The AM Forum

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

The following link has a lot of info:

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

73s
Mike

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.

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

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

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".

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

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  8)

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

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.

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

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.

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

Carl

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.

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.  ;D )

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.

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

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

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.

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

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.

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

Nice modulator Mike. Thanks for posting.

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.   

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

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. 

Thought I had problems with a shorted cap or something. I had reworked my bench supply to run both the BC 454 recvr and this ARC tx for testing etc. I kept finding it keyed when I hadnt keyed it so before Christmas I set it aside till winter really hit. Today because of that thread Tim VE6PG had on the old Bellradiophone I was tempted and succumbed to the urge to pull my single tube 45  TNT transmitter out, fire it up and put it on air. I used the same supply. Strange, it was live even though the key was up. Same trouble I had when I last worked on the ARC 5. After some voltage checking and finding my new glasses I discovered a solder sliver from the cathode or B- pin of the PS octal socket down to the #7 pin just below it .( I use octal beam power tube wiring for all my power plugs. 2 and 7 for heaters , 3 for B + and 8 for B- . That way any radio can be plugged into any shack power supply without fear.) Anyway when I rewired this one for a separate plug for the ARC 5 stuff I had a solder drip making the connection between cathode B_ and pin 7 of the heaters. So the heater was grounded and it just happened to be the right one by chance. The PA cathodes were grounding through the heater as well, bypassing the CW key and jack completely Probably nothing wrong with the radio. Just all caused my this accidental goof on my part when I went from twist together test set up to properly connected and soldered. I had set the radio aside for a better time as I didnt want to risk ruining it. Good thing I did. I will dig it out again next week and get it going. First with Cathode modulation (after a bit of CW of course! ) and then perhaps build a Heising type modulation set up.  Why my resistance to plate modulation? Good question . I know it is the best  but I just cant bring myself to use two radios(one Rf and one audio with similar tube counts and probably even the same tubes) to run one. There is another way to do it as well I had heard of but never tried. It does not use a transformer. I may try it on the TNT first to see if it works at all. If it does I will say so. If not no one will ever know.
Don VE3LYX

Mike,

Is there supposed to be a blocking/coupling cap between the plate of V1a and the grid of V1b?

Just checking, I like the circuit!

Greg

[/quote]
Mike,

Is there supposed to be a blocking/coupling cap between the plate of V1a and the grid of V1b?

Just checking, I like the circuit!

Greg
[/quote]

Nope Greg,

You can direct couple low level audio stages with tubes or transistors as long as you bias them correctly and you have enough voltage compliance for linear operation.

Mike

I'm running Navy (black wrinkle finish) ARC-5 transmitters at one of my stations on 160, 75 and 40. They are powered by a power supply/modulator
I built  with 807's for the modulator. Running 650 volts to the rig and 807's I get 60-75 watts out 100% modulated. I have 250 volts regulated on the VFO and the PA screen comes from a 47K dropping resistor off the modualed B+. watch that plate blocking cap in the PA tank if you plate modulate, best to replace with new before you get to agressive with the B+ voltage. other than that I have no failures in 3 years and the little rigs work great with only minor mods- improved RF bypassing of VFO plate, PA screen and cathode, per an old CQ magazine ARC-5 conversion manual. I chose to leave mine set up for 28 volts, and run 28VDC on the fils and relays. much less invasive conversion that way. 

