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Saturday, February 29, 2020

A Dingey size Boat Anchor

I first started on this project over 15 years ago. I wanted a 6L6 based transmitter with a spotting function. Over the years it has gone through several iterations to the "Dingey Size Boat Anchor" I use today. Along the way I added a loading control, regulated the oscillator plate voltage to 108 volts, shifted the output circuit to a pi network and repackaged it in a metal cabinet to better match my HRO Sr. Like many projects it has been a journey. Here are a few more details about this transmitter as it stands today.

The circuit itself is fairly standard using a separate 300 B+ power supply. About the only thing I don't see in most designs is SW, the spotting switch. It allows me to check my crystal frequency against any station I might want to work or against QRM. As an important serendipity it also allows me to run the oscillator continuously while keying only the 6L6 output stage. This cuts down on chirp caused by the crystal restarting at the beginning of each code element. C10, the loading capacitor, is really two capacitors in parallel. One is a typical 300pf variable. The second is a 300pf high voltage "door knob" capacitor that can be switched it/out with a toggle switch.
Coils are what I could find. The 80 and 40 meter coils are commercial Bud coils that I had. I'm using only the tank portion, not the link. For 30 I found an old plug-in coil that looked about right and tried it. It works. In general look for coils that  dip and load to around 60% efficiency (ie. 6 to 7 watts output for 10 to 12 watts input).

Inside I arranged everything so that it made sense.  Across the back, left to right, is the 6J5 oscillator, the 6L6 final and then the plug-in output coil. Just to the right of the coil is the toggle switch that switches in/out the additional 300pf  loading capacity. The crystal socket is near the 6J5 and the 0C3 108 V voltage regulator is just to the right of that.

When changing crystals or, especially, coils it is important to recognize where high voltage is exposed. In this case the unprotected meter  terminals have 300 VDC, a lethal voltage, on them. Power this transmitter down when under the covers. This is even more important when working under the chassis. Don't take chances. Power down before working on it. As a safety modification consider placing the crystal socket on the front panel.

Are you looking for something more modern looking? Try  W1TS's 6C4-5763 MOPA in the October 1968 issue QST. Just add a resistor and 0B2 to regulate the oscillator B+ to 108V and include a switch to run the oscillator stage while keying the final.

Thursday, February 27, 2020

A Dingey size Boat Anchor on 30 mtrs

After putting my Drake 2B on 30 mtrs I immediately started looking for a matching transmitter.  The trouble is, the WARC bands like 30 mtrs weren't created until about 1980, long after the novice requirement for crystal control had gone away. By then solid state VFO controlled transceivers were becoming the typical rig. I'd need to homebrew a transmitter if I wanted vacuum tubes and crystal control on 30 mtrs.

A search of the QST archives brought up three articles that helped me decide what to do. The first was WD8DQT's article in January 2012 showing how to use today's HC49 crystals in simple MOPA (Master Oscillator - Power Amplifier) transmitters. The other two, W1TS's in October 1968 and WD8DAS's in January 2003 each show 6C4 to 5763 MOPA designs that can be modified per WD8DQT's article to use HC49 crystals.

What next?

I already have a crystal controlled MOPA transmitter on my desk. See https://w0vlz.blogspot.com/2008/02/further-6j56l6-transmitter-developments_5846.html . I did a quick check and found that the 6J5 I used is a close match to the 6C4 WD8DQT based his article on. I could start with the 6J5-6L6 transmitter I already had.  First I added a VR-105 and 12K 5 watt dropping resistor to power the 6J5 oscillator at 105 VDC. Next I converted the PA to a pi network instead of link coupled output. The tank capacitor is now 140pF and  the output/antenna capacitor is now 300pf plus, optionally, an additional 300pF. The coils on 80 and 40 are the same B&W output coils I've been using but without the output link connected. For 30 mtrs I searched through my junque box and found a coil that loads up fine on 30. Finally, I found HC-49 crystals, including 30 mtrs, five for $7.50 on ebay. See https://www.ebay.com/sch/possumlodge/m.html?_nkw=&_armrs=1&_ipg=&_from=

My "new" 6J5-6L6 transmitter uses FT-243 and HC- 49 fundamental frequency crystals. It runs almost 12 watts input and about 6 watts out on 80, 40 and 30 ... outstanding! 

See ya on 30.

Sunday, February 23, 2020

Putting the Drake 2B on 30 and 17 meters

Recently I noticed the optional crystal frequency chart in my Drake 2B manual. It shows that a 14.0 MHz crystal allows you to listen to WWV on 10 MHz and 30 mtrs on 10.1-10.150. Nice, but wait, buy one get a second one free! The chart in the manual also shows that by retuning the preselector that same 14.0 MHz crystal also covers 17.5-18.1 MHz. This includes the lower half of the 17 mtr band. It looked like shifting the crystal up 100 KHz would shift coverage to include the entire 17 mtr band and still keep WWV and 30 mtrs on the dial. Outstanding!

A quick check of AF4K's website at http://www.af4k.com shows that he stocks 14.1 MHz crystals in both the HC49 and HC6 style holders. Being cheap I bought the HC49 version for $12 instead of the HC6 plug and play option for $24. I mounted my new crystal on a hacked off HC6 base and plugged it in. It works fine.

Range C on my 2B now covers WWV, 30 mtrs and 17 mtrs.






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