The AM Forum

I've entered the piss-weaker challenge! I completed the OSC, Driver and Final (Class-D) today. She's making 5 Watts CW @ 12 volts. Tomorrow I'll get working on the Class-A series modulator.

(http://users.wpi.edu/~smithw/Piss-Weaker/KE1GF_Class-D_Piss-Weaker_RF.jpg)

(http://users.wpi.edu/~smithw/Piss-Weaker/KE1GF_Class-D_Piss-Weaker_Stage1.jpg)

Sorry that the second photo is lousy, I can't use the flash when taking a picture of the scope.

Man, Bill, you don't waste any time!   You said two days ago you're gonna join the PWer's and I didn't hear from you - and here it is.

You'll be one of the first ones to do a solid state PW version. Please post the final version's schematic and I'll bet others will build it too.  Looks like a strapping and durable PisWeaker for sure.

You'll have more fun with that thing on the air than the big rig.

Great effort, OM!!

T

I bought a big box of TO247 FETs if anyboby needs any 48 volt FETs.

Nice Bill!... What Fet's are you using?.... I think I have some IFR840's laying around...

schematic please!

Glenn, I'm using a single IRF610 and 2 IRF9610's. That's what I had here for small RF transistors in the parts bins.

You may have the LTC SwitcherCAD files for the output network if you wish. I don't have the models for all the parts so I had to use a pulse train with a current sourcing resistor to simulate the FETS.

Sounds good Bill. Looks like a trifilar wound toroid in the photo,

Sure, I have autocad here at work so I imagine I can import your file and print it out..., actually just a rough napkin drawing would be fine...  8)

IRF 840s are a good little FET I use 14 of them push pull parallel
in my 160 meter rig. It will bap the 2 KW peak power output peg
on an atomic yea-lo. I usually run about 300 watts carrier so 20 watts
carrier per device will be a good number. Case temp hits 100 F on a hot day during a buzzard transmission.  I run about 28 volts carrier and around 75 volts peak mudulation.

Well Crew, things didn't go as planned on Monday so I didn't get a chance to build the modulator. I started school today, back from spring break. It's still sitting there like in the last picture. Hopefully I'll get some quiet time this weekend to finish her up and do a write up for you guys...

Hi frank,

Yep, they sure are! I did quite a bit of experimenting back in the mid 80's with mosfet PA's used in homebrew LF CW beacons (the "LOWFER" band). I'm going to see if I can find my old drawings tonight. I could really use a simple modulator circuit for my PW rig. Of course I could do it the traditional way, with a transformer, but I'm sure you guys have a better idea, right?  :D

I'd be VERY interested in hearing the details of your 160 rig!  8)  8)  

Glenn, just go to Linear Technology's website and download SWCADIII it's a LTC SPICE 4 backend with a GUI frontend, real easy to learn. I have to thank Bob N9NEO for getting me hooked. It's free and it automatically updates it's parts libraries from the internet! You can use regular spice models with it so you're not stuck with just what's in the libs...

Glen,
QIX H modulator would be an easy option. Pictures of my rig on amphone
with write up. fc

Well Crew here she is, complete except for a PTT switch. Class-A series modulated, Negative peak limiter, Single Ended Class-D Final; ready to rock.

(http://users.wpi.edu/~smithw/Piss-Weaker/KE1GF_75M_1Watt_Piss-Weaker.jpg)

Catch you on the air.

Excellent, Dude!!

Let's get on PW tomorrow, Bill.

Will you be ready to operate?   If so, let's meet between  3:30- 4:30  tomorrow afternoon between 3870-3890 - I'll have the 2 watter on...  Any other Pisweakers please join us.

T

Okay dude, we'll give it a shot. I've got rocks for 870 875 880, I need to order more of them. I had 885 and 890 but they got lost in the rat's nest.
I'll be attempting to call on whatever's clear if circumstances allow.

Hi Frank;

Where is the write-up on your rig? can't seem to find it.

Thanks!

Glenn, I haven't done a write-up yet, The final is presenting a non-linear load to the modulator right at the moment. But it's not too too bad. The power output craps out at very low voltages. I think driving the gates below ground will solve the problem, I'm not sure. Once I get the rig working up to my satisfaction I'll post some schematics. Who knows I may have to scratch the design all together.

