Arc 5 on AM

[VE3LYX]

I have aquired an Arc 5, T18. Was working wiring it up for a in house power supply. Also worked at converting PA output to 80M. Now for an easy way to modulate it. A Apir of 1625s whichare just 12 volt 807s should have some stomp. I have an 800 volt B+ supply but I will start them off around 250v till I am happy . Need to wire the screens and the heaters  from the plug to the PS. unit already has a octal conversion at the rear. Any hints or tips?
Don Ve3LYX

[w5rkl]

The following link has a lot of info:

http://www.w7ekb.com/glowbugs/Military/arc5pages.htm

73s
Mike

[kb3ouk]

Those things were originally meant to be screen modulated, so the simplest modulator would probably be 1 or 2 of something small (think receiver audio tubes) driving a small modulation transformer . Another possibility, I haven't looked at an ARC-5 schematic recently enough to remember how exactly they were wired, would be to stick the secondary of a transformer between the cathodes and ground (or whatever the cathode is tied to), and cathode modulate the thing. You should be able to get more power out of it that way than you would screen modulating it, but it wouldn't need nearly the amount of audio power compared to plate modulation. Usually, cathode modulation requires audio power equal to 20% of the dc input to the final, and gives about 50% efficiency.

[VE3LYX]

I must say cathode modulation really interests me. I now have a schematic . It is doable. However the screen is already equipped with is soley external connnection. I have done HB screen modulation with a carbon mic circuit electro mechanical and 1/2 a 12sl7 triode. Unlike many of the current AMers  (BROADCAST QUALITY!)I really like the communications sound of the old military carbon mics. Clear distinct , a wee bit tinny. I am thinking. Good sugggestion! I see a key jack has been installed. If it is cathode keyed then it will be cathode modulated!
Don Ve3LYX

[kb3ouk]

If that jack is for cathode keying the final, then this work work perfectly, all you need is power for the filaments and plug the output right into the key jack, if it really is cathode keyed. Most of the parts probably can be found in your junk box. If you need more power, just put another 6Y6 in parallel.
www.w7ekb.com/glowbugs/projects/SimplestModulator.pdf

[k4kyv]

The 6Y6 or similar tube(s) in series with the cathode and ground is not true cathode modulation.  The modulator tubes effectively serve as a variable cathode resistor, nothing more than a form grid modulation, with typical grid modulation efficiency of about 30% when the modulated stage is adjusted properly, not counting the voltage drop and plate losses in the modulator tubes themselves.

True cathode modulation in a combination of grid and  plate modulation.  A modulator stage using a modulation transformer with tapped secondary is used. One section of the tapped winding is configured to modulate the grid, and the remainder simultaneously modulates the plate. The only voltage drop is from the DC resistance of the section of the winding that modulates the plate, just is in the case of regular plate modulation.  With optimum grid/plate modulation ratio, the plate efficiency of the modulated stage may approach 50%, and IIRC under those conditions it requires audio peaks of about 15-20% of the DC input to fully modulate the carrier.  The grid/plate modulation ratio is determined by  the placement of the tap (not necessarily at the exact midpoint of the winding). It is a compromise that allows for greater efficiency than pure grid modulation, but requires less audio than does pure plate modulation.

Cathode modulation is fully described in some of the handbooks and periodicals from just before and after WWII, and I believe also in West Coast Radio publications. After the war, a number of articles on the variable cathode resistor method were published, under the misnomer "cathode modulation".

