The AM Forum

Hi All,
New to the forum and want to build a small transmitter to get my feet wet with AM. I found this on the web and I think it's from the 1965(ish) Handbook. It looks approachable enough and I have a lot of the parts.

Does anybody remember this design or have any comments about it? I'm curious how well the screen (only) modulation works. I have the entire article, but it's too big to attach. If anyone is interested I can email it to you.

Tnx and 73,
Bill

Scream modulation works very well.  There are a few write-ups of people who've done scream mod here on amfone.  Use the search bar and you'll find some of them.

Scream modulation is not very efficient.  You don't get the plate modulated efficiency, or near it.  That said, it's VERY good audio quality wise.

I used a PWM that I had high voltage FETs in to scream modulate a 4CX250 with.  I got about 80 watts of carrier.

The biggie is loading.  You don't tune for maximum output like you do with a linear amplifier.

That said, it's a great way to get good modulation.  It's not the most efficient way, but in this day and age, who cares.  We aren't limited by power in, only by power out. 

--Shane
WP2ASS / ex KD6VXI

Hi Bill,

If I were to build up a 6146 based screen modulated rig, I would duplicate a combination of a DX-60 (or better yet, DX-40) and Steve/QIX's design.  Or two homebrew 6146's will work fine with his screen modulation circuit.

You would get rid of the audio transformer, run solid constant carrier  and have exquisite audio, assuming you load the final correctly.  Listen to the audio recording on his post he made below.

The other way out is to buy a DX-60 and modify it as shown... a relatively easy procedure.  Also, the DX-40 is less expensive, more room and also a good choice to modify with similar circuits to the DX-60..

A single 6146B will put out about 8-10 watts of carrier with decent 150% positive audio peaks headroom. Double this for two tubes.  A good power level to drive many linears.  Notice he has the negative peak limiting all set up as well as carrier level control adjustment.

Good luck.

Steve's DX-60 project:
http://amfone.net/Amforum/index.php?topic=26603.0


MP3 Recording:
http://amfone.net/Amforum/index.php?action=dlattach;topic=26603.0;attach=24297


Are you sold yet?  :-)


Tom, K1JJ

*Heck, I've been thinking of building something like this myself...  Maybe a single 4D32  (or a DX-40) screen modulated with this screen circuit and using a Chinese DDS VFO.  This would be the perfect driver for my pair of 4-1000As linear.  A few hundred watts carrier with huge positive peaks.  

DX-40 YouTube Videos -  going thru it for repairs:
https://www.youtube.com/watch?v=TtgQaX2_5Ts
https://www.youtube.com/watch?v=mIqsWC7-s5s


DX-40 schematic below:

If you have the parts, go for it. Screen modulation is an interesting modulation method.

Phil - AC0OB

That recording does sound good. You've convinced me and I think I'll go ahead with this project.

I was thinking about using one of Jim Hagerty's DDS boards as the VFO/exciter. A little pricey, but quality stuff. Also makes a good sig. generator around the shop.

Just a word to the wise - I have one of the Chinese DDS boards - all the PC software for configuration and set-up is in.... Chinese. Wrote to the seller. Got a response in ...... Chinese. Complained to fleaBay. Got no help. Threw up my hands and said "lesson learned".

Bill AI7CH

Sounds good Bill -

I use one of those Chinese DDS boards as a VFO for my 4-1000A plate modulated rig. (I made it into a broadband solid state 100 watt VFO unit)   The DDS board itself had some spurs down only -40 dB.  However, after it passed thru the 4X1 grid and plate resonant circuits, the spurs dropped down to about -80 dB at the 4X1 output. So it appears FB to use that way as long as a "filter" is present.

Post your progress and pictures to this thread as you go.  

Good luck with the project!

Tom, K1JJ

Here is another design using screen modulation but using two 813 finals.

https://worldradiohistory.com/Archive-Radio-Electronics/40s/Radio-Electronics-1949-02.pdf  Page 22.

I was able to develop 175 Watts output with one 813 in my design.

Phil

I decided to follow the design in the article for the time being. As interesting as the modified DX-60 screen modulator design looks (and sounds), it required a power rail that was available in the DX-60, but not in the article's design. And it looked to be a stretch to add the DX-60 mod and keep the general purpose linear amp aspect of the design. Once I get it built and working, then I may look into modifying it.

As to the original design, one of the parts that looked to be totally unobtanium was the tiny little modulation transformer - the Triad M-1X (and it IS tiny). Was only $1.50 back in the day, but not at all common now. I even contacted Triad to see if they might have one stuck in their obsolete parts inventory. Nope! But they did send me the drawing of the color-coded wiring and dimensions (dated 1974!). Amazing company!

Anyway, thanks to a club newsletter from October 2020 (the Beaver Valley PA ARC) which appeared on the web, that little beast was listed for sale (thank you Google). Since the seller's email was listed, I contacted him to see if, by some stretch of the imagination, he had ever sold it..... No! He still had it and was more than happy to find a buyer. Go figure.... The seller's even gotten the bug to get back into AM.....hmmm.

So, just a word to the wise - if it seems like a long-forgotten 'For Sale' is just too old to be possible, ya' never know... Take a chance, it may work out...

Bill AI7CH

I'm late to this party, but I built a screen modulator (also based on '60's ARRL Handbooks) for my 80m and 40m ARC-5 transmitters. I will have to look and see what I used for a modulation transformer, all I can recall is a 12AX7 speech amp and 6AQ5 output stage.

I think there's about 10-15W carrier, not pushing the 1625's too hard. But it's much improved going through my homebrewed AB₁ 4x4-125A amp with 250W carrier  ;D

I'll second the "word to the wise" above. Use a scope to set up a screen modulator. It will look like crap and sound like it too if you just tune and dip. The loading is not where you'd expect for clean AM.