MOPA Transmitter Build and Mo’ Success!

The next project has turned out to also be a success.  I had built a MOPA or two (Master Oscillator Power Amp) some years ago (more than 20 years ago) so I didn’t figure building one now to be much of a task.  It turned out pretty well like I planned with only a minor hiccup or two in the process.

I had three main goals in mind before I started:

1.    make it simple
2.    make it using all tubes (even the power supply)
3.    make it unique (as much as possible)

My design ended up thusly:

No power transformer for the filaments of the tubes or the main B+ supply, vacuum tube rectifiers wired as a voltage doubler, I used tubes that I had on hand in the junk box, and voltage regulation was done via a tube as well.

Here is the tube complement that I used:

2 each, 35W4 tubes (normally used as half wave rectifiers but I wired them up as a full wave voltage doubler)

1 each, 12BA6 oscillator tube (I wired it as a triode by tying together the the plate and the screen grid)

1 each, OB2 voltage  regulator tube – 108 volts (for good voltage regulation on the oscillator and the screen grid of the power amplifier)

1 each, 35C5 beam power tube (the power amplifier tube in the MOPA) Note:  this tube is designed as an audio output tube with a power output normally of about 1.5 watts.

The 35W4 (x 2), 12BA6, and the 35C5 tubes’ filaments wired in series came to a total of 117 volts.  All of the tubes in that series are 150 ma filament draw tubes so they worked nicely powered by the normal line voltage.

With the two 35W4s doing the work of rectifying in the doubler, I was able to obtain 265 volts (under load) in the power supply.  I powered the doubler with 120 volts AC.

The rig is crystal controlled, using the 40 and 30 meter band crystals I have in the junk box. The oscillator is a Pierce type (capacitively coupled from the plate to the crystal via a 500 pf capacitor)

Result:  I can use the rig on 30, 40 and 60 meters as wired.  Output power:  two possibilities – 2 watts and 5 watts (2 watts if I use the voltage regulator in the circuit to regulate the oscillator voltage and the power output’s screen voltage, or 5 watts if I run the screen grid of the power amplifier at 140 volts instead of 108 volts , by removing the voltage regulator circuit)

The tank circuit is the simplest (and cheapest) method I could come up with:  a tank circuit tuned with a variable capacitor. The inductor is wound on an old baby aspirin bottle.  The antenna is link coupled via a 3 turn winding located a little below the tank coil on the form – the baby aspirin bottle)

I run the output of the MOPA through an old antenna tuner which reduces out of band harmonics to a respectable level.

At present count I have worked more than 60 stations with the MOPA transmitter on 40 and 30 meters so far.  The signal tone is better than with the last Jones style rig because the crystals that I have are better suited to the very low crystal current levels in the MOPA design in comparison to the higher crystal current in the twin triode Jones style rig.

A schematic of the transmitter is shown below:

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Second Jones Style, Push-Pull Transmitter Success!

A few days ago I finished building my second Jones style push-pull transmitter.  It uses a single tube, dual triode, 6CG7 tube (unlike the two separate pentodes used in the 60FX5 transmitter I have mentioned in earlier blog posts)

This transmitter design departed from the first one in several ways:

1)  it uses a full wave voltage doubler instead of a half wave power supply

2) it has 270 volts on the plates of the triodes

3) it has a filament transformer for the 6 volt filaments in the 6CG7 tube

With a good quality tube and a decently active and appropriate crystal, it puts out about 7 watts.  That’s above QRP levels but it is easily tunable to 5 watts or less (or just use a weaker tube 😉

I have worked approximately a dozen stations with it already, with all but one station giving me good tone reports (the one station that mentioned chirp in the signal was when I was forced to use a wonky crystal to get near enough to the frequency that he was on)

I think with careful planning and fine tuning the output (tank) circuit I could get better efficiency and more power out but I am not trying for perfection (yet) and am happy with the results, especially so when I am able to make contacts with the transmitter and get good reports on it.

In time I will post a schematic of the transmitter on this blog. Maybe even some pictures, etc.

For the next Junk Box transmitter project, it’s going to be a MOPA type transmitter. (MOPA being the abbreviation for Master Oscillator Power Amplifier)  More about it in the next blog.

72 de AK4JA

Update: the 60FX5 Jones style homebrew transmitter does Transpacific Australasia DX!

Update:  12/31/2016

I worked Gary, ZL2IFB, with the little homebrew Jones style rig tonight.  What a thrill to work him in New Zealand, he being over 8,000 miles away from me.

