The AM Forum

I know some people on here have heard me for years talking about how I'm going to build a 24 FET rig. My plan is to build a 160/80 rig and do PDM and I plan to use a flex 1500 which should give me my 1 watt of drive. I have a flex 1000 which will work too...anyways. Over the years I have just been collecting parts. I plan to follow the classeradio.com website.

So I am going to lay out the FQA11N90 and the IXDD614 and C2, C4 and C6, I see they are .47uf, but what should I be looking for on voltage rating? I see 12v 2a, would a minimum 16v cap be OK?

Also same thing on the the shunt cap 1500 pf I see a ATC100C with microstrip termination, Where are people getting these? I am striking out using google.

I guess this will be step one for me. When I have a completed RF deck I think there enough people in New England, hopefully I'll be able to bring it to someones house to test it.

Ok, I have made some minor, but very important modifications to the circuit since the initial schematic was published.

So please get the new schematic for the 24 FET RF amp from the class E web site.

http://classeradio.com/24_fet.htm (http://classeradio.com/24_fet.htm)

The differences involve putting a 300 ohm resistor in series with each driver input, and documenting the fact that a single 12 or more amp power supply may be used for the drivers, rather than an individual supply for each module.

The resistors are very important.

I'm guessing that the reason for the 300 ohm value is that there are 6 drivers connected to one coax feed.  300 ohms / 6 modules = 50 ohms.  

So if a person were to have 2 drivers connected to a single coax feed, they'd use 100 ohms resistors instead (both in parallel with the feed and a 100 ohm resistor going in series to each driver.  Is that right?  I guess the series resistor value wouldn't matter too much since the driver is a high-impedance device.  It's just some amount of resistance to fight parasitics.

And building a 24-FET rig for your first class E rig?  Cool beans!  I built a single FET version for my first try.  It was very useful to experiment with and learn how to operate and tune these rigs.  I tend to take baby steps and am generally cautious in life... I guess that's why I decided to go with the Walmart drone version rather than the F14 fighter jet for my first transmitter.  

Anyway, best of luck with the 24-FET deck!  One of these days I'll build one too.  I mean, why not?  The parts cost like $100 or so.

Jon

More or less, yes.   For 2 drivers the resistors across the feed lines for the drivers would be 100 ohms eash, so as to terminate the junction of all of the short feed lines in 50 ohms.  But, the series resistors are still 300 ohms.

OK so I have tbd schematic, but I still can't answer my original question  from my original post.

I've had trouble locating ATC capacitors.  Instead, I've used Russian doorknob capacitors.  They're not nearly as compact, but I mount them above the FETs using #14 solid wire since it will hold them mechanically.  They're also dirt cheap and will handle about a million KW. 

My 2 cents on the .47uF's is that 16 volts or above should work fine.  Just don't go too big since space is a premium and it gets really crowded on that little copper strip really fast. 

...and just so you know, I'm not the expert on this stuff.  I've just built a few that happen to work.  That's all.

Jon

Oh, on the driver bypass caps, 16V would work.  Make sure they are high current capacitors with a low ESR.

On the shunts, I would stay away from doorknobs.  I have had numerous failures of doorknobs - the good ones - and 100% failures with the not-so-good ones.

If you can't find ATC caps (I get them directly from ATC or on Ebay), you can use 2 of the Russian silver-mica 500V capacitors.  They work quite well.  Get 2 750s or whatever you can get.  A 1000 and a 500 will work in a pinch.  I've used them on a number of class E rigs and they work.

I just found a listing for these on Ebay 750pF 500V.  Perfect.  2 per module.  Try to buy a lot of them - you get a better price.

(http://www.classeradio.com/russiancap.jpg)

Not sure why ATC caps are hard to get.

RF Parts sells them directly.

Now, if you meant, hard to get at a realistic price, I feel ya.