[VE3LYX]

Your point it well taken however I am not worrying about true this or that "I jess wanna use my radio." 10 percent 30 pecent 120 percent, I dont really care. A readable signal and not interfering with anyone ham or neighbour is my only goal. I have given up on perfection since 2 birthdays ago and am working diligently on having some fun before they pop me in a box.
Yes I read a lot of this info in my 1940 Radio handbook. (Not ARRL)
In that vien I did some experiments here with a low power CW transmitter.
I also wired and powered up the ARC 5 today. Filaments work properly. Power is everywhere it should be . I used only 140 B+ just for safety. Relay wont work on 12 volts . I had hoped it had been changed since the filiments had been paralleld already. I did not disturb the rear plug but inserted my power supply wiring through the tube pin holes and soldered it on the lug inside. To return to original just unsolder the wires and pull them back through the holes.
Don Ve3LYx

[WB3JOK]

I built a simple screen modulator for my 80m ARC-5 using a 12AX7 for speech amp and a 6AQ5 driver. Basically copied a low-power modulator design from a 1960's ARRL handbook except for not using a modulation transformer... The final B+ comes from a 480:120v control transformer run backwards into a FW bridge, about 650-700v. Also an OA2 (150v) or maybe it's an OB2 (108v) VR tube for the oscillator B+.
It's a real PW rig, about 10 watts carrier, but I did make a 200 mile contact with it, with a trap dipole about 25' up 

[VE3LYX]

Thanks I have the crcuit in a rig here but used a 6sl7 instead of a 12ax7. I will just install a jack in the circuit to feed the ARC out the side of this unit and use the modulator on both.
Don

[k7mdo]

Here is a photo of my Heathkit AT-1 uswing the variable resistor cathode modulation from ARRL in 1955 or 56 handbook.  6SN7 and 6Y6 if I remember correctly.  Works great, just 4 watts out but contacts say it is quite respectable.

[VE3LYX]

I love that kind of stuff.
I am curious , on the Arc 5 set what kind of plate voltage on the PA? Same as the rest of the tubes or can one put 750 to 800 on them  since an 807 (or 1625) should be able to handle that. I have an Hb power supply tat would give me 250 volts and 580 volts right now.
Don Ve3LYX

[kb3ouk]

If I remember, the 1625 plates are seperate from the low B+ for the other tubes. I want to say they ran about 600v on the plates, but I'm not sure. 807/1625s should be good for something like 750v if you really wanna push them.

[KM1H]

While many used screen modulators a plate modulator was available, I believe the models are MD-5/BC-456 and MD-7.

Carl

[VE3LYX]

I reworked the cathode grounding circuit so I can do it with an insulated key. I do the same for my Hb Pa so it is no big deal. Previous owner had the key wired to run the relay except of course it wont run on 12 volts.  The other side of that relay runs the VFo B+ I believe. Should be somewhere in the neighbourhood of 180 to 200 volts I think. I will try that and see if the VFO works.  So I have heaters working on 12 volts. , cathode keying of PA stage done. I need to get B+ to the vfo  and need to do something about the PA screens . Usuaully a 24 to 25 K resistor from the PA plate circuit before the RFc is what I use with the tubes I am used to. Should work here I would think. And I need to rig a dummy load for this radio.

[W3RSW]

The final B+ comes from a 480:120v control transformer run backwards into a FW bridge, about 650-700v.

Good idea.  I have one of these too, a Micron.  It's pretty hefty and looks like it could do more than a 10 watt rig.  (That's not really PW, it's just lite.  )

Let's see; mine has 110/115/120 "secondary" winding; 220/230/240 or 440/460/480 other side in the two "primary" windings.  Micron tag claims .075 KVA.   Seems about right,  Core is 3 x 2 x 1-1/2 inches thick. Heavy little dude.

[k9ing]

I ran a pair of Arc5s (40 & 80) which I built a modulator for of 2 807s; this was in '57 with a BC312 rcvr. I probably had more fun with them than any rigs I have ever had; but I was13 and easy to please :-) Bob K9ING

[VE3LYX]

I have it on 80m now and running well. Time to decide on a modulator and build it.
Don Ve3LYX

[kb3ouk]

If you are running cathode keying, you might be able to modulate it with an audio amp coupled through a transformer. Put a key jack on the secondary of the mod iron and plug it into the key jack on the ARC-5. The impedence of the cathodes should be around 478 ohms. You need about 25 to 30 watts to modulate it, and should be able to get about 66 watts output, assuming you are running at 600v 200ma. You can use any transformer (an old power transformer will work) as long as it can handle the cathode current (around 225ma), and the turns ratio brings the input impedence (which is whatever the output impedence of the audio amp is) close to the 478 ohms of the cathode.

