This year’s Field Day transmitter employed a 59-59 (ECO/Buffer) exciter and Push-Pull 804 amplifier. While it worked relatively well for the circuit in use, I wanted to get better performance out of the exciter. Tuning was very critical at each stage due to insufficient isolation (incomplete screening in the 59s), causing chirp and in some cases a raspy note when optimal settings were not used. A particular “good” setting of the tank tuning at each stage was also only good for 10-15 kc before re-tuning was needed to clean up the signal again.
After experiencing some undesirable receiver behaviors during a station demo at