Unelco also makes the same style.  Unelco Metal Clad Mica caps.

https://www.surplussales.com/capacitors/UnelcoMtl.html



Hope it helps.  Surplus sales has about any value you want below 600 puff.  And in damn near every case style imaginable.

Ebay metal Clad Mica for better deals.

--Shane
KD6VXI

OK all, I think I've found what I need, for some reason .47uf caps at 16v seem to be hard to find in surplus sources. I can find 25volt, but 16v not so much.

I use 100V caps in my own rigs because that's what I had around.

OK well I've been working on the heat sink for the last couple of nights for an hour each. I've got most all of the parts needed to complete the heat sink. I'll post a picture where there is something worth looking at.

Things are coming along nicely, at least it feels/looks like it. At this point I have a big hunk of heatstink and and copper bus strips. Everything is laid out and then holes are marked with pencil and sharpie. I'm getting ready to drill and tap. I was looking at the FQA11N90s and I see they need a thermal insulator, I noticed in my searches there is a also a little washer. Should I also be drilling my holes out to accommodate this washer/insulator? Or more importantly are people using them? I can't seem to glean this information looking at the class e website or my internet searches.

Hi Bob,

FQA11N90s require an insulator and nothing else.

I use 4-40 screws to hold down the FQA11N90s.  Some guys drill out the hole in the FQA and make it accommodate a 6-32. I don't recommend this, but it appears to work.

 

Try these guys http://www.passiveplus.com/

Is it done yet? :o :o

I got some more questions on my build.

1. On the 75m inductor using number 4 wire I only was able to make 8.5 turns. I noticed the value is 7 uh. My meter will not measure below 2mh. So I'd have to get a new meter. What I noticed is those meters use a certain frequency and obviously I'm concerned about 75m. Doea anyone know how crucial this is?

2. Also the there isn't any spec for the 160m coil. Such as diameter and how many turns, which leads me to believe I need a meter to measusure inductance.

3. Lastly I was going to energize my relays for 160m off the driver power supply, anything to watch out for here?

I think there's more but I can always post more later.

I use 4U rackmount enclosures, if i am counting correctly I think thats 10 winds on 75 meters 2 inch form,#8 wire.

Another picture iots of room for my big hands.

That should be fine for the inductor, if you have enough series capacitance.

OK guys, so I can use the same piece of PVC that I wound my 75m inductor on to wind my 160m inductor. I just need to double the windings and I'll be using #8 wire as called out in the schematic.

Lastly where can I find those nice ceramic stand offs that you guys are using to tie your inductor into?

i find most of my standoffs at Hamfests.

You can use the same *type* of PVC, but not the same physical piece - because there will be coupling between the coils, and the shorted 160 meter coil section will get hot when you're on 75 meters.

Typically I recommend putting the coils at right angles to each other, or one next to or above the other to minimize coupling.

So I got my monster soviet vac cap for the tuning. I need some ideas how to mount it and mounting hardware. I measured and the "collars" are 60mm and I also need a shaft coupling. It's 12mm. When I measured with my caliper and thats what the decimal converted to.

So if you giys have anything in your hoard heres your chance to thin it down and help a whipper snapper homebrewer.

Hello from the metric side of the ocean.
I made mounts for my rusvaccap from copper strips (copper pipe split in half and hammered flat) as top clamps, and bottom clamps/mount, from a piece of hardwood. Made a nice bed for the capacitor. Connections were made directly to the straps.
As far as shaft coupler went, I used a piece of 1/2" sturdy fuel hose (metric right...) as coupler to another shaft..

I use hose clamps, home made copper brackets and big stand off insulators.  This works quite well.

Pictures:

For some reason, the BBS software will not allow in line pictures.  So, they are attached.

The pictures show two views of the back of the cap and one of the front.  The mounting is copper and insulators.

Note the copper braid is the conductor.  The bent copper does not make a good enough connection to the capacitor.  The braid does.

The front of the capacitor (where the adjustment shaft and insulated coupling are located) is the low voltage end.  This connects to the loading capacitor.  The high voltage end is the back of the capacitor.

