Cathode Modulating a Knight T60?

[W5EFR]

I would like your input on a project of the AM persuasion...

I have a Cunningham CM-50 AM broadcast band transmitter. You may not have heard of this company, the guy that built them was a retired AM Broadcast Engineer that designs and built these for religious broadcasting in under-developed countries and things of that nature..

any way...

It uses Cathode modulation by either a TIP100 (Darlington) or a standard modulation transformer, switchable on the transmitter...

I also have an old Knight T-60 Ham transmitter that uses screen modulation of a 6DQ6B sweep tube... and since screen/grid modulation, or the Knight T-60, is not consider to be "all that" by the powers that be...

My question is...

Do you think I could use the Cunningham-style Darlington transistor modulation circuit to Cathode modulate the Knight T-60?

Both schematics are here:
Cunningham:
http://riley-music.com/BowsStuff/CM3050/14CunninghamSchematic.jpg

Knight T-60:
http://riley-music.com/BowsStuff/CM3050/t60sch.gif

My Idea:
http://riley-music.com/BowsStuff/CM3050/T60CathodeMod.gif

NTE253 Datasheet:
http://www.nteinc.com/specs/200to299/pdf/nte253.pdf

Am I on the right track?

[KM1H]

IMO, the carrier control modulation can be a very good performer.

Do a Google on the subject and particularly check out the DX-60 mods. By balancing the amount of idle carrier with improved audio Id say its not worth the effort to change to anything but plate modulation.

Carl
KM1H

[W5EFR]

I was just thinking of trying something different... and Darlingtons are much cheaper than iron...


I just fired up the T-60 into a dummy load with a 40M crystal I have...

I am starting with the transmitter stock and grid modulated, except for some cap changes I did a couple years ago in the audio chain. I want to make sure it is working correctly before I try to modify it.

Tuned up, I am getting these numbers in AM mode on the scope:

Carrier: 11.2v RMS/30v P-P

I used a signal generator set at 50 Hz and 50mv P-P into the mic input, adjusted the mic gain until I get as close to 100% neg peaks as I can get without the positive peaks distorting, and I get these numbers
Modulated: 18.5V RMS/70v P-P

The positive peaks are over 100% modulation (O'scope set for 2 divisions with the carrier, and getting 4+ divisions modulated)

The weird thing is, as I increase the frequency of the signal generator, I start to lose my negative peaks and the signal is under modulated... why is that?

At 1000 Hz (and still 50mv p-p), the output is clearly around 50% modulated... 
 

[Steve - WB3HUZ]

Check the frequency response of the audio stages and also check the value of any screen bypass cap(s) on the RF amp.

[KC4VWU]

Hard to beat plate modulation - BUT - if you wanted to play with cathode modulation, it wouldn't hurt. Look here--http://www.mines.uidaho.edu/~glowbugs/tx/index.html for a simple circuit that costs hardly nothing and can be built in a stand alone enclosure. Plug it into the CW jack and your in business. Just get a small tx and want to modulate it? You're ready!
Phil

[KM1H]

Could be those "cap changes" from the past. As Steve said test the audio chain.

Carl
KM1H

[W5EFR]

Hard to beat plate modulation - BUT - if you wanted to play with cathode modulation, it wouldn't hurt. Look here--http://www.mines.uidaho.edu/~glowbugs/tx/index.html for a simple circuit that costs hardly nothing and can be built in a stand alone enclosure. Plug it into the CW jack and your in business. Just get a small tx and want to modulate it? You're ready!
Phil

You mean this one? http://www.mines.uidaho.edu/~glowbugs/Simple%20Grid%20Modulator.pdf

Pretty cool. But my tube junk box is lacking any large bottles...

I wanted to try mixing some cheap solid state parts and doing it that way...

I guess it isn't a popular thing to try?

I have a question about those Modulators that plug into the Key Jack...

How does the Oscillator get isn't ground? The same way as the final tube?

On the T-60, it seems the 6HF8 Oscillator circuit and the 6DQ6 Cathode are tied together, and grounded through the key jack, so how do you keep from modulating your Oscillator along with your final?

I was going to tap the 6DQ6 Cathode ground for the Darlington modulator after the 6HF8 had it's ground... would that be correct?

[KC4VWU]

Yep, that's the one. The tubes really aren't that bad. The 6Y6 is equivalent to the 6L6, just higher gain. ESRC gets $3 a piece for the 6Y6GT's and the 6SL7 is $7. Stan is a really nice guy to deal with; he's always did right by me.
The whole deal would need a switching arrangement. I set mine up with stereo phone jack for PTT energizing a relay. Let the relay ground the oscillator cathode, and the PA gets it's "theoretical" ground through the modulator.
As far as the solid state circuit, I really don't know. However, there are guys on here that knows solid state theory REALLY well. They might have some input for you. I got a copy of the schematic; but I'm too far into other things to play right now. It is interesting.
You have a good basis for experimentation. I have a few small tx's here that I will get back to after I get a few things finished first. If it intrigues you though, give it a try. That's what this hobby is all about; to learn and have fun.
Phil