.com/ and you might want to put a link to this site on your homepage. just an idea.
/D

Hello Stan,
Many, many thanks for the information. Densil has not responded so I think I should.
It was good of you to calculate the values and give some practical information, this is really appreciated. May I update my homepage 2-Tube TX project with your information? If so, then can I include your contact details in the project in case anyone has any comments or questions?

You wrote:

Stan AK0B wrote:Kits and Parts have this size - 5 pcs at $6.00  ($1.25 per core)  A 114 core is 1.14 inchs in dia.

From where can one purchase these "Kits and parts"?
5 pcs at $6 (1.20 per core) - does this include postage?
From where can one order them?
Is there a link to a website on-line?

If you are running an on-line shop, or if you have a source of uncommon components, then you are invited to post the occasional advert in the "Wanted and found" section for radio/electronics related items. If not, but you see anything that could be of interest and "on-topic" then please share the info with us. I am always combing the internet for components at reasonable prices, from vintage IFTs, and Tunnel diodes, to CRTs, etc.

https://sm0vpo.forumotion.com/f3-wanted-and-found

FYI - I have noticed that Densil seems to visit the forum for a few days at about monthly intervals. He is the only member NOT to have a registered e-mail address, so he does NOT receive notifications about replies to his posts.

Very best regards from Harry - EA/SM0VPO (until Sunday, then SM0VPO)

PS - As a registered ember you may post pictures and links.

Yes ferrite will work.  I just went through the calculations for a 2 tube 50 watt xmit.

for 2.5 mH use size FT-114 material #43 and 64 turns, I would use 20 ga or 18 ga

For 1.0 mh use size FT-114 material  #43 and 41 turns again I would use 20 or 18 ga

Kits and Parts have this size - 5 pcs at $6.00  ($1.25 per core)  A 114 core is 1.14 inchs in dia. 

Stan AK0B

Hello Densil,
Nice to see you back on the board. Great that you got the 2-valve TX working, and 25-W is quite impressive.

I am a bit concerned that you wrote:
> i only got two chokes so i missed the 3rd one as it doesnt really do anything

If you mean the choke that is connected to the antenna circuit then that is VERY important. It does not affect operation at all, BUT it is necessary for safety reasons. If the 1nf capacitor connected to the anode of the final stage should become leaky, or short-circuit, then it will put 250 Volts DC on your antenna, which could be a danger to you and anyone who comes in contact with it.

So please be careful.

Best regards from Harry - SM0VPO

Hi harry thank you for info. my 2-tube tx is working now and it needs to be tidied up a bit. I had to do a few mods cos i was using different tubes. I used your trick of connecting 2 transformers 240volts to 6-0-6 10A and put the 12vols sec 6+6v  into a 12volt sec of another tranformer and got more than 330volts after smmothing.
It drops to under 300volts and gets warm on full power and i got 25 watts from a pl519. thanks for the info.
i only got two chokes so i missed the 3rd one as it doesnt really do anything.
i also put a 100pf variable cap and 40t inductor in the crystal so I can pull it a LOT. its an old trick for rubber-banding crystals. the cheaper the better it works.
/Densil

Hi Densil, I have not seen your text here for a few weeks. Hope you are ok?

Strange that you should ask that very question! Several other people have asked me questions about that project in the past three weeks, one of which is the same as your question (see below). Has it been re-published anywhere? Can you send me the link? I thought I gave all the info in the original project.

Σπηλιόπουλος Κώστας

Hi Harry,
Me and friend try to build your schematic 2 valve TX but we have some questions about it.First question is about 3 2.2Mh RF Chokes,what kind of ring we can use or can we replace them with another rf choke?If you have more details about schematic that you update it please send to us the denotes to start assemble soon.
Regards from Greece Kostas-Stavros/SY3AUB.

When I built that project I just put my hand in the junk box and took the first coils I had that were ready wound. They are rated at 2A and have 17+17 turns. They are designed for 50/60Hz mains "common-mode rejection". Frequency of operation is supposed to be 50Hz to 10kHz. I just put the two coils in series. Internally the coils look like those below.

The actual wire is about 0.3mm Diameter. According to the manufacturers specifications "Core material = ferrite".

If you are making a tuned circuit and need a specific inductance, and you need to calculate it, then yes, the ferrite material is important. Some ferrites are no good for VHF and UHF. But for 50Hz to 30MHz it makes no difference at all what ferrite you use. The only criterion is that is should be grey.

The number of turns should be about 5 turns to 200 (or more) turns since it is nothing more than a simple choke that needs to have a high impedance to RF in the range of 1.8Mhz to 30MHz. The more turns so the higher is the impedance. Mine is 17+17 turns (34 turns total, as in the pictures below). Former outside diameter can be from 5mm to 25mm, more if you have the space.

In other words, the specifications are:
- ferrite ring coil
- more than 5 turns (30 for good measure)
- MUST be a ferrite ring (not a rod)

The third coil (to the right) in the circuit is a DC-block(shunt). If the EL95 coupling capacitor (1000pf) becomes leaky then this inductor will short-circuit DC and prevent 250V DC getting to the antenna.



Have I answered your question(s)?

> sorry for the dumb questions.
There are no dumb questions.

Very best regards from Harry - SM0VPO

(Found these pictures on the web of a similar product)

Hi harry. i saw your 2-tube tx and i think about building it. the project uses a 2.2mh choke. where can i get them?

does it have to be the same, can I use any other 2.2mh choke?
can I make one with a ferrite ring?
what grade of ferrite?
how accurate does the 2.2mh have to be?

sorry for the dumb questions.
/densil