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Don Dulmage
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VE3LYX Ham Since 1980

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Other Callsigns
First issued VE3LYU

My Ham Radio Interests
Homebrew tubes CW and AM. Minimalist style.
Skeds for low power QSOs with homebrew stuff.

50C6 Cw rockbound with matching 12sn7 regen.
01 TNT 40M with matching 01 regen
40M hartley CW
80M x 40M Am transceiver HB
Two No 19 Wireless Sets
6l6 Breadboard Cw TX and matching 6sl7 regen.
BC454 recvr Operational 80M
ARC 5 T18 transmitter which now covers all of 80M band in CW and AM (cathode modulated through CW key jack)
Twin 811A HB linear Amp.

Other Interests
Speak Deutsch at a functional level, can get by in French but it is hardwork for me.
Make fiddles and play them. (gospel and country)
Drag racer . I run The Senior Dragster. Slant six powered.
I race it at Picton Airfield
Am a writer. I have two books published and many magazine articles in both the USA and Canada. I used to have my own opinion column in a local paper for several years.(View from the Ridge)
Cast aluminum for fun.
Also raised cattle for several years and trucked them. I also bought and sold for others. (Dolmetsch Beef Farm)

In the interest of allowing others to enjoy their clubs I now refrain from joining.(does not play well with others)

Cathode modulating


3/1/2013 6:27:13 PM (1 comments) Add a commentAdd Comment

Over the past couple of years I have been very interested in vintage AM equipment, both commercial and homebrew. My basic AM rig is a DX60B and a HB10R receiver with a homebrew VFO for the DX60B. I must confess I am not in love with this set-up.
The person who decided crystals should go in the rear should not have had a job. The phono plug coax connector defies reason. Better than nothing radio in IMHO.
I have an ARC5 rig as well. A T18 that I have moved to 80M . All changes I have made to this rig are reversable in a hour or so. I wanted it on AM but did not want to buy or build a big modulator. I choose cathode modulating (some call it grid modulating. Personally I dont care what you call it. It hooks in series between the B- and the rig's cathode or cathodes.)
These simple little deals work surprisingly well. Any CW rig cathode keyed can simply feed the tube filaments of the modulator and the rig supplys the B voltage. You simply unplug the CW key and plug in the modulator. That, aside from any drawbacks, is the big attraction. I have built four versions so far. The first was from the 56 ARRL handbook and while it works is overly complicated and does a marginal job at best. Especially on a rig of around 100 watts or less input. Ben, a friend from another site gave me a simple transistorized schematic for one he uses. Immediatley I realized it could be adapted easly for tubes. I built the first with a DX235(similar to a 171A)and tested it on my 45 TNT. It worked very well but stole more power then I think it should. Probably a 00A or 01A would have been a better tube choice. Next I tried it on my ARC5. It worked well on the 1625 pair but I didnt like using a 1920s tube which cannot be replaced on a newer rig. I made one with a 12sQ7. It worked well but also stole more power then I think necesary. I searched for a tube with low internal resistance or characteristics that would indicate that. I came up with a 6J5 as my best choice. It turned out good so far but I am still testing it. Unlike the ARRL design all mine use carbon mic circuits with a battery. I like that set up very much. Simple reliable and doesn't rob the transmitter of power.
What is the big advantage? Why am I attracted to this idea? Because one can take a simple basic CW rig and as long as it has cathode keying, one can with a small handful of parts turn it into a usable AM rig.
A tube socket
A triode tube
a 2.2k resistor
a 2500ohm pot
an .01 cap
an old audio transformer froma table radio
a battery (9 volt)
Carbon mic.
Push button switch .
Wire the filments so they can be fed from your rigs power supply.
Wire the triode plate to the rig's PA tube(S) cathode(s)
Wire the triodes cathode to the B- or ground circuit.
Install the 2.2k resitor in series with the 2500 ohm pot from the grid to cathode. (giving you adjustable grid bias from 2.2 to 2.7K)
Connect one end of the primary from the audio transformer to the triode cathode.
Connect the other end of that same winding to the grid through the .01cap
Connect the carbon mic in series with the push button switch and the battery to the other winding (formerly the 8 ohm secondary of the audio transformer.
Turn the rig on. Wait for warm up with the modulator input jack unplugged. Using the CW key tune the transmitter. unplug the CW key ad plug this modulator into the same jack. The transmitter finals will be on at a reduced level. Push the button and talk into the mic.
Adjust the pot for modulation that suits you. The push button save the battery.
You might wish to add a switch to break the cathode circuit on standby. I have one or a method of doing that for all my cathode modulators.
Pick a triode of moderate power so it doesnt suck up too much power.
The triode and modulator essentially acts as a modulation variable cathode resistor. Perfect? No. Practical? Yes!
Seems too simple? Never let that stand in the way of success.

Read the 1 comment after the break below.


by VE3LYX 3/10/2013 12:03:52 PM

I have come to a few conclusions during this process and would like to share them as comments.
The cathode style modulator is a very easy and relatively simple way to add AM to an exisiting CW transmitter by simply tapping into the cathode circuit. Is it ideal? Doubtful, but it certainly is practical. The biggest problem is it steals power from the transmitter to run the modulating tube. This really hurts range in a moderate or low power rig. Is that the only way to do this?Certainly not! But it is the easiest way. So if you have an ancient CW rig , be it a TNT or and old style Hartley etc or even as in this case,a old miltary set without AM provision in its cabinet then this is a workable solution for the occasional fun night on AM with other Hams. It could be improved by raising the B voltage to adjust for the extra used up by the inserted modulating tube, however, there goes the simplicity of it out the window. On the other hand there is essentially nothing wrong with it if one is willing to accept its limitations. It has been for me a most interesting experiment to say the least.

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