In the last two weeks, I have connected up  a clean audio feed consisting of my old 1968 vintage WB2ZPS Shure Sphere-O-Dyne dynamic mic, octave band EQ, and a small mono utility amp with a 25 volt line output into a military RM-12 remote control unit, which was used to connect remote land line  field phones into things like the BC-191 carbon mic input.  This is sent directly into the all original BC-456 screen mod/T-19 setup.  The result is extremely clean, wide band  near 100%  modulated audio at about 15 watts carrier output.  "Test audio" testing into a dummy load sounds pretty good, better than some of the local FM stations around here (especially WMOS 102.3, where they constantly crank in too much bass and distort the heck out of their audio).. I swept this setup with my HP-201 and it was clean from about 100 Hz to way above 10K at close to 100% peak modulation.  Nice symmetrical envelope, no flat topping, near full 100% positive peaks just prior to hitting the negative peak clipping point .  Another thing that I think helps keep the waveform clean is the use of a impedance matching RF  network that presents a very low impedance antenna feed to the transmitter output , which was what it was designed to work into in the aircraft installations. The network consists of a parallel tuned LC circuit, with the transmitter fed into the coil about 3 turns above the cold end via a 200 pf series cap. Antenna tap is at 6 turns above the cold end. This circuit was presented by Walt,KJ4KV back around 1990 in one of his ER in Uniform columns about the Command sets.  Using this network, I can get about 55 watts in CW out of the otherwise stock dynamotor powered system.

I am reading this with great interest. So the screen resistor change I assume (rightly or wrongly) cuts it approx 50% which is then added back in at full modulation. I could do that easy enuf if so. I am no whiz on the ARC5 , just learning but it seems to me screen supply is external and I had thought I could cut whats coming in and add it back in audio or have I missed something there.
Don Ve3LYX

   I read these posts with interest since I have an original ARC-5 which covers 80m. I do wonder though about the MO-PA configuration, which might have a tendency for the oscillator to 'pull' with PA loading changes, and also some PM or FM when the PA is plate modulated?

   With the advent of SDR receivers, it turns out that some of the vintage AM transmitters DO INDEED have one sideband reduced in amplitude due to oscillator 'pulling' during modulation. My Gonset G-76 does this on 15m with the internal VFO with a MO-Buffer-PA configuration. Using a different VFO or a crystal, and the folks using SDR receivers say the sidebands are equal. I confirmed this issue using my selective Icom R8500 receiver; my LSB was about 30 DB below the USB. So regarding the ARC-5 Xmitter in MO-PA configuration, there has to be an effect here. Has anyone looked at this? For sure the antenna swinging in the breeze will 'wiggle' the VFO.

Jim
WD5JKO

The ARC-5s MOPAs do wobbulate a little when you modulate, but nothing serious to be concerned about.  With a BFO off, you don't notice it.  Even the BC-191s and BC-375s SBE effect can be tamed by careful neutralization.

Set up a simple audio monitor circuit today to listen to test audio through the BC-456 screen modulated T-19 using a small signal diode tapped off the O scope and  counter pick up link on the coax between the watt meter, and in this case, dummy load.  Fed the diode output right into the aux input of the duty bench Fisher Studio Standard amp and speakers.  All I can say re the results is whoa!!!.  I thought this little transmitter sounded good through the R-392, but being able to hear the full spectrum without any bandwidth limitations or distortion in the receiver feeding the amp really gave me an appreciation on how good these things can sound. I guess when the entire modulator consists of two transformers and a single ended Class A 1625, there's not too much you can do to screw up what you feed into it!

After an hour of so of pumping 15 watts carrier plus continuous test audio to the Bird dummy load, the transmitter and modulator themselves were not too warm, but the DM-33 dyno was pretty hot, about 165 degrees on the case as read with a infrared surface temp scanner.  What's the typical rated temp rise limit for a continuous duty dyno?

Interesting thread.  

I'm in the later stages of putting together three sets of Tx/Rx units for 160/80/40.   For a modulator I'm using an old SS stereo amp and a 220VAC Filament transformer with a 10V 10Amp C.T. secondary.   Feeding it backwards I get plenty of audio for hi level modulation.  I use a National NCX-5 power supply as a source, and  designed SS regulators (http://s670.beta.photobucket.com/user/mikeinkcmo/library/Radios/ARC-5) to provide all the required operating voltages.   The transmitters put out about 45 watts which is good enough.

I've had the 80 Meter setup on the local morning net with good reports, but need to insert an attenuator in line so I can run it through the NC-2000 linear.  So far, its been a fun project.

