More 1934 Transmitter

After collecting all of the parts comes "playing checkers" to find the best layout given the set of parts located. In this case I found suitable 30's parts for all of the major components except the chassis itself. For it I'm using a 17"x10" sheet of 18 gauge aluminum. It will be fastened to a 4 1/2" tall wooden frame and then frame and aluminum painted black winkle.

A 1934 Style Transmitter

Since the AWA Linc Cundall OT CW Contest in January I've been thinking about building another transmitter, this one falling in between my 1929 TNT transmitter and my Thordarson 100 or late 30's 6L6 transmitters. I want to use it with my National FB-7 so it needs to be an early to mid 30's design.

A popular design in the early 30's used a type 47 as a crystal oscillator driving a buffer and final on 160 through 10 meters. Typical power was anywhere from 20 watts on up depending on the tube lineup. The Gross CW-25 was one example. It had a 47 oscillator driving a 46 buffer/multiplier followed by two 46s in parallel. Plug-in coils were available for all bands 160 - 10. This looks like a pretty neat transmitter. More about the CW-25 can be found in Bill Orr's "Antennas" column in the February 1977 issue of CQ magazine.

I wish I could find a CW-25 available. Does anyone have one they will part with? In the mean time I'll have to homebrew something.

The cover story of the November 1971 issue on CQ magazine is a Bill Orr construction article describing a 160/80/40 meter "1934 Style Transmitter". Circuit-wise it is a close match to the CW-25 but without the buffer/multiplier stage. This circuit design fits my early/mid 30's requirement but the CW-25 chassis sort of construction better matches the "modern" look of my FB-7. I'll combine the two using the circuit from Bill Orr's article but building it to look more like the CW-25.

Notice that the schematic scanned from the original CQ article is missing the connection between the coupling capacitor C6 and the 46 tube grids.

QRP FYBO Contest

Today I participated in the annual QRP FYBO (Freeze Your B___ Off) contest. See http://www.azscqrpions.org . It happened that we are into a little warm spell after several weeks of below freezing weather. The temperature was 40 at 10AM when I got started, rose to 46 around lunchtime and then dropped into the 30s by the time I got off the air at 3:15. I'm not a big contester. With on and off operating and marginal band conditions I made only 5 contacts on 40 and 20 CW for 800 points. Most hams I worked were relatively warm, only VE3RRP in Ontario was below freezing.

The FYBO Contest did give me a chance to get out of the basement shack. One of the neighbors interrupted a snowman build to find out why I was lounging on my deck in February. Even with temperatures hitting the mid 40s it is still winter in Minnesota.

I found the FYBO Contest a challenging and fun break from my regular winter boatanchor activity. I'll do it again next winter.

AWA Linc Cundall OT CW Contest (Part 2)

The second half of the AWA Linc Cundall OT CW Contest was this past weekend. Between Saturday evening and Sunday afternoon I added 4 contacts to my log including W0LGU(MN), WA9QNN(WI), WB9WHG(MN) and NG9D(IL). While not a lot of contacts this year my push-pull 6L6 transmitter seemed to work pretty well. I suspect my low antenna (only 10-15ft off the ground) resulted in a lot of RF going straight up. I did OK for nearby stations but heard/worked no east coast stations.

AWA Linc Cundall OT CW Contest

The first half of the AWA Linc Cundall OT CW Contest ran Wednesday evening through Thursday late afternoon. This coming Saturday is the second half of the "contest".

My station this year is a 1938 vintage NC-101X and a homebrew crystal controlled push-pull 6L6 oscillator. Electrically this transmitter is based on the QSL Push-Pull in QST, June 1940. I added cathode current metering, a load control and regulated the screen voltage to 150V. I ran mine with only 300V on the plates for about 20 watts input and around 10 watts output on 80 and 40. Cosmetically this transmitter is based on what I had on the shelf to work with, an old amplifier build in an even older Meissner Signal Shifter cabinet.

Here in SE Minnesota using my low hung 105' inverted "L" antenna I heard no AWA stations on the air Wednesday night but did work W0NYQ(MN), KB0ROB(MN), VE3AWA(OT) and AA9DH(IL) late Thursday afternoon.

An Iambic keyer rated for cathode keying

When using my vintage transmitters I've missed not having a keyer. Many older tube transmitters are cathode keyed. The voltage across the key terminals can be high enough to fry the output transistor of a typical modern keyer. In some cases the voltage can even be high enough to be a safety problem.

I already had a K1EL K10 keyer board ( http://k1el.tripod.com/K10.html ). I added a Keyall from Jackson Harbor Press ( http://wb9kzy.com/keyallhv.htm ) to make a keyer capable of keying up to 500V. It works great! I even installed a jack for a straight key so that I use the K10 CW monitor plus the Keyall isolator on Straight Key Night.

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 http://www.io.com/~nielw/tnt/tnt.htm ) while the receiver was a National SW3 three tube regen (see: http://www.io.com/~nielw/nat_list/sw3.htm ). See http://www.io.com/~nielw/1929Stn/1929Station.htm 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.

A Simplified CW keying monitor

My homebrew CW station shown at http://www.wiegandfamily.org/2tube_xmtr/2tube_xmtr.htm
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.

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.

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.