[VE3LYX]

Now we,re talking! Thanks for the tip.
I have the iron already wired to the key jack and have a speech amp built(double triode 12sl7. I just need a power tube for the speech amp deal. I suppose a 6l6 would be a bit light? I could pop a 6sl7 or 6sn7 in place of the 12 and series the heaters for 12v same as the rig.I also have a  couple of stand alone electro mechanical single button carbon mike circuits which I have gotten used to working with over the last couple of years. Still need a preamp and a power tube though. This is on my stuff to do today list.
I also remember hearing years ago of a cap coupled modulator rather then a transformer coupling. I might play with that idea on my PW homebrew and if I get it working to my satisfaction, hook on to the screen grid feed and try it out.
Don Ve3LYX

[kb3ouk]

as long as you can get at least 12 watts of audio, it should work, you need audio power between 10% and 25% of the dc input to the final for cathode modulation.

[WU2D]

You can also plate modulate which gives very good results. A 12AX7 and a pair of 6L6's work fine. A 12SL7 and a pair of 807's or 1625's work fine with a preamplified microphone or add a 12SG7 or 12SK7 speech amp on a low level microphone. You can use a transformer interstage coupling for phase inversion or an old HiFi phase splitter like this.

I have had the best luck with 2:1 or 3:1 modulation transformers like 6000 CT to 3000 Ohms running higher voltage on the modulators than on the ARC-5.

Mike WU2D

[Steve - K4HX]

Nice modulator Mike. Thanks for posting.

[AJ1G]

The ARC-5 versions of the Command Set transmitters were in fact plate modulated using the MD-7, which included the DM-33  dynamotor for the TX high voltage, which was about 500V DC.  I am running my SCR-274/ARC-5/ATA components mash up using a T-19 or T-22 ARC-5 transmitter, and the SCR-274 BC-456 screen modulator/dyno unit.  I recall that the screen dropping resistor value for the finals is different between the 274 and ARC -5 units, but they are otherwise very similar.  I changed the screen dropping resistor in the ARC-5s to the value used in the SCR-274Ns.   

The MD-7 used a pair of 1625s for modulators, along with a 12J5 ? speech amp.  Track down the ARC-5 and SCR-274 info from the BAMA web site, you can learn a lot about how to work with them and emulate the modulation and T/R switching schemes by carefully perusing the schematics.  Mike,WU2D is the local Command Set guru on this forum.  Can't recall which which units, the transmitters or the receivers, have different rear connector pin outs between the ARC-5 and SCR-274N versions, always be sure to use the right schematic.

If you do go with screen modulation, tune for max output in the CW mode for a start, then tweak the loading/output coupling for best sound/waveform on an O scope in the voice mode, best sound will be at less than max attainable  carrier output in screen mod voice mode.  My setup makes about 40W out on CW and about half that on voice using the DM33 dyno and the stock screen mod setup.   

[VE3LYX]

I lost a cap. Old age I guess. I will check them all and repair before I run it anymore. Thanks for the modulation tips all. What I wind up with is anyone's guess but I have always wanted to try so called cathode modulator. I guess because I can use it on many of my Cw rigs. Shouldnt a pair of 807 be able to make a bit more power than 40W or am I dreaming.

Don Ve3LYX

[AJ1G]

Sure you can raise the HV on the 1625s and crank out some more watts.   The 40 watts I am getting on CW is from an  original system setup,  right down to the dynamotor at 500V, which does not push the tubes to the limit.