The AM Forum

Ladies and gentlemen,
I have a very nice Allied Radio Knight Kit T-150 AM/CW xmtr.
The manual clearly states that you must tune the finals for maximum output.
In my case that is about 100 watts.
I want to drive a linear amplifier but 100W will exceed legal limit on AM and
it can't exceed about 20 watts of carrier to comply with the factory recommendations for my amp. which would produce about 800 watts P to P.
My question is how do I adjust the T-150 power output while maintaining resonance.
Maybe I'm missing something, any help would be greatly appreciated.
With band conditions as they are barefoot just doesn't cut it.
Thanks
Dave
W4PBU

I built a fixed, heafty attenuator sometime ago to place between my transmitter and amplifier.  It is best to always properly tune your transmitter and not detune to reduce output.  A simple t-attenuator will work.

A convenient place to do it would be to remove the jumper in the accessory modulator plug and add enough resistance to drop the screen voltage.

As mentioned earlier, an attenuator would work.  Use it when you drive the amp, don't use it when you're not QRO.  However, you'll need to use appropriately sized components to dissipate the energy (80w).  Here is a link to a cookbook, which also gives power ratings.

http://home.sandiego.edu/~ekim/e194rfs01/pi_atten1.pdf (http://home.sandiego.edu/~ekim/e194rfs01/pi_atten1.pdf)

7db loss sounds like it gives what you want, if you go this route.  73 Dean NW2K

Dave, Take a look down in Transmitters:

http://amfone.net/Amforum/index.php?topic=34680.0

Phil

THANK YOU VERY MUCH FOR YOUR INPUTS.
AFTER REVIEWING YOUR COMMENTS AND DOING SOME FURTHER RESEARCH I HAVE DECIDED TO GO THE ATTENUATOR ROUTE.
A SPECIAL THANKS TO DEAN NW2K FOR THE CALCULATOR INFO.
73 TO ALL
VERY HELPFUL
BLESSINGS
DAVE w4PBU

Easy peasy. Varying screen voltage on final tube. O volts, no output. 1/2 voltage, some output but less than full. For class C legacy radios, the fix is in. Use a WW pot etc. Not sure what final tube(s) are. 6146, 807 or ?

The Knight T-150 uses two 6146's.

i would recommend studying the full schematic before making that mod.

http://bama.edebris.com/manuals/knight/t150-2/

"Adjust" to me infers a variable attenuation scheme. A fixed attenuator is not variable.


Phil - AC0OB

I'll look later at screen voltage current and check tube data. I'll post some ideas later. Possibly a wire wound pot on the rear apron would do. But lower screen voltage is normally used to vary output. example, if transmitter was placed in tune mode. some commercial radios used that technique in boat-anchor transmitters. I did this on my old DX-40 with a single 6146. I was able to lower out output from 50 watts full to 5 watts, no problem and any value in between.  ST

How would you do this safely and reliably unless you change the grid bias on the second stage triode in the modulator? Since the original post is posted on an AM Forum the original poster must be concerned with AM power output.

And if you up the screen voltage, does the power supply have enough current capacity to feed the finals. More screen voltage means more plate current and increased screen current.

These are just a few of the things you have to consider when doing modifications. You can't just willy-nilly throw in a wire wound potentiometer and expect things to last.

See schematics in the file below.

This thread is about the Allied Knight T-150 and not a DX-40 and the two have very little in common. You have to careful with the DX-40 about boosting screen voltages since the transformer is underrated and is operating on the ragged edge.


Phil - AC0OB

When modifying a transmitter internally for variable power, keep some things in mind. Will this work when running CW? Will the AM modulation be able to track the power output, or does it slam the modulated envelope on the baseline? Does a class C operating point get maintained, or does the amplifier stage become more inefficient?

My opinion, is that lowering the plate voltage inside is an alternative. How you do that is another story. On my Gonset G76 I added a 1K 25W in series with the B+ attached to nearby sheet metal with a binder clip. At 150ma plate current, that drops 150v B+. Then I still use an attenuator after the rig, about 5 db worth.

Another alternative is to remove one of the 6146's, and perhaps dial in the rest from there to get proper circuit behavior.

As to exceeding the AM allowable carrier power, much of that is a myth. For example, on FM and CW we can run 1500W RF output. On SSB we can run 1500w PEP RF output. Where does FCC say 375w Max carrier for AM? It does NOT. Why cannot you run 500w carrier, or 750w carrier, and turn down the audio till the PEP is no more than 1500w PEP? Heck, if your voice has asymmetrical peaks, make the larger amplitude peak go downward. That might allow 1000 watts AM carrier power. There was a time when a big carrier that was only partially modulated was called "East Coast Sound".  :P

Jim
Wd5JKO