Mike, have you tried just lowering the output using the antenna coupling control?  OTOH, not sure how that will throw off your loading and linearity of final amplifier.  I assume since you are making 45 watts of carrier that you are plate modulating.  I have a few more 3-4 MC transmitters around, and a few tube mono hifi amps, notably a Newcombe 75 watter running 7027As that i should try plate modding with.   Also going to get the SB200 going again to hang on to the end of the screen modded T19/ARC-5.  That little thing sounds really sweet, but needs a boost from the 10-15 watts carrier it can put into the antenna.

It was the cap at the top of the coil which isolates the B+ tothe plates from ground. I subbed in a 2kv .01. Hope that value will work. Also during y hunt for the probpelm i discovered one of my1625s is no good. Top where cap is is broken from the glass although one couldnt see that by just looking.
As to modulation I i understand how they were modulated in the past . As for plate modulation , as much as everyone is in love with it, I refuse to build a radio to modulate a radio. And that is about what one s doing. Huge power supplies, Same tubes as finals driver tubes , transformers etc etc. No thanks.
I built a cathode modulator for it as per 1956 ARRL handbook. Some cant say cathode modulator so it is a simple grid modulator which goes beteeen b- and the cathode in any cathode keyed rig. Call it what you will. I heard from many who have used that method with success.  Ben from G call land also gave me a high voltage transistorized version. For a carbon mic whch ilike.
Don VE3LYX

Almost sounds like you had a bad experience with a plate modulator or something Don ;D with the refusing stuff and all ::)

Your choice of course. Have fun.

Al VE3AJM

Don said,


I agree!! Hi-Quality, High Level "Goldburg" Modulation is extremely difficult to achieve.   I think I have about half an hour and 2 bucks in this setup, since both the transformer and amp were given to me.   ::)

I'll have to get a '56 handbook and look at the cathode modulator.   A fellow on the morning AM net uses one for several different HB and Mil rigs.  

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/ARC-5/Mod1_zpsfb41e0af.jpg) (http://s670.photobucket.com/albums/vv62/mikeinkcmo/Radios/ARC-5/)

(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/ARC-5/Mod2_zpsbbccb3e9.jpg) (http://s670.photobucket.com/albums/vv62/mikeinkcmo/Radios/ARC-5/)

I suppose it would be extremely difficult if you only had a half hour and twenty bucks...I've built many a HB plate modulator with very good quality and fidelity noted. :) 

Al VE3AJM

But Al that is you. I am interested in low level modulation of all types.  To me it seems two power supplies, two sets of finals , two drivers  add infinitum is not something I want to do right now. Doesnt float my boat . It does yours and I have no problem with that if it brings you pleasure. Same BTW goes for me with PP circuits. A waste of one tube to my Schwaben brain.
Now I know it is not that simple and that both those have advantages and very good advantages but currently my interest lys elsewhere.
I got the cathode modulator working on my TNT even.  (See cathode modulation thread.) So it should be no problem for the ARC 5.  I also have a single, modulated double triode working on 10M. Still working on it to see how far it will go. Got new tube comong for the ARC 5 and I believe I have sorted out the snag.
Don Ve3LYX

Of course Don. Thats what makes ham radio and AM fun. We enjoy different things more and less. Pulling your leg. BTW St. Kitts hamfest was very good.

Al VE3AJM

Buy any ARC5s there?
Don

Close..there was a HB pair of 807s tx for $75..I got the Lysco 500/600 VFO/tx..its a cool old rig.

Al VE3 AJM

I've used two different PS/Modulator combinations with the 80m setup.  One using a more or less stock Gonset power supply for the old Twins, and the second using the NCX PS and stereo amp.

Using the Gonset supply/Modulator, It supplies all the correct voltages for the Tx/Rx pair as is, and I only needed to add a fet, source follower after the OA2, to boost the current to handle the receiver and Tx Osc/Buffer.  I also have to pull one 1625s  so the power supply doesn't over load.   This gives me about 20 Watts out.
(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/ARC-5/80MSta1stTime.jpg)

The NCX setup with the various regulators, gives me 40+ Watts with the Amp/transformer modulator.
(http://i670.photobucket.com/albums/vv62/mikeinkcmo/Radios/ARC-5/Mod5_zps49a1c725.jpg)

The Gonset powered arrangement, with an attenuator, is what I'll use to drive the linear.

