The AM Forum

Gentlemen:
     I've been doing some CW on 160 meters lately and having some fun with it. My old Globe Chief Model 90 works on 160 but I've been having some problems with it and now want to build a simple 2 tube transmitter for CW on 160. A monobander. To avoid the cost of air-wound coils, I thought I'd simply wind toroidal inuctors for the plate circuit of the oscillator and for the pi-network of the amplifier circuit. Toroids are smaller and easier to mount also.
     The plans for the transmitter are from Lew McCoy's article in QST, August 1969 entitled "Building a Novice Rig From an Old TV Set" . This is a simple 2 tuber and was designed for 80, 40 and 15 meters. I've built one in the past and it works great on these bands. It used, however, air-wound coils. I still have this rig and it still works well, but I don't want to tear into it and modify it for 160. Hence a new rig, and will pull power even from an existing power supply in another rig.
     Any advice on toroidal inductors in this application? I suspect the Q of a toroidal inductor is quite a bit different than that with air-wound inductors.
Thanks in anticipation of some ideas and advice.   ---Marty, KK4RF---

I have used them for year. There field is more confined and the Q is good if you use type 6 or type 2 for HF work. I have used T50 and 80-2s to wind tuned coupling transformers that worked great.

I used a T400-2 in a 2000 watt 160 amp 15 years ago with a 4-1000A and it never even got warm.

Pat
N4LTA

Pat,
     Thanks for the report on your experiences with toroids in tube circuits. I've got a number of suitable cores here and will be giving them a try. Thanks again.
---Marty, KK4RF---

ETO/Alpha used toroidal inductors in the output network of many of their amps to support the compact size.  I picked up a 274 (the early dual 8874 version) for $150 at a hamfest and it works nicely with my Signal One CX7A, tunes smoothly and the RF network seems to work fine.  Those coils definitely allow for a very compact RF compartment.

Hi Mark. A good resource is

http://www.micrometals.com/

They published a manual on Q Curves for Iron Powder Cores. Not sure if its still available. -2 material can provide unloaded Q values above 200 and larger sizes well over 300. So their competitive with solenoid air core. Since they are in the shape of a torus, they are self shielding. The lower perm materials, say 1-7 are probably preferred for keeping the heat down, losses are lower. The lowest TC cores are -6 and -7 and of course -0 which is phenolic. Hope this helps.   

Most of the current and some past amp manufacturers use/used toroids at least for 160 and sometimes for 80 and even the L inductor in a Pi-L.
Ive used T200-2 thru T400A-2 singly or stacked for 1000-5000W levels for decades.

For a 50-75W rig the T80-2 would be a minimum size singly or a T94-2 is better since there is a considerable jump in Al value. The T106-2 would be my choice since the Al jumps again and it and the T94 both are restricted to the same number of turns.  Use #18 or 20 Teflon wire to minimize heat which will cause tuning drift and since there will be high RF voltages.

Carl

Gentlemen:
     Thanks for all the info/ideas. I've got several of the T106-2 and T200-2 cores here and some fairly heavy enameled wire, but no teflon-coated. I'm hoping to get about 60 watts out of the rig. I've been reading old ARRL Handbooks to calculate the components in the pi-network output circuit. This will be a fun project and even better if it works , HI! Thanks again to all.---Marty, KK4RF---

Use some Scotch 27 or similar HV tape. If the wire isnt too heavy Teflon sleeving will do.

Carl,
    I'm hoping I can find a local supplier of the Scotch #27 tape. If not, I might have to order it. Thanks for the advice.
---Marty, KK4RF---

A friend of mine overhauled a Multi-Elmac (AF-67?) a couple of years ago and as I recall replaced the pi coil or part of that network with a toroid.  He may have been following an article in Electric Radio.  I will try to get him to chime in here.

I had a message from Tom, WA3KLR about the toroid use in tube PA Finals.  Yes I did replace the 160-20 part of the Pi coil in one of my Multi Elmac AF67's with a tapped toroid inductor. The original design appeared in Electric Radio but I am not at home and I do not remember the issue.

