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Thursday, December 18, 2008

1929 AWA QSO Party

The last two weekends I participated in the AWA 1929 QSO party. The main requirement is that the transmitter be a 1929 or earlier design. I ended up using my two tube TNT (tuned plate - not tuned grid) transmitter (see ) while the receiver was a National SW3 three tube regen (see: ). See for more information and videos.

I was only on 80 and worked 8 stations, 7 in MN and one, WA3FFC, in PA. MN contacts were W0DGU, K0KCY, W0CWU, WA9WFA, W0NYQ, K0PK AND KB0ROB.

Sunday, April 27, 2008

A Simplified CW keying monitor

My homebrew CW station shown at
includes a control (speaker/power supply/TR switch) box. I found, though, that I still needed some sort of CW monitor. The June 1952 issue of CQ magazine describes a simplified CW keying monitor using only 6 parts. Basically it is a neon bulb type audio oscillator keyed by a second neon bulb energized by transmitter RF. I built the circuit into my control box coupling the monitor output to the grid of the 6F6 audio output stage. I added a 12 Meg resistor from B+ to the junction of the two neon bulbs to help reduce motorboating when the circuit is not energized.

It works fine. Now I can silence the receiver on transmit and have the CW monitor to listen to my own fist.

Saturday, March 15, 2008

Grinding Crystals

As I played with my 6J5/6L6 transmitter I saw that I had some holes in my 80 mtr crystal selection. I had several crystals at 3550 but practically none between 3550 and 3570. Before stable and well calibrated VFOs were plentiful, grinding crystals was a common activity. Military surplus crystals were obtained cheap and ground to the frequency of interest. I decided to grind some of my 3550 KHz crystals up a few KHz to fill in the gaps in my crystal selection.

I used 400 grit wet sandpaper face up on a flat surface as my grinding "station". I removed the quartz crystal plate from the holder and, pressing on two diagonallyopposite corners, ground in a figure 8 pattern. I'd typically grind for 5 to 10 passes and then rotate 90 degrees, repeating four times. I then cleaned (with rubbing alchohol) and dried the crystal, reassembled it and checked the frequency. If I hadn't shifted it far enough I repeated the whole process. To insure that I ground only one side of the crystal I marked a side with a dot of ink.

I found that my Millen grid dip oscillator and a frequency counter made a handy crystal checker. I plugged the crystal in place of the GDO coil and capacitively coupled one pin to my frequency counter. The counter then read the crystal frequency and the grid current indicated crystal activity.

Additional tips:

- Try to limit the frequency shift. I've never gone beyond about 30 KHz.

- Use a large binder clip to temporarily hold the crystal holder together when you test it. This saves screwing and unscrewing three bolts every time you test for frequency.

Monday, February 25, 2008

Further 6J5/6L6 Transmitter Developments

Using my 6J5/6L6 transmitter in the January AWA OT CW Contest reminded me that my original intent for this transmitter was to have some sort of loading control. In theory L2, the output link, can be wound to properly match a 50 ohm load. In reality I never quite got the power I expected when I used a fixed output link. The solution is to switch to a pi network or add a loading capacitor between L2 and ground.

In keeping with the vintage of the design and the typical output networks of the day I added C10, a loading capacitor, between the output link and ground. C10 is actually a 300pf variable with an old television 300pf doorknob capacitor optionally in parallel with it. The photos show a toggle switch next to the output coil that controls whether the extra 300pf is in the circuit of not. It is needed on 80 but not on 40.

Tuneup is now the standard "dip and load " sort of procedure. Plate current is dipped using C8 and then increased by increasing C10 capacitance. This is repeated until adjusting C10 does not increase plate current. The last step should be to dip the plate current using C8. With a 300 volt (under load) power supply mine easily loads up to about 50mA or 15 watts input.

Tuesday, January 29, 2008

AWA OT CW Contest Operating

Last week was the AWA OT CW Contest. I had hoped to be operating using my 815 transmitter but it continues to run off on it's own as a tuned grid-tuned plate oscillator. I switched to my 6J5/6L6 transmitter ( ) and a NC-101X ( ). The pair worked well together and I had four enjoyable QSOs including WA9QNN, W0LGU, WU2D and AA4RM.

Wednesday, January 16, 2008

More on my 6L6/815 Transmitter

Next Wednesday, January 23, is the AWA Linc Cundall Memorial OT CW Contest. It is about time I get my 6L6/815 Transmitter on the air. Two big stumbling blocks had to be worked on: 1) three different high voltage requirements, 815 B+ around 500 VDC, 815 screen around 250 VDC and oscillator B+ around 150-200 VDC and 2) a tendency of the 815 to run off on it's own as a tuned grid-tuned plate oscillator.

The original transmitter design called for voltage dropping networks to get the right screen and oscillator voltages. In the end I added a VR tube to even out the voltage to the 6L6. In the 1941 design the 6L6 plate voltage would drop from 500 to 250VDC at key down. The oscillator would over drive the 815 as the voltage dropped, creating harmonics and spurious RF output. It was bad enough that the SWR would bounce up at the beginning of each dit or dah indicating RF temporarily not on the design frequency. The VR 150 at the left rear of the chassis along with a 10K resistor network underneath now keeps the oscillator B+ at 150 VDC, key up and key down. I found the 815 screen voltage to be more forgiving than the 6L6 plate voltage. There I added the recommended power resistor network to drop the screen voltage to an acceptable level. Neither of these solutions are elegant. Between them they waste 25 watts of power but they do work.

The instability of the 815 amplifier is still a problem. A tin can (actually Wolf brand chili can) shield around the 815 grid coil helps a lot, especially if the final is lightly loaded. I still have to tune up carefully, though, or the output looks pretty trashy.