The AM Forum

I am thinking about a low power AM transmitter using a pair of 807 or 1625's in the final.  What would be a good tube pair and transformer for this task? I plan to put about 700 VDC on the plates.  Also, want audio response out to around 20 or 30 KHz.  No, this will not be on the air.  :-)

Thanks!

Matt

Hello Matt,

If you want decent response to 20 or 30 Kc, then a transformer probably is not the best way to modulate.  I would suggest you consider one of three other options.... Screen modulation, Series modulation, or Class-E.  Any of these eliminates the phase shift and frequency rolloff issues associated with high-level plate modulation.  All these methods are well-covered in other threads on this forum.

Just curious about your application, if you will not be putting it on the air?  What is your intended RF frequency?

73, Rick

Yes, screen modulation of something like a 4cx250b or 4x150a, very simple, 100 watts out, lots of audio with no phase shift or response problems.

Why the need to go so high in frequency? No one can HEAR that high except bats.

Brett

Makes for good a "guard band" to keep slopbuckets from encroaching.  JUST KIDDING GUYS!!

Don't bother with screen modulation. There is modulation iron available, although it might be 50% bigger than you need because it is for higher current.

Two 225-400 MHz military radio sets have nice 120-150W modulation transformers that go from 200Hz to 20KHz. The mod iron has a high DC rating in the secondary because of the lower efficiency of the finals at 400MHz.

The modulation transformer from an MD-141 is rated 120W and 200-20000 cycles. I think the one from the MD-129 is 150W and the same HF response. They are designed or 850V B+

The AN/GRT-3 consists of the T-282 transmitter and MD-141 Modulator, and accessories. Accompanying receiver: the AN/GRR-7 consists of the R-361 Receiver, and accessories.
MD-141:
T201
Z: pri: about 8k to 2.8K, voltage ratio about 1:1.18
Eb 850V
mod: two 811A
RF two 4X150, 100W carrier, 290mA meter reading
aux winding: C.T.'d winding for voltage feedback or AMC
screen modulation circuit: http://amfone.net/Amforum/index.php?topic=35905.msg276191#msg276191




The AN/GRC-27, AN/GRC-27A, or AN/GRC-48 consist of the T-217 transmitter, MD-129 Modulator, and R-278B Receiver, and accessories. The modulator has collins iron, not that it matters whenit is military.
MD-129:
T1404
Z: 5500CT to 2400 Ohms, voltage ratio 1:1.32
Eb 850V
mod: two 4-65
RF two 4X150, 100W carrier, 350mA DC
aux winding: C.T.'d winding for voltage feedback or AMC
aux winding 2: 600 Ohm output 50W MAX.  for remote operation sidetone
screen modulation: dropping resistor in TX from mod. B+


manuals are here:
http://www.bunkerofdoom.com/lit/mil_uhf_01/index.html

I have spare modulation transformers from some parted out MD-141s. If you want one PM me.
I have one of the little T-282 screen modulators as well, if wanted as well as three T-282s in fair cond.

The only pic I have of any of them is of the 150W one in the MD-129, just so you know what to look for.
I have only complete MD-129 modulator/power supply units, no parting out.

Thanks all!

This would be for a plasma tube.

The tube is a dynamic load; when it first lights, impedance is probably around 5 to 25 ohms.  At steady sate, probably around 500 to 1K.  This tube will handle about 125 to 150 peak.  200 tops.

Carrier will be varied from about 200 KHz to 1500 KHz.

I have considered some of the old Taylor circuits as they do not need a big tranny.  Thinking of using a HP-606A as a VFO.

Maybe someone sees where I am going............:-)

I want tubes, for transient reflected waves would likely fry FET's.  I am thinking about using a link coupled, parallel LC circuit to feed the plasma tube.  This circuit would help to "even out" the load seen by the transmitter, given high enough Q.

Thoughts?

I got a picture of the military 200Hz-20KHz modulation transformers available. It would be nice to see one used. They are large, scale shown by octal audio unit on top, 17 lbs bare, and the power rating is probably quite conservative. Insulator types may vary.


Since you are doing unusual experiments, there is also another transformer from a Sonar system that may be of interest. It is the gray one. It was driven by two 4-400's, but is not so big due to the high frequency range up to 70Khz. You can therefore shift gears.. No way for me to check impedance or really know its power rating on that but it is an unusual one.

I had to put the pictures here as they don't go into PMs but will reply to your PM.

Very interesting application !  Non-resonant would be good if you can do it.  You could realistically use MOSFETs because the frequency is so low.  A source follower may be a realistic solution.  You could also put a fast recovery diode in the output configured such that anything going "below zero" would be shunted.  The body diode protects the other way (above the supply).  You might even be able to use complementary symmetry FETs - they are available and again, that's pretty low frequency.

Then modulation would be a breeze.

Tube-wise, screen modulation would certainly be your friend.

I designed a prototyped a MOSFET "transmitter" that was used to drive ultrasonic transducers in a medical application.  Similar to what you're doing.  Not as much power needed for the medical app.

I have one of those 200 to 20,000 Hz mod transformers, has screen taps also.
I tried it on a rig and it sounded VERY bad for voice.
Must be a lot of phase shift through it as well.
120 watts audio I think its good for.

My screen modulated rigs will do 20,000 hz easy, 1 Hz to over 20 KHz, no transformers in line at all.
I suppose the screen and plate bypass caps are the only thing to limit response.

They are military aircraft band modulation transformers not hi-fi transformers and produce the sound appropriate to the application. They work best and produce a penetrating 'space shuttle' voice quality when used in a circuit with AMC like in the manuals.