That is quite a set up Mike,  WOW.
Al I just bought 10 1625s NOS. I needed one for the ARC5 (cap broke (glass also) Now I have a lifetime supply probably. I am thinking a fresh pair in the ARC5 and a quad 4 Linear.
Actually some of these not restorable ARC txs would make an excellant HF PA. Everything is there.  Tuned input (use osc tank) and tank with a pair of 1625s which at 750 volts can give good results. And an antenna tuner included in the package. Wish I had a spare as I would give it a whirl. This one is a functional unit in good condition so I am not going to butcher it although if my triode modulator like I built for the TNT performs  I will remove the calibration Crystal octal plug in and install the whole deal right in the same socket using a 6 volt tube and resistor so I can steal the carbon mic voltage accross the resistor with a diode and 470uf x 35v cap. If I use a metal triode no one would know the difference. Would be a nice compact selfcontained AM /CW rig.
Don VE3LYX

Yeah, thats is quite a Rube Goldberg setup that actually works well in those pics apparently...lol....hopefully the 220v/secondary winding thats being used as his mod transformer, and that carries audio and HV B+ has good insulation properties for the obvious reasons.

Sounds like a good score for you Don. So it was the 1625 tubes and more ARC5s?

Al VE3AJM

HMMmmmm  Test equiptment porn.......

That is really a nice set up. The 'cubbies fer the stuff, and the table looks like its on wheels.

klc

I dont know about more ARC5s. I have been working on my ARC5 project today already though. 6SQ7 tube I think will do the trick. Although I could use a 12SQ7 and just use 12 volts on the carbon mic too. Probably wouldnt hurt it and I could just pop some R in there to cut it back a bit.
Just seemed to me 1625 which are as you know just 12v 807 have a lot of potential. Ancient perhaps but good power at moderate voltage. If one has ten  Well seemed like a good idea anyway. I needed one for sure and it would be smart to pop in a pair keeping the oldy for a spare. We will see. Really I love homebrew and its challenges. I got into the command set stuff as you remember at Durham Hamfest when that fellow gave me a badly battered BC454. It was quite easy to convert to 12 volt heaters and I had it running in a couple of hours. I made a tuning knob and PS connector and I was hooked.  I bought the T18 for $17.50 and had it shipped to my older brother in NY state. He sent to me here. It was not real hard to get it into 80m quite aways although not all the way in . About 3700 I think with no parts mod, just a tune. Soldering a couple of wires, OSC coil turn and PA tank coil turn would get it at least to 3725 I think. Conversion to cathode keying had already been started so I completed it. Now if I can modulate it with either the 56 ARRL modulator or ideally with the homebrew version I got to work on my 45 tubed TNT I will have a decent power AM rig for probably under $100. Parallel 1625s at 700 volts  with everything else as is should have a decent poke.
I try not to get carried away. I already have far too many radios both tx and rxs not to mention three HB transceivers. If I had my druthers I would like to find another No 11 tank set. I had a complete one as a teenager and when cars and girls got my attention I left it behind in a move. Any more ARC5s? Probably not.  I might someday try for a ART 13  but at the moment I am a busy lad with what I have. 

Al said,


Look back up the thread to the pic of the transformer, and you'll see it has Hi-pot rating of 2KV.


Kevin said,


Yes it is. (http://s670.beta.photobucket.com/user/mikeinkcmo/library/testgear/Bench)  That makes moving gear around a fairly easy task.   The shelves are fixed, but the vertical dividers can be moved around to accommodate the various widths of gear.

I'm making a second bench so I can separate the T.E. and the radios.

I think you're running 550v on the finals. At 100% modulation, the voltage doubles as does the current. I would want to have a rating closer to 2500v on the secondary winding of the transformer, but you use what you have. Taking the DC component off the secondary would work using a modified heising arrangement. That way that transformer would only have to handle the AC/audio, with less possibility of saturation/distortion on audio peaks and peak voltage breakdowns. You would just need a suitable choke and blocking/coupling cap.