The original AF67 pi network used mini-ductor material and in most cases the plastic has either melted or deteriorated over the 55 years since in was built. It was a mobile rig and space was limited so the wire size especially on 160 and 80 meters was marginal. The 160 meter coil was actually mini-ductor that you would use in a receiver and very easy to bend and distort. Using the toroid made a lot of sense and actually increased the Q and efficiency. I get 50 watts output on 160 and 75 meters and close to that on most of the other bands. The AF67 uses a 6146 modulated with a pair of 6L6's or 5881's.

Most restorers like to keep things stock and since there was no toroid material, at least not what we have today, at the time the AF67 was constructed it would be a no-no to use it for some. But from an electrical standpoint using a properly sized toroid works very well. I would not hesitate to use one in another rig of that vintage should the need arise.

I think the material I used was a T-200-6  but again I am not home and I do not have that info a my fingertips. It was a red toroid and nicely covers the HF range down to 160M. I wound a few layers of electrical tape over the toroid before winding the enameled wire. I happen to have a very large quantity of #15 enameled wire left over from my company (we wound large magnets with it) so I used that as I remember. I did send photos and info into the Yahoo Multi Elmac users group though and it is still on file there. If you need more detail I can give it to you when I get home in a few days.

I have a Multi-Elmac section on my web page but I have not put up the AF67 as yet. I do have my A-54 restoration on there and I did use a toroid for the 75 meter coil. The A54 uses separate coils for each band.  http://www.crompton.com/hamradio

73 Doug


   

Tom, and  Doug,
 
     Thanks for all the info. I'm trying to track down some Scotch 27 electrical tape for the project. This has been a very instructional subject thread for me. Thanks agin for the shared experience.  73s, Marty, KK4RF---

Marty,

   I don't think you need to use a special type of tape. There is virtually no heating in the toroid in my AF67 at 50 watts on AM key down for a long time.I just double (overlap) wound the entire circumference with standard electrical tape. Mine happened to be grey in color.

If would also be a good idea to not have the HV DC on the coil. Some old rigs like the A54 put the plate blocking cap after the Pi coil (antenna side) and fed the DC from that end. I changed mine around to be the more conventional arrangement.

73 Doug

Doug,
     Thanks for your observations. It looks like I'd have to order the Scotch #27 on-line.
     I got to thinking about toroid cores and high voltages. There won't be any DC voltage on the inductor as the DC blocking/coupling cap is between the plate of the 6146B and the pi-network.
     My antenna for 160 is an end-fed half-wave dipole in an inverted-L configuration and I feed it at the bottom end, about a foot above ground. I match the very high end impedance with a link-coupled parallel resonant circuit consisting of a toroid (T-200-2, about 31 uH) and a parallel variable cap. I suspect there must be a few thousand volts RF at the high end of the LC circuit. I don't try to put more than 60 watts or so into the antenna as I've feared arcing by the variable capacitor.  I should have thought about that before I made the thing as I don't remember putting ANY kind of tape around the core. So far, no problems, but I'm thinking of ordering some of the Scotch #27 for future use.
     Thanks again. My project is slowly coming along. I'm building the xmtr in the remains of an old KnightKit T-150. It was way too far gone to fix up as many parts were missing when given to me.     73s, Marty, KK4RF---

I have used Teflon tape purchased at the hardware store. The type used for sealing pipe threads in plumbing. Easy to apply, stays in place, low cost. Application though at power levels under 250 W.

     I've used the teflon stuff on pipes in the past. I'll check it out at Lowe's next trip out. Thanks,  Marty, KK4RF

Gentlemen:
     Getting quite a bit of output on the rig. Fully lights up a 60 watt light bulb and the  rig keys pretty well. Using lots of clip leads at this point.  Still need to wire in the meter switch and the function switch and quite a bit of other wiring before I can put it on the air. No arcing on the pi-network toroidal inductor. I used a T-200-2 with 37 turns of #18 enameled wire, measuring about 24 uH inductance. I never did get around to putting any teflon tape or #27 Scotch tape on the core. If it arcs or smokes, I'll let you all know. Many thanks to all for the comments and ideas.  ---Marty, KK4RF---

I've used a T200 core for a pi filter coil as well. It was used in a GG SSB amplifier test set up. Worked without getting warm.

PA0FRI, who has an interesting website with lots of home made QRO projects, describes such an inductor on:
http://pa0fri.home.xs4all.nl/Lineairs/Frinear750/FRI750uk.htm
(look for the coil at the bottom of the page)

And while you're there, you can explore the rest of the interesting site.