It would be interesting if you were to brew up something out of what you have kicking around in the junkbox, following the true spirt of the PisWeak operator.

Look for "articles" maybe it is there. I had the 75 meter rig there but Gary had problems when he switched server or ISP a while back. I think the file was too big. Send me a message withan email address that can handle large files and I can mail the 75 meter stuff. I don't think I have a copy of the 160 meter write up anymore. years ago....

Hi Bill;

Ya never know what I'm going to end up with.... the design I'm drawing up right now uses a pair of IRF840's in push pull, biased for linear service, single FET driver, low level SA602 balanced AM modulator, heterodyne type VFO... The final will sit at 10w carrier with enough headroom to produce 4 times the carrier power on mod peaks.

Hi Glenn,

Sounds like a nice rig.

Consider putting in a switch that will select 1, 5, or 10 watts.

Most of the time 10 watts is too much power and produces a full quieting signal at 3:30-4:30 PM on 75M.. The fun is when everyone has some noise mixed in indicating they are near PW threshold.... :D

I have 1/4, 2 and 5 watt positions here and use 2 watts most of the time.

BTW, Bill KE1GF became an official PW member tonight. We made a 30 minute contact with our PW rigs and his qualified by being homebrew,  10 watts or less and successfully communicated with it in a QSO lasting at least 30 minutes.  He now can induct other PW candidates into the PW Society..

73,

T

Greetings master Vu san,

Yes, no doubt about it. I will put in switch selectable (or maybe on a pot) power levels. 250mw sounds like a challange!... The 10w position will only be used when I'm driving a linear in non-pw mode.....when those mud duck slopbuckets come on the channel caw mawn!

Glen, that NE602 modulator looks like an outstanding basic building block.  I was considering making up one using the internal '602 oscillator as a VFO, then driving a MC1350 --> 6CL6 leenyar --> 1625 or a pair so I gte about 20 watts to drive the big leenyar.

What I'm concerned about, using the internal 602 oscillator, is variance in the RF level thruought its range, which would affect the mudulation percentage.

Let us know how it works out.

73 John

Glen good luck with the positive bias on the 840s. I've never tried it
myself but always wondered what happens. I wonder if you will do temperature compensation or just rely on internal heating?

(http://www.qsl.net/wb1gfh/cakepan.jpeg)

Notice the groovy coil winder!!  8)  8) and the pie pan receiver!!

Too bad the photo is on QSL.net .......loads oh so very, very slow  :)

About ten years ago, I built a 40 meter linear amp using a single IRF840. If I remember, I was able to squeeze about 25 watts out. I used a positive bias on the gate, through a resistor. I might try this circuit;

http://homepage.tinet.ie/~ei9gq/80pa.html

which looks to be a push-pull design with feedback. I'll try it with the 840's

Not sure what you mean about temp compensation.. on the 840's?, I'm not using the internal oscillator in the sa602 if that's what you meant

HA!!!
The Cakepan PW guys will love that shot, Joe!  

T

Glen,
Very cool. I was talking about FET bias temperature compensation.
I wondered if you plan to fold the bias back if the stage gets too hot.
I see the Fairchild FETs QIX found as a big brother to the 840s.
Imagine a big linear amplifier using them.  fc

That's my next project as soon as I finish the PW exciter, a mosfet linear for 160-80-40... I imagine I'll go with just some sort of temp indicator or maybe a shut down circuit. You couldn't fold back the bias and still continue to operate, well, you could but you would no longer be linear, right?

Yes Glen but push pull may allow you to get closer to class B. Distortion would go up. I think motorola used a little in their Fet linears.
iIhave a bag full of MRF150s waiting for me to build them into a big ss linear when I have nothing to do. Still it would be interesting to see how well switching FETs work in the linear mode. I read the TO220 package had less series L so were better devices.  I'm not sure if that is still true
but sounded good at the time. I never grew the BAs to apply positive bias to my 14 PPP IRF840 rig. Same is true with my 22 fet 75 m rig but it would only take lifting 1 jumper. .......keep us posted

As a cakepan chassis fan, I LOVE it!  Many thanks for posting that, Joe.  Too cool.