You may notice another vacuum capacitor - a 300pF fixed capacitor.  This is connected across the 1000pF capacitor, to give some extra capacitance for 160 meters.

Oh, this transmitter is not my main 24 FET rig.  This is a different transmitter that uses vacuum caps in the output network.  My 24 FET rig uses air variable capacitors.

My vacuum variable arrived and this is how I mounted it.

Jon

Take great care with those clamps Jon. A little too much force close to the seal and the glass - metal seal will break./ I always make them of thin copper foil to prevent local force or use the original clamps. But also the original clamps should be mounted such that force near the seal is prevented. . I speak from experience... :'(

Thank you for that tip!

I think I was fairly careful not to clamp too hard when I mounted it. Either way, I'm not going to touch it. Even if I broke vacuum (which I don't think I did) I'm probably not in danger of arcing over. But, definitely noted! These capacitors are expensive!

Jon

Hi Jon,

If the vacuum failed you would certainly have arcing with a 24 FET rig power level.   The vac cap spacing in very small and would barely work as a loading cap, never mind as a tuning cap once the vacuum is gone.

The mechanical set up you presently have should be fine if the stress loading is adjusted and balanced right. Because you have it supported in two places, it will have less stress than the many we see that are mounted in the front only.  As already said, much will depend on the stress you introduce. The problem comes when tightening things down and one strut is shorter (or out of alignment) than the other putting a load (like a lever) on the seals/ glass. Or, the RF connections pull on the glass, sometimes from tightening or even thermal expansion or contraction. Or the tuning shaft interface is not flexible enough.  We must baby our vac caps just like you would a vac relay or the pins on a big tube.

Also, I would get rid of the bare wire that connects the RF tuning cap and associated RF circuits. Use copper or silver plated strap for all RF connections for function and aesthetics.

I see you have a Plexiglas 1/4" rod to RF isolate the floating tuning cap rotor shaft from burning your hand... :-)   Nice machining job on that adapter assembly.

Your construction techniques have greatly improved from your first 3-500Z amplifier and other early projects. Looks very FB now.

T

I wish I could claim credit for the machining job of the shaft coupler. I got it from eBay.

Noted on the copper strap. I’ll make changes.

I have a variac to bring carrier voltage slowly up. I’ll see how the vacuum variable behaves. If I get arcing, I’ll know that I’ve let the gas in. I do however plan on running this at just the 400 watt carrier level. I’ve always had a knee-jerk interest in QRP.

Thanks for the compliments too! Hopefully this rig lives up to more than just aesthetics and belches out some RF too.

Jon

That whole thing looks like something on a laboratory table in Star Trek TOS. Maybe it's the big vacuum cap but it all looks great.

Open frame construction does let the techie-ness show!

I moved the back vacuum variable mount outward so that things fit better now. I also added copper strapping on the output network and added the transformer.  All that's left is to complete the heat sink assembly and test.

I'll make future updates on the other thread.

Jon

I was working on the project for an escape from yard work and heat. So be nice, I enjoy building, but I am not the prettiest builder.  ;D I have the heat sink mechanically attached to the box, hopefully the loading cap fits! I was just laying things in the box to see how things looked. I'm getting closer.

So I still need...
Loading cap
Mount for the tuning cap with shaft coupling and knob
Meters and switches
Rewind the 160m coil to a different diameter amd mounted out 90 degrees from 75m coil as Steve suggested.
Mount/connect T1-4
Of coarse interconnect everything
I'm sure I'm missing something.

My goal is to have this done by the end of the summer and have it tested. Then I'll start the PWM!

Looks good!  My only suggestion is that you turn the 160 meter coil 90 degrees, and possibly put the relay directly underneath or directly next to the coil.  If underneath the coil, the relay can actually support (or help support) it.  In this way, the coil isn't directly coupling into the 75 meter coil, nor the transformers.