ARC5 cathode modulated

[VE3LYX]

I have been working on a cathode modulator for my T18 ARC5.(now on 80M)I finally have it working now to the point of making a contact with my regular 80M group this morning.
Here is the set up. The power supply (homebrew) in the middle. It was quite at thrill to have it working as I wanted to. Problems encountered were.
The original handbook schematic and parts list had a mistake in it. Once I fixed that the whole thing came alive. I wonder how many built it and figured ok it works but is crap before me. I found it accidently and was unsure but it didnt make sense to me so I tried what I thought it should be and it worked instantly.  
Secondly this type of modulator and speech amp steals all its power from the B supply being in series between ground
or B- and the cathodes of the 1625 finals. The voltage drop caused by this drops output dramatically in this case since it is a high % of the total B volts available. An increase in B supply between 150 to 250 volts would help big time as the ARC5 has good power on CW with full voltage.
Also the ARC 5 was designed for a longwire not  50 ohm coax. It occurred to me I have a longwire so I unplugged it from the tuner along with my super duper make it 50 ohms stuff and hooked it direct to the ARC 5 which tuned and loaded it easily with what devices it had for that purpose when it left the factory.
Band conditions were bad today. Lots of noise and static crashes and signals were not great so if it worked today
I am happy. I may try adding a  transformer and rectifier of 150 volts or so in series with the main supply to see if I can recover some of the lost power due to what the modulator steals.
Still it is to me anyway and interesting way to use to modulate a rig simply by plugging the device into the cathode keying circuit. Future ideas are to build it all inside using the calibration crystal socket and the tuning eye socket (both octal). Took a lot of fooling around to get to this point. I will have to record all the new values I came up with before I go further now that it works well.
What was wrong with the old handbook diagram? (simple grid modulator) Check the value of the first resistor in from the Mic jack.
And to the morning AM group a big thank you for giving the spot to try it! Great bunch of fellows to say the least.  
Don  

[K6JEK]

Now that is nice.

[W3GMS]

I have been working on a cathode modulator for my T18 ARC5.(now on 80M)I finally have it working now to the point of making a contact with my regular 80M group this morning.
Here is the set up. The power supply (homebrew) in the middle. It was quite at thrill to have it working as I wanted to. Problems encountered were.
The original handbook schematic and parts list had a mistake in it. Once I fixed that the whole thing came alive. I wonder how many built it and figured ok it works but is crap before me. I found it accidently and was unsure but it didnt make sense to me so I tried what I thought it should be and it worked instantly.  
Secondly this type of modulator and speech amp steals all its power from the B supply being in series between ground
or B- and the cathodes of the 1625 finals. The voltage drop caused by this drops output dramatically in this case since it is a high % of the total B volts available. An increase in B supply between 150 to 250 volts would help big time as the ARC5 has good power on CW with full voltage.
Also the ARC 5 was designed for a longwire not  50 ohm coax. It occurred to me I have a longwire so I unplugged it from the tuner along with my super duper make it 50 ohms stuff and hooked it direct to the ARC 5 which tuned and loaded it easily with what devices it had for that purpose when it left the factory.
Band conditions were bad today. Lots of noise and static crashes and signals were not great so if it worked today
I am happy. I may try adding a  transformer and rectifier of 150 volts or so in series with the main supply to see if I can recover some of the lost power due to what the modulator steals.
Still it is to me anyway and interesting way to use to modulate a rig simply by plugging the device into the cathode keying circuit. Future ideas are to build it all inside using the calibration crystal socket and the tuning eye socket (both octal). Took a lot of fooling around to get to this point. I will have to record all the new values I came up with before I go further now that it works well.
What was wrong with the old handbook diagram? (simple grid modulator) Check the value of the first resistor in from the Mic jack.
And to the morning AM group a big thank you for giving the spot to try it! Great bunch of fellows to say the least.  
Don  

Don,
Nice to hear your success story!  To help others, please pass along the handbook issue and from that, I guess its obvious about the 1st grid resistor. 
Joe, GMS

[VE3LYX]