My power level was 2 watts output and the antenna was the 140 feet long doublet up at 35 feet above ground level.

There are very few (if any) thrills to match working a DX station at distances like those with a homebrew rig.

Here’s hoping that the new year will bring more homebrew and DX thrills.

AK4JA

Putting the Jones Style Transmitter to the Test

This weekend I put the little Jones Style transmitter to the test.

The SKCC (Straight Key Century Club) held a Weekend Sprint this weekend (August 13-14th, 2016)  Their theme this month was “Boat Anchors” – an endearing term used by ham radio operators occasionally to describe old, tube based radio equipment.  Extra points were given for working tube type rigs, depending on how many tubes the rig had in it. For more information on the SKCC group, which I highly recommend, go to this URL :

http://www.skccgroup.com

I was able to work 23 stations during the WES (Weekend Sprint), with two of those being DX with the little Jones Style 60FX5 transmitter described in the previous post.  I found that the best tone/keying occurs at less that full power output which had the rig running at about 3 watts.  I ran the output of the transmitter into an old MFJ tuner and fed a 140 foot doublet antenna with 300 ohm Radio Shack twin lead.

I only operated a few hours of the total Sprint time, which is approximately 36 hours in total.  I could have made more contacts but I had family obligations (for one thing) and other promises to keep that prevented me from participating more.  I also only operated in the WES on one band (40 meters) even though the WES was open to all the major HF bands.

Also during the WES I got a chance to work someone I had been wanting to for some time now (AA4GA).  Lee was participating in the WES as well and I made a brief SKCC contact with him, both of us using tube rigs with extra points capability.  I had seen and spoken to Lee earlier in the day when I attended my first North Georgia QRP Club meeting in Atlanta.  The meeting was a great experience and I plan to go back, now that I am an official member.

It was great to put the little Jones Style transmitter to the test this weekend.  I would give it an A+ score.  It never failed me, I never got any bad reports about tone, etc. and I worked enough contacts that it kept things interesting.

Now on to the next tube project.

 

Addendum:  August 15th, 2016

While on an organizing mission to put some order into my tube junk box chaos, I came across a couple of old 50C5 tubes (the audio output tube used in hundreds of thousands of “All American 5” design radios)

Both of the tubes looked worse for wear and I had my doubts about whether either one was actually of good quality (I don’t own/don’t have access to a tube tester)

I checked the pin out of the tubes and they are the same as the 60FX5 tubes, so on a lark I put them in the Jones Style rig, added a dropping resistor in series with the filaments (to drop the additional 20 volts – at 150 milliamperes) and gave it a try.

Lo and behold the two tubes were good enough for virtually the exact same power output into a 50 ohm load as the 60FX5 tubes gave: right at 4 watts for the best tone during keying.  In a push pull circuit like the Jones Style transmitter uses, the two tubes need to be fairly evenly matched to work well and those two were “close enough” (plus adding identical resistors in the cathodes of the two tubes helps to balance the currents in the two tubes) It’s back on the shelf again with the Jones Style 60FX5 transmitter.  Sidetracked mission / diversion accomplished.

That was fun and educational. Now back to my current project.

 

 

 

Hello and Welcome

Welcome to my QRP based blog.  Glad you stopped by.  I will post things related to the QRP items that I build for operation on the HF ham bands (amateur radio bands)

At the present I am building a QRP Jones style transmitter for use on the 80, 60, 40 and 30 meter bands.  It is a very simple transmitter that has many advantages.  I have been working on a schematic and my first draft follows below:

 

I built this transmitter with the idea of making a QRP rig using old tubes from my junk box that I have had for over 30 years. I wanted it to be simple and inexpensive.

To me, it is both.

Jones style transmitters have very few parts compared to some other transmitters and they have excellent second harmonic rejection.   The tube I chose is one that requires very little drive and I can use cheap, “computer” style crystals in it with virtually no problems with the crystals heating excessively.

It is a thrill to make contacts on such a simple and inexpensive rig and to do so reminds me of the joy and satisfaction I felt as a younger ham,  when I first got into ham radio.  To me there is almost nothing in ham radio that I enjoy more than making contacts with other hams,  using equipment that I made for myself from scratch.

I’m so glad to be playing around with tubes again and finally using some tubes that I have had in my junk box for far, far too long. I have plans for many more of them as well.

 

73 de AK4JA