CHALLENGE - SS rig for the AM PW (QRP) net.

[Ed/KB1HYS]

Solomon try this IRF 510 based amp. a Class C QRP amp for CW use.  It should be something you could Drain (source?) modulate.  Google "MINIBOOTS QRP AMP" for details.  Schematics below. 

I built one for my 40m QRP CW rig and it works FB with one watt or so input providing about 10 watts out.

Also has T/R switching built in as well.  Very neat and compact design, just needs a good power supply.

[ssbothwell KJ6RSG]

thanks frits and ed!

i have a semi-operational pushpull irf510 amp i have been working on in the last thread. i really dont want to give up on it and go back to a single ended design if i can make it work.

is there anything different about how one would modulate a push/pull pair versus a single fet amp stage?

[W1FVB]

thanks frits and ed!

i have a semi-operational pushpull irf510 amp i have been working on in the last thread. i really dont want to give up on it and go back to a single ended design if i can make it work.

is there anything different about how one would modulate a push/pull pair versus a single fet amp stage?

Not really, in both cases you can modulate the final (IRF510) in a "high level" modulation fashion. In my earlier design I was trying a "low level" scheme, which produces the AM in an earlier stage. Which certainly can be done, but the amplifier stages after that need to perfectly linear to reproduce the AM.
Guess, I took the easy way out  all though when choosing the "high level", way you have to build a modulator that can deal with the higher currents.

Frits

[Ed/KB1HYS]

Apply the audio signal to the input voltage of the Push-pull stage, you could use a 12 or 24 volt power transformer (yes it would work, simple but not hifi) as a modulation transformer and pump in audio   Remember you'll need a half as much power in audio as in RF final. 

[Ed/KB1HYS]

worked up a Voltage Regulator for the VFO using an LM317 adjustable regulator, for 9volts inot the VFO.   I have the VFO feeding the IRF-510 fet.  0.2 volts input into the FET and I am getting ~2 volts out of the FET (all peak-peak on the scope).  The FET input voltage is 15 volts, Gate bias was adjusted with a pot to be around 8 volts or so to get maximum stable output.  This looks like a class A amp to me (device is always ON, output varies with 360 degrees of signal).   

Methinks I need a driver device? do I need to boost the VFO output somewhat to get more OOMPf out of the FET?   I have a monster 2N350x mounted in the metal can with mounting tabs on it.  If the final needs 5v on the Gate for full on, do I need to kick up the input?

[W4AMV]

I agree. I would like to have available 1 W max to drive the 510. You can pad it back as required, improves the load isolation for the VFO. So a driver would be nice. Say a couple of 5109 units in //.  I have a boatload of them and placing them in parallel with small emitter R's (similar to ballast R's in power devices) works fine. Say 4.7 ohms.

[Ed/KB1HYS]

I also have some NTE 197s which look promising.  and for some reason a boatload of 2n6111... probably could work something up with one of them I suppose.

[kb3ouk]

i think that if you use the 12 or 24v transformer you may want to use something like modified heising, that way the transformer doesn't become saturated by the dc on the one side.

[Ed/KB1HYS]

yep, that would work quite well.  and components to handle these voltages won't break the bank either.

[KD7EDW]

Excellent challenge:  I got my General class this year and I am working on a station now.  I will be interested in the replies to this challenge.

Question: why limited to a VFO?  A VXO circuit with a switch and two crystals would possibly cover the whole 80 meter AM portion, be easier to adjust to get working and be more stable as well.

Also has any one ever attempted to modulate an LM317 regulator (like they described in CQ Aug 2002)  I am wondering if it could provide modulated voltage for an FET or Transistor final stage for a transmitter.

[WU2D]

I like the old CMOS 4049 with all sections paralleled up. You can run them with 12V on them and they will swing hard and drive the FET.

 http://www.ebay.com/itm/5-x-CD4049-4049-Hex-Buffer-Converters-IC-TEXAS-/220710920552?pt=LH_DefaultDomain_0&hash=item3363656168#ht_1352wt_1164

[Ed/KB1HYS]

Excellent challenge:  I got my General class this year and I am working on a station now.  I will be interested in the replies to this challenge.