I will find the book and page and tell what I changed and why. Mic wouldn't drive properly till I did. It was last spring when I did this so I have to screw my ham radio head back on to remember what I changed and why. I know I was very unsure about the change but had nothing to lose. Anyway it works now. Circuit is in 1956 handbook page 250 although I don't think I used that book . I have several.
I rubbed on it some more this afternoon and now have it showing about 25% on the Field Strength Meter which may mean nothing more then it is more then it was. But more is good, right.
I am tuned up on 3725 at the moment. I will try another freq to see if I can work south east  a bit.  
Don
Ok I remembered. I couldn't get enuf response from any of my mics. I guessed the 2.2 meg resistor across the input was causing some of the trouble perhaps because it was intended for a different style of mic then we use today. I put in a 82 K instead and right away the whole thing came to life. Since best op was properly within the pots range of control (dont run it full or off but somewhere in the middle is best. )I left it there as it works well.
Resistors are always a problem for me as I am severely colour blind. Because of that I tend to second guess myself thinking I have simply used the wrong one. Now I just measure. I  was sure I had probably just made an error so I never considered the circuit would need a change till I ran out of ideas and solder. 

[W3GMS]

Don,

Thanks for doing the research. 

I really am surprised that the 2.2M caused the problem.  Those high value grid resistors  were mainly chosen for crystal microphones.  Saying that, dynamic mic's generally work well into a very high impedance also.  Dynamics being lower impedance and generally lower output work fine into a hi Z input. 

Joe, W3GMS 

[VE3LYX]

It could be just my particular mic.  All I can say is it was less output before then it is now. I had zip to lose by trying it and it worked for me.
I have two other cathode modulators built and running. One uses an old 4 pin triode and a carbon mic battery/ audio transformer circuit and will modulate this rig as well although I built it for my 45 tube TNT. I have another carbon mic triode for this rig as well but this modulator works so much better (now) I parked the other on a shelf.
Don

[W3GMS]

It could be just my particular mic.  All I can say is it was less output before then it is now. I had zip to lose by trying it and it worked for me.
I have two other cathode modulators built and running. One uses an old 4 pin triode and a carbon mic battery/ audio transformer circuit and will modulate this rig as well although I built it for my 45 tube TNT. I have another carbon mic triode for this rig as well but this modulator works so much better (now) I parked the other on a shelf.
Don

Very good Don and maybe I will hear you on with it.  Speaking of modulating self excited oscillators, brings back memories of working Bob, W2ZM Heising modulating his Hartley.  As long as he kept the audio around 60% modulation the FM'ing was not an issue.  I forget what the Harley used, but it may have been a 203A or something similar maybe modulated by a 211.   

Have fun!
Joe, GMS     

[steve_qix]

Hey, looks good!

I don't have the schematic, but it appears to be a 6V6 or something like it as the cathode modulator.  If you increase the DC voltage, you must make sure you do not exceed the plate dissipation rating of the cathode modulator tube.

At least half of the total power input to the system will be dissipated in the modulator tube, depending on how it is adjusted.  With cathode modulation, depending on the circuit and how things are set up, you can have some grid modulation of the RF amplifier as well.  As an example, with pure cathode modulation (no incidental grid modulation), and if the system is adjusted such that you have equal voltage across the RF amplifier and across the modulator, you will be dissipating the same power in the modulator tube as you have power input to the RF amplifier.  If things are adjusted such that you have MORE voltage drop across the modulator than you do across the  RF amplifier, then you will be dissipating more power in the modulator than you have DC input to the final.

Anyway, calculate the power dissipated in the modulator tube.  This will be the voltage across the tube multiplied by the RF amplifier plate current (because the tubes are in series).  You may end up having to add another modulator tube in parallel with the original modulator to keep things within dissipation limits.

Good luck with it!  I run series modulators (a cathode modulator is a series modulator) all the time on a daily basis.  They can work very well.

[w7fox]

I ran this exact set-up in 1965.  The modulator worked fine, and as I remember, when the modulator was plugged in, the plate current dropped to half.  This is what its supposed to do, remember that your carrier should be 25% of what it is on CW.  PEP on AM is four times the carrier.  Power is not getting "absorbed" by the modulator, you need head room for the voice peaks.

