01/26/2007 | AA4LR Although last year, I skipped SKN, I found something to inspire me. In my junkbox was the chassis and remnants of a transmitter. This was one of those projects that never quite worked. It had originally been a 6DQ6 oscillator / PA transmitter, but was later revised into a two-tube 6GK6 / 6550 design (that didn't work), and further into a hybrid transistor VFO / 6GK6 / 6146 design. This barely worked, but was really chirpy and had low output.

Many of the useful parts has been stripped away over the years, but it seemed like a good basis for a project. So, just after Christmas, I spent a few hours each day building it up into a working transmitter.

The result is a crystal-controlled 40m transmitter very similar to designs popular just before I received my Novice (1975). Power supply uses an ancient TV transformer in a full-wave bridge that provides 800v to the PA and 400v for the oscillator. An 0A2 tube regulates the screen voltage for the oscillator.

A 6CL6 is used in a crystal-controlled electron-coupled oscillator, a design very similar to the AA8V One-tube transmitter. The ECO keys very cleanly through the cathode, without any attempt at shaping, with no chirp or drift.

PA is a 6146A, biased for class-C. Since the PA is in cutoff without drive, I didn't bother to try and key it. This means that the key only has the oscillator cutoff voltage across the contacts, which I measured at 9 volts. Very safe. PA bias supply comes from a low-voltage transformer reverse-connected to the filament winding of the primary transformer.

The 6CL6 generates enough drive to give almost 2 mA of grid current in the 6146. Plenty. Cathode current maxes out just shy of 90 mA. WIth 760v on the plate under load, that's nearly 70 watts in, and I measured about 40 watts out. I think the plate circuit has insufficient L, which is why I'm not seeing the 90 watts or so input that is typical of 6146 designs of this type.

With a crystal-controlled transmitter, you can't tune up and down the bands much, so for SKN I ended up calling CQ. I have four useful crystals for 40m CW: 7001.25, 7010, 7075 and 7121.5 kHz. I ended up using 7075 exclusively.

Spent about two hours New Years Eve calling CQ and ragchewing with the callers. This netted six QSOs. After two hours of pumping the straight key, my arm was sure tired. Also operated a little bit on new years day for three more contacts. All signal reports were pretty good, and no one had any bad comments about my keying, so I'd guess the transmitter is pretty clean. Not bad for a rig that was junk box parts a couple of weeks ago.

I've enclosed some pictures of the transmitter. It doesn't look like much.

Key I used is one from my Novice days, still mounted on the same board as it was 32+ years ago. You can even spot a bit of masking tape used to level the board off on the table. This is an inexpensive Japanese-made key from the 1970s. Curiously, I found a new one just like it at a recent hamfest and bought it for $2.00. It's on the table to the left.

Great fun for SKN. I still have a few improvements to make to the transmitter, and add things like a bottom plate and top cover. Perhaps next year I'll build a companion receiver to go with it. (I used my Elecraft K2/100 as the receiver. T/R switching was accomplished with a manual antenna switch)

See you next time. -- AA4LR