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.