Al VE3AJM

Agreed, but since we are talking transformer here, and the source is not specified, I'm presuming the Hi-Pot test was AC not DC, so peak would then be on the order of 2.8-3KV.   I could be all wet here, but so far so good.

Of course a transformer isnt the "only" way to plate modulate. We just think it is cause most are done that way.
Don VE3LYX

Use what you have to work with, and see what happens and have fun. Hopefully if you get the rig on the air, the guys will be able to hear you and complement you at the same time. :)

Al VE3AJM

Spent the whole day in studio B playing with two rigs and two styles of modulation . After a frustrating morning I got the one little rig running on transformerles plate modulation. I wound up so close to what I had started with it surprised me. One little change and a couple of improvements on that and it is working. I need a small audio choke to keep the audio signal stuffed in the radio and not leaking out into the power supply but it works fairly strong.
However
had the TNT going today, 1929 special (copper tubing coils , 45 tube , 1920s parts. I had have been working on a cathode modulator for it which is an outgrowth of this ARC5 project's modulator only much simpler and uses also a pre 1929 tube. I am still smiling. DEEP, rich, full, sound now coming from that ancient radio. Got carried away and tuned up on 7194 and called CQ. Today I got a proper battery for the modulator tube and I sorted through all my unused audio transformers testing each one in the circuit till I found the best one. The effort paid off! It worked so good I invoked "the Cabbage Rule" (quit while you're a-head)  and came upstairs for the night.  Still grinning from ear to ear!  
Don

Great to hear your day in the lab was a success, days like that are very satisfying. 

Mike

Hardly slept all nite! ;>)
Today I am going to install the same system in the octal crystal socket of my T18 ARC5. I decided to leave the carbon mic transformer and battery outside  in the mic base. The cathode connected triode though which needs no B power since it steals from the PA can be left in place or a key can be just plugged in the same jack and full Cw ops return. Doing it that way keeps the set almost as produced and loses nothing but a crystal calibrator of somewhat useless frequencyanyway . I can install a crystal socket descretly somewhere else I think. I wouldn't mind having a bottom of the band crystal and I do have a vintage 3500kc rock
Don VE3LYX

Unless you are deliberately going for that "BANDITS AT 2 O'CLOCK, I'M ON EM!" audio quality, can the carbon mic and feed some clean line level audio from a good mike and an EQ into that slick little modulator of  yours.  You will be amazed at how good these rigs can sound using low-level grid type modulation.

I can vouch for what Chris says about the ARC-5 having the potential for some impressive sounding audio. I chatted with Chris for a few minutes on 3.710 late last night and the little ARC-5 was very easy copy through the static crashes even with its relatively low power output of about 15 watts. It sounded smooth with an impressive balance of lows and highs and he had plenty of audio. The short MP3 attached to this message will give you an idea of how nice it sounded.

Rob W1AEX

Well I did a rethink this morning I asked myself, "Self. Why would you install a modulator into that Arc 5 when you already built one to plug into it?" Long silence. no answer. So I will leave it as is and perhaps make a 3500kc plug in calibtion crystal from an old tube base. Encourging to hear your success, but to be straight up honest, yes I really do love that "Bandits at 12 oclock sound." However I spent the morning instead on the other modulator. (1920s style) And I have a good mic already to go for the plug in 56 ARRL modulator.
I think my 1625 tube is in so we shall see what tomorrow brings.
Don VE3LYX

Just a note to say. It is done and working fine. Used it to talk to Carrot Creek Wed. Not huge power but that is a reasonable haul just the same. Best report was with my homebrew modulator which consist of little more then a carbon mic, a battery a .1 cap , an old audio transformer and variable pot (bias control) pushing a parallel connected 12sn7 which has plates to the 1625 cathodes and cathodes to ground. I tried it with a 12bl7 and got more output. Will try a 6080 next. Bottom line is it works and is usable. I tested the transmitter on my Dummy load. In CW mode it is quite strong . Loss in AM comes mainly from the voltage drop across the modulating tube which is in series with the 1625s. Still it works reasonably well. Going to experiment with a homebrew screen modulator next.
Anyway just thought I should give a final report.
Don

