Arc 5 on AM

[VE3AJM]

Al said,

...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.

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

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

[MikeKE0ZUinkcmo]

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.

[VE3LYX]

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

[VE3AJM]

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

[VE3LYX]

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

[MikeKE0ZUinkcmo]

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

Mike

[VE3LYX]

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

[AJ1G]

I decided to leave the carbon mic transformer and battery outside  in the mic base.
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.

[W1AEX]

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

[VE3LYX]

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

[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

[VE3LYX]

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

[VE3LYX]

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.

[WU2D]

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!

[k5myj]

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 

[VE3LYX]

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