[steve_qix]

I ran this exact set-up in 1965.  The modulator worked fine, and as I remember, when the modulator was plugged in, the plate current dropped to half.  This is what its supposed to do, remember that your carrier should be 25% of what it is on CW.  PEP on AM is four times the carrier.  Power is not getting "absorbed" by the modulator, you need head room for the voice peaks.

Well, yes/no    With a linear amplifier, which has a finite power output - yes, definitely.  With a high level modulated system (such as a series modulator), you can run almost the same power on AM phone as you can on CW with the same set of output tubes - and that is assuming you are running the maximum ratings on CW.  If you are running less than the maximum the tubes will do on CW, you can run exactly the same power on AM plate modulated phone as you can on CW in most cases.  Certainly, this is the case with the 1625s, etc.

All analog series modulators (such as this one) do dissipate a lot of power.  If the system is set up for pure high level modulation (in other words, no incidental or intentional grid modulation), the modulator will dissipate as much or more power than you are actually running into the final RF amplifier.  This is because the modulator must have sufficient voltage drop across the output tubes (or whatever you are using) to allow for positive peaks.  To achieve 100% positive modulation, the voltage must be divided evenly between the modulator and  RF amplifier.  If you want to achieve higher than 100% positive modulation, even more voltage must appear across the modulator than is across the RF amplifier, further reducing the efficiency of the modulator (and increasing the power dissipated in the modulator devices).

Series modulation is a very good way to go, and there are methods such as class H, and pulse width modulation that can be employed to dramatically increase the efficiency of the series modulator making series modulation very practical at high power.

For low power such as this application, straight analog works quite well, and is easy to set up.

[VE3LYX]

OK, Yes I understand how it works. After having built three and tried them all they are earning a place in my heart.
Yes it is a 6v6. Could try a 6l6 tall boy just for fun since I have some.
But to be honest the last little bit of work I did on it brought the power up a bit (on the transmitter ) and if I find it serviceable for use on decent propagation days I may just invoke the cabbage rule and "quit while I am a-head"
It seems to be working quite well.
How did I get the power up. I got rid of all (ALL!)the stuff I had made to be able to use 50 ohm coax and turned it back to how it was made. Since I use a long wire it is very happy tuning and loading it.
Yes I hope to work some of you.
Having one on the TNT was a lot of fun too. I used a DX235 triode to modulate the 45 in the TNT on 40 with an old telephone carbon mic driving the electro mech circuit to the DX235 tube. Trouble is I am so far from any AMer s that a couple of watts is about useless here. Got to get some other lads interested. Mentioned AM to some hams at Field Day this year as they were set up in the park nearby. Looked at me like I was a dinasoar.
 
Don

[W3GMS]

OK, Yes I understand how it works. After having built three and tried them all they are earning a place in my heart.
Yes it is a 6v6. Could try a 6l6 tall boy just for fun since I have some.
But to be honest the last little bit of work I did on it brought the power up a bit (on the transmitter ) and if I find it serviceable for use on decent propagation days I may just invoke the cabbage rule and "quit while I am a-head"
It seems to be working quite well.
How did I get the power up. I got rid of all (ALL!)the stuff I had made to be able to use 50 ohm coax and turned it back to how it was made. Since I use a long wire it is very happy tuning and loading it.
Yes I hope to work some of you.
Having one on the TNT was a lot of fun too. I used a DX235 triode to modulate the 45 in the TNT on 40 with an old telephone carbon mic driving the electro mech circuit to the DX235 tube. Trouble is I am so far from any AMer s that a couple of watts is about useless here. Got to get some other lads interested. Mentioned AM to some hams at Field Day this year as they were set up in the park nearby. Looked at me like I was a dinasoar.
 
Don

Build a MOPA with a 204 and we will hear you just fine OM!

73,
Joe, GMS

[AJ1G]

Also the ARC 5 was designed for a longwire not  50 ohm coax.

Actually it was designed for an electrically SHORT wire, i.e, like those rigged from the upper rudder to the radio ops position on a B-17.  Low impedance and lots of capacitive reactance.