Question: why limited to a VFO?  A VXO circuit with a switch and two crystals would possibly cover the whole 80 meter AM portion, be easier to adjust to get working and be more stable as well.

Also has any one ever attempted to modulate an LM317 regulator (like they described in CQ Aug 2002)  I am wondering if it could provide modulated voltage for an FET or Transistor final stage for a transmitter.

Actually, if you can make a VXO cover the phone portion of the band and keep it simple/low cost go for it!   

I admit the LM317 idea is interesting...  would that be a really simple form of series modulation?

[KD7EDW]

I admit the LM317 idea is interesting...  would that be a really simple form of series modulation?

Yes series modulation is what I am thinking.

[Ed/KB1HYS]

Schematic of a very simple VFO, Buffer, and Final Amp.   The FET in the amp is a IRF-510. 

Not much power out, needs to be optimized a bit, but it is SIMPLE and would work OK.

[w3jn]

You need a 50-100 uF bypass cap at the B+ feed of your final if you're gonna modulate that, otherwise the audio will get into the rest of the circuit.

I'd also suggest a L/C tuning network between the VFO and the driver - it'll help get rid of harmonics and make the thing easier to drive.

[ssbothwell KJ6RSG]

Hello Joe,
It's an old linear power supply. I just pulled the old electronics and build in a pretty
simple modulator circuit (see attached image..my version is slightly different though)
Very simple analog design, but sound pretty good!

hey frits. would you recommend this series modulator setup for a an AB push/pull irf510 amp. i know you already recommended to me a low level modulation circuit driving the gates of the IRF510s but i am really interested in learning about high level modulation.

[W1FVB]

Hello Joe,
It's an old linear power supply. I just pulled the old electronics and build in a pretty
simple modulator circuit (see attached image..my version is slightly different though)
Very simple analog design, but sound pretty good!

hey frits. would you recommend this series modulator setup for a an AB push/pull irf510 amp. i know you already recommended to me a low level modulation circuit driving the gates of the IRF510s but i am really interested in learning about high level modulation.

Hi Solomon. Well, I wasn't very succesfull with the "low level" modulation scheme and thats why I went for "high level" modulation.
Doing it the high level way, you don't have to worry much about how linear the amplifier stages are. The downside is that if you modulate the final, you have to deal with modulating higher current.
Take a look at the attachment of my latest RF Deck and you realize how simple it is to drive a FET.
For that particulair RF Deck , I'm using an old converted 13.8 Power supply as a high level modulator and I'm driving the deck with a DDS VFO that I've build in the past.

If you like,there are more pictures and info on my website / blog :
http://my.opera.com/Frister/blog/

[ssbothwell KJ6RSG]

hi frits. i've got a (almost entirely) functioning 35W irf510 AB amp. i just need to get it modulated.

is this the modulation circuit you are currently using for the amp you have attached in your last email? if so, how does it sound? can it reach 100% modulation?

by the way. your construction photos look really nice! great job on the board layout.

thanks,
sol

[W1FVB]

Hi Sol,
Thanks, yeah I like the old "dead bug" construction method. And for RF work its great to have a chunk of grounded copper clad board 
That schematic is a little bit old and I've made a few changes since, but that
is indeed the basic design ( Attached is a newer design). In my case , there are
two 2N3055's to split the current. The modulator has no problem reaching 100% as long as the carrier voltage is set correct and the power supply can deliver the current needed.
Congratulations on the push pull AB amp! did you use two IRF510's?

[N4LTA]

I think you may have trouble VXOing a crystal at 3.8Mhz very far  - anything further than a couple Khz will be difficult. That has been my experience anyway.

I have been recently experimenting with a NE602 as a VFO and am inpressed with the quick start up stability and overall stability.

I used a T68-6 and a small C0G 390pf ceramic cap and got a very stable 3.2 Mhz VFO which I am using for a High Fidelity 3.7 Mhz AM receiver.

