AM modulator design suggestions...

Discussion in '"Boat Anchor" & Classic Equipment' started by W6ELH, Jul 9, 2018.

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  1. W6ELH

    W6ELH Ham Member QRZ Page

    I succumbed to temptation recently and bought a clean looking Meissner Signal Shifter EX. It will be here in a couple of days. I can't say I had a firm plan for how to make use of it, I just thought it would fit in with my vintage station, but now that I'm researching, I belive I might like to build a simple AM modulator for it.

    I've seen a number of references to doing just this with the "Signal Drifter' but I failed to turn up any plans specific to use with this low power transmitter, even searching the old QST archives. I found a simple looking design in my 1956 handbook. I like the looks of it as it requires no HV transformer, but don't know if it is suitable. (see photo below)

    Any of you have a design you would suggest? Thanks in advance! Jim W6ELH

    Screen Shot 2018-07-09 at 12.38.32 PM.png IMG_3008.jpg
     
    KE0ZU likes this.
  2. G3YRO

    G3YRO Ham Member QRZ Page

    Never heard of this, but I see it gives about 6 watts output, so presumably runs about 10 watts input.

    Our legal limit on 160m was 10 watts for many years . . . so I built several AM/CW rigs for that band back in the day.

    So why bother using lousy Grid Mod, when you can easily have proper high level Plate & Screen mod? All you need is 5W of audio, so a single-ended Class A valve like a 6BW6 would do fine. (or 6V6 if you want to keep it vintage.) A double triode like a 12AX7 would drive it fine.

    A small 5 watt mains transformer would also be fine as the mod transformer . . . or even just use a choke. (the great thing about choke mod is that it's proper high level mod, but you can't over-modulate!)

    EDIT: I just saw the title of that article you posted . . . but having looked at the circuit, maybe you meant to use a circuit like that as a Plate & Screen modulator for your Tx . . . and yes, that kind of thing would work fine.

    For Choke Mod you just connect the Anode of the Audio output valve to the RF PA HT feed, and feed BOTH through an audio choke from the HT+ rail. (or again, just use the primary of a small mains transformer as a choke)

    Roger G3YRO
     
    Last edited: Jul 9, 2018
  3. W6ELH

    W6ELH Ham Member QRZ Page

    Thank you Roger-

    I'm way out of my element here, having never built such a thing. But if I can find a suitable schematic, I believe I can cobble one together and learn as I go. That's why I was searching for an older article with a design specic to use with the Meissner.

    I did notice that design I posted mentioned being, "capable of modulating any CW transmitter from about 100 watts input..." Perhaps not suitable for a low power TX like the Meissner?

    So, grid modulation bad, plate and screen modulation good? I'll go back and look at the handbook and some of the vintage QST aricles again. Sorry for being so ignorant! Jim W6ELH
     
  4. G3YRO

    G3YRO Ham Member QRZ Page

    Yes - the point is that although that circuit would NOT be enough to modulate a 100 watt transmitter by Plate & Screen, it would be enough to provide Grid Modulation.

    BUT - you only need about 5 watts of audio for Plate & Screen Mod of your 10 watt Tx . . . so that circuit would be fine. (it's pretty much what I have used in several transmitters, except mine used miniature valves - 12AX7 feeding a 6BW6 )

    Roger G3YRO
     
  5. KE0ZU

    KE0ZU Ham Member QRZ Page

    Very nice find Jim. The cathode modulator would be an easy way to get it on AM.

    Your example is much nicer looking than my example, but it will have it's turn on the bench later this year, I hope.

    Enjoy your new toy!!

    Mike
     
  6. W6ELH

    W6ELH Ham Member QRZ Page

    Thank you Mike! I should see it today. The auction photos looked very clean and unmolested, but those can be deceiving.

    I know I should employ it with CW as the Good Lord intended, but shamefully, I still lack that skill. I REALLY need to knuckle down and get back to learning CW, but every time I undertake that I seem to get distracted by some other radio-related shennanigans! :(
     
  7. W5INC

    W5INC XML Subscriber QRZ Page

    Even the very best CW OP had to start somewhere. I would listen to the Left coast version of W1AW as 1 possible method of learning the code. Free and it will get you use to coping CW in less then perfect condx. on the bands. The Straight Key CW club can be useful for folks learning or building their CW skills up. Rome wasn't built in a day, so don't expect to have the CW skills of a OT, who has spent the last 50 years as a CW only OP, overnight. Learning CW is easier for some then others. But it looks like you have some musical talent according to your Zed page, that should help make the learning curve pretty short in your CW learning project. Good Luck and hope to catch you on the CW side of things in the near future. :)

    Nice CE Xmtr next to the A-4!
     
  8. W6ELH

    W6ELH Ham Member QRZ Page

    Thanks for the encouragement, Michael!

    I was just beginning to make some progress on CW when my Dad passed, and it took the wind out o' my sails for a spell, but I really MUST make time to start that learning process again (slams fist against open palm in gesture of determination).

    The Meissner just arrived. Boy, somehow I pictired it as being fairly compact - it's 80% as big as the 75A-4! Going to have to build a much larger bench at my new QTH after the move!

    Cheers... Jim
     
  9. KE0ZU

    KE0ZU Ham Member QRZ Page

    Yep, something on the order of 8 feet would be a good start.:)
     
  10. W6ELH

    W6ELH Ham Member QRZ Page

    Piker... ;) It's a ways in the future, but I'm looking forward to building a new larger operating bench as well as a dedicated work bench for radio projects!

    Hey Mike, I saw the photo of your Meissner - did you puzzle out the function of the control added to the right of the eye tube? Meisner 3_zps16cmgq31.jpg
     

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