I think I may have previously posted on matchng an unbalanced 50 ohm coax fed antenna to the command sets, If not, here it is again...works very well for me, originally published by Walt, KJ4KV in ER in Uniform early 90s/Late 80s...

Use a parallel L/C tunable resonant circuit that is fed about 3 turns from the cold (grounded end) from the transmitter via a 200 uuf cap in series with the transmitters output  loading coil/ adjustable link.  Tap the antenna feed line in at about 6 turns up from the cold end. Makes a resonant RF autotransformer that provoides a step down impedance transformation fro the aqntenna to the transmitter.  Tune the L/C network and transmitter loading coil for max soup on  a wattmeter in the antenna feed line, and then tweak the transmiiter loading link .  Should peak out at about 2 oclock on the link.  I'm gettiing about 50 watts carrier outpuit on CW with a BC-456s scrfeen modulator dynamotor putting out about 560 VDC for the final plates. For my coil, I use an Eico 720 Pi Net coil assembly  (all of it on 80/75,  I shunt out the smaller section for 40).

The BC456 stock screen modualtion seems to like the added loading afforded by the autotransformer - I get good symmetrical near 100% mod peaks at about 20-25 watts output in AM voice mode.

[VE3LYX]

Well it tunes and loads fine on my antenna. 132 feet single end feed hanging over the cliff to a high tree.
Here is one of my other cathode modulator. My design. Carbon mic circuit running a triode. Homebrew directly out of the work I did to get the DX235 to modulate the 45 tubed TNT. It works but I haven't tried it on air. Bit more carrier with this one . Sounds good when testing. Listened on 3885 tonite but was very noisy. So other then a quick attempt I let it be.
I have a screen modulated rig I built not to mention my DX60B as well. I wanted to try modulating this without going inside. I thought an 811 TNT would be fun! Don't even have to change the socket!
Don

[VE3LYX]

A 204? I am not familiar with that but will look it up. Now you started it. After a few AM QSOs today (80M and 40M)I pulled the TNT off the shelf and looked it over to see what I need to put it back on 40M. A 45 can be pushed pretty hard especially the new ones (600v even)but I will check into the 204. And if I need to make it a MOPA can.  I also dug out the modulator for the TNT. Still appears to be intact and working.
Don

[VE3LYX]

Thanks for inspiring me to fix the  TNT. Here it is with its modulator. I rebuilt the coil and a new RFC for it this aft. Tried it a couple of times today on 40.  Anyway enuf about this. I will be on with the ARC5 in good conditions on 80M .
Bottomline? It works.
Don

[W7TFO]

Hi Don,

The 204 is a 1920's vintage 250-Watt glass triode:

http://frank.pocnet.net/sheets/111/2/204A.pdf

It would indeed be a good amp for you, but getting a good one is tough, and the socket set for it is even tougher to find.

73DG

[VE3LYX]

Oh, Too much for me. I was thinking that a #10 tube was a step up from a 45.
Don

[VE3LYX]

Found a bit more today. Probably 20%.
Don

[VE3LYX]

Just as a matter of interest to those who may be into this. Today I took a handful of tubes down and swapped them for the 6v6. Here are the results.
Best by a wisker  6l6gc. So close it was hard to call compared to a 6l6EH tall boy. Next was the 6l6ga and finally the 6v6. I am rating them for carrier output. Modulation seemed about the same % AGE. Not enuf difference to call. However the carrier power was significantly different. Well over 30% between 6v6 and the 2 best 6l6 variants. Also as far as modultion goes there is quite a difference between a 6sn7 and a 6sl7 in modulator use.  I had to rewire the filaments in my modulator . I was using it with the Arc5 and paired the filaments to use the 12 volt. However because of the mismatch with current the tubes would not heat evenly . I wired them for 6 volts and all is well. And of course that enabled me to make this test.
Don

[VE3LYX]

However I discovered today a big carrier does not mean a great signal. Best performance was obtained on air with the weaker tubes. I fear the big ones were labouring my PS to the point it was showing up too much on the signal. I also bypassed the one mic today which got rid of a bunch more crap. used an .02.
Don