The receiver sounds very nice with a discrete transistor 2 watt audio  amp driving a decent quality 8" speaker. I am using a 455Khz IF with Mouser IF transformers and a high fi detector followed by an audio filter. It is modeled after Robert Batey's high fidelity AM tuner for the BC band. It was made with the intend of being a monitor for the local net on 3715 Khz. Right now a HF 15 variable cap moves it about 60 Khz.
My plan it to put it in a box with a 6" good quality speaker and keep it tuned to the net frequency. Right now the bandwidth is about 10Khz or a little less. I may add a ceramic filter that is switchable to narrow it down when needed.

The audio uses a complimentary pair BF140 and BF139 and a 2N3904 and 2N3906 and (I think) sounds much cleaner than the IC audio amps.

[ssbothwell KJ6RSG]

Hi Sol,
Thanks, yeah I like the old "dead bug" construction method. And for RF work its great to have a chunk of grounded copper clad board  
That schematic is a little bit old and I've made a few changes since, but that
is indeed the basic design ( Attached is a newer design). In my case , there are
two 2N3055's to split the current. The modulator has no problem reaching 100% as long as the carrier voltage is set correct and the power supply can deliver the current needed.
Congratulations on the push pull AB amp! did you use two IRF510's?

very cool. thanks for the updated schematic. indeed i am using two irf510s. i'm attaching a photo of the board. you inspired me and i rebuilt it yesterday to try to clean it up a bunch.

i need to build a 30dBm gain amp stage to drive my amp (i have been testing the amp with a little 1w qrp transmitter that i dont want to use in the long run). i have some 80m and 40m hartley oscillators already assembled and buffered/amplified to 0dBm. would you recommend a transistor to use for 30dBm gain operation and what class should i run it in?

after i do all that i will be putting together your series modulator circuit and i will have an entire homemade AM transmitter!

[W1FVB]

very cool. thanks for the updated schematic. indeed i am using two irf510s. i'm attaching a photo of the board. you inspired me and i rebuilt it yesterday to try to clean it up a bunch.

i need to build a 30dBm gain amp stage to drive my amp (i have been testing the amp with a little 1w qrp transmitter that i dont want to use in the long run). i have some 80m and 40m hartley oscillators already assembled and buffered/amplified to 0dBm. would you recommend a transistor to use for 30dBm gain operation and what class should i run it in?

after i do all that i will be putting together your series modulator circuit and i will have an entire homemade AM transmitter!

Sol,
I've attached a schematic of simple drive based on a design originally by W7EL.
It will get your oscillator up to about 1 to 5 W , which I think will be enough to drive
your amplifier. Now the L network in the schematic is designed for 3.9 Mhz , for any other
frequency just plug the details into you favorite (online) calculator like,
http://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html or
http://www.changpuak.ch/electronics/calc_18.php
..You might have to experiment a little bit with the values.

Nice work on your amp, I hope you will attach all the leads to the FETS? 

[ssbothwell KJ6RSG]

oh wow. thanks for the schematic frits. i think i might have everything i need for that driver in my scrap bins. do you think i could substitute a J310 for the VN-10?

the FETs in that photo are disconnected because they are burnt out! i am waiting for new ones to arrive in the mail from china. it might be a week or more  before i can get the rebuilt amp running.

[W1FVB]

oh wow. thanks for the schematic frits. i think i might have everything i need for that driver in my scrap bins. do you think i could substitute a J310 for the VN-10?

the FETs in that photo are disconnected because they are burnt out! i am waiting for new ones to arrive in the mail from china. it might be a week or more  before i can get the rebuilt amp running.

I don't think the J310 will work. But you can certainly give it a try. And for
your parts, may I recommend http://www.mouser.com
The usually have a decent selection and reasonable shipping

[ssbothwell KJ6RSG]

hi frits. i assembled your driver example. i was hoping i could get away with parts from my junk box. i put the circuit together using a J310 but im not sure if it is working properly.

what kind of output should i expect with 0dBm input?

my output is 1.7V Pk-Pk with a dc offset of about 6.7V. the waveform is heavily clipped as well. i'm not doing any filtering.