Martin

this is a very nice thread...

once the pi-net inductor is calculated, i.e., the required uH is known, where does one look for formulas to compute the number of turns for a given core ?

Also, is there a list somewhere that gives the power handling capability of
the various cores available today, and an appropriate frequency range for these cores ?

Peter

Hi Peter, here is a link to Amidon:

https://www.amidoncorp.com/pages/specifications

Phil   AC0OB

I believe most single coat solderable "enamel" wire is good for around 500V, dont know about the red paint on the core or what happened at the wire bends.

You can easily calculate the peak RF voltage at resonance and realize that well off resonance it can be a lot more. High Q just adds to it.

GL

Carl

Many thanks !

The Amidon inductance chart appears to low-ball the inductance for the
T-200-2 coil measured above (37 turns measured at 24 uH).

Time to buy an L meter...

With advancements, what is the highest frequency that a toroid can be used in a high powered amp?

Gentlemen:
     A lot of neat postings in the past day or so. I'm still working on my 12BY7A-6146B transmitter. I'm measuring about 70 watts out to a 50 ohm dummy load  using an MFJ wattmeter, about 600 volts on the plate of the 6146B with 150 mAmp plate current. So far no problems with the T200-2 toroid despite no Teflon tape (or Scotch #27.) I certainly would have wrapped the core if I'd had these tapes readily available, but I was in such a hurry to see if the rig was working, that I threw all caution to the wind  and wound the toroid without that protection (haste makes waste?) Anyway, no arcing yet. We'll see what happens when I connect the rig to an antenna and have to do some loading up.
     I do remember smoking a toroid in my Kenwood TS-440 26 years ago. It has the built-in antenna tuner option and I tried loading up a mismatched antenna for 80 meters. I didn't understand then that if you exceed the SWR by 3:1 or so, that you could damage the tuner. I remember seeing smoke coming out of my then brand new $1200 rig. A call to the dealer impressed upon me the reality that this was my fault, not the rig's. I took it apart and wound a new toroidal inductor and put it back together. It still works fine today. I try being more careful with its limitations, however.
     I see ads for the MFJ auto-tuners with all their toroids inside. Does anyone use one? Do they wrap their toroids in teflon or scotch #27? I'd be curious to know.
     Thanks to all. Martin, I liked your posting. Carl, your advice is very reasonable. Maybe I'll luck out at the 70 watt level. If it smokes, you guys will be the first to hear about it.  73s, Marty, KK4RF

This is a pretty hand calculator:

http://www.dl5swb.de/html/mini_ring_core_calculator.htm

it really is...thanks.

Am I right to think that for higher power you stack cores ? or is it better to use the higher # cores (300 or 400) ?

Gentlemen:
     Got the rig finished enough to try on 160 tonight. Made a qso with Bob, W8PC, somewhere in South Carolina. He said my rig sounded good there.
     Needs a lot of work to get it all together and enclosed. I hope I get it all done someday. But it was a fun project and if anybody wants to build a simple two-tube cw rig, I highly recommend this easy to build transmitter from 1969 QST, by Lew McCoy.
     Many thanks to all who helped me in this project.   ---Marty, KK4RF---

Congratulations. Hope to catch you on the air with the new rig soon.

Marty, all the 1500W+ amps Im aware of use 27 or similar thick tape on the core and Teflon insulated wire or sleeving.

At least one commercial amp used them to 40 but at that power level you need one core per band. Command Tech tried 160/80 with one core and customers were burning them up.

And I converted many SB-220's to add 160 using a pair of T225-2's back when 1500W became legal, tape and Teflon of course. Sold kits also.

Thanks, Steve. Hope to catch you on 160.
Carl, thanks for the info on the linear amps and the protective tape. No argument that this is a best practice guideline, i.e. using the protective tape on the cores. I found numerous other examples on the internet.  i plan to get some of the Scotch #27 soon and rewind the  amp pi-network inductor. Curiously, for now, I've not noticed any heating whatever on the core itself. Thanks again, Carl.
     73s, Marty, KK4RF

If that core was heating you wouldnt have any output :)

Carl,
     Good point. 73s,  Marty