UPDATE.
Rebuilt the carbon mike modulator using a paralleled 12sl7 driving a Hammond universal deflection transformer. I use my RX PS to power the 12sl7 B+ and filament. The 160V for that feed thu the primary of that transformer to the plate of the 12sl7 while the secondary completes the cathode ground circuit for the 1625 tubes of the ARC5 PA which now run at 700 to 750 volts. Power out is very good rivaling my hot-rodded DX60B. Modulation seems strong from in shack monitoring but will test this morning on air I hope and see what it sounds like to others. I am not looking for broadcast sound but clean audio with 100% readabilty  The more I play with this rig the more appreciation I have for it. Wish I had a couple more. One for 40M and one for 160M. Carbon mike circuit is a bit hard on 9 volt batteries so I may eventually make a AC based small PS for the whole deal with  well filtered 5 or 6.3 volt supply for the carbon mic circuit. BTW I had some PS noise (hummage) on the TX power supply. Traced it down to the series resistors bridging the 450v caps. Was able to reduce it to a minimum yesterday but if I didn't need to bridge the caps (ie 800 volters) it would be almost silent. It is the ECONOMY PS from the 84 Handbook.

Don

Just a short update.


I now have three cathode modulators built and working on the ARC5s Yes ARC5s. I bought another T18 and put it on 160. I can drive my homebrew 811A GG linear with these rigs and it works fine. I have also added absorption loop modulation to both of these transmitters(as well as all my homebrew stuff , VFO and Crystal controlled) learned a lot about ARC5s too. I now run them at 770 volts plate. They are a very good little transmitter. They also have a great VFO. Have one eye open for a parts chassis so I can use the VFO as a shack VFO and maybe put a pair of 811As or 572bs for a neat matching in shack GG linear.
Thanks to all who helped with ideas and suggestions.
And Yes I am still a big fan of "Bandits at 2 oclock" audio however one of the cathode mods uses a modern audio mic and has good quality audio. I feel a bit goofy using it as it sounds like the local FM broadcast station. Not a sound I like. The Absorption loop modulators sound good audio wise since the mic even though it is a carbon mic is not producing the sound but stealing the difference.

The ARC-5 design is simple and prone to FMing, but it is minor, as discussed.

You can always improve pushing by regulating the voltage on the oscillator. I also regulate the screen on the 1625's ( separately with higher voltage).

Neutralization can not be over stressed on simple MOPA transmitters when it comes to pulling. If you do not believe me, temporarily disconnect the black wire going to the neutralization gap cap assembly and simply key the radio and observe the CW note. Now reconnect. Some Command transmitters  are better neutralized than others and you will notice that the capacitance required to handle two 1625's in  parallel is very low!

A couple of years ago I bought a 75M ATA transmitter with the intention of using it on 3870.

One of my problems is I live in a 4th floor 20X20 retirement closet. No outdoor antennas permitted.

I have a real 40M dipole about 6" below the ceiling. Yes it is bent to follow walls. But it is a real 40M dipole.

There is no way to have a 75M dipole unless it has loading coils. That has been an option.

What I had in mind is that the Command Set transmitters were designed to be used with short antennas I would give them a try. Also without modification the Command Set transmitters don't work properly with a 75 ohm dipole.

The ATA transmitters used screen modulation. So did the SCR-274 transmitters. My plan is to make a controlled carrier modulator for it.

But I don't plan on doing anything with this stuff until I can hear the NW AM group(3870) again.

Bob Macklin
K5MYJ
Seattle, Wa 

Well its winter again and ham radio time. I spent the afternoon touching up the ARC5 and its modulator. it appears to be working better then ever. Band was not good tonight so I will have to wait for a good day to try it again. This cathode modulation favoured in the 40s by Frank Jones is very interesting. I have tried both parallel Cathode modulation and series cathode modulation. Both work quite well and require no mods to the rig once it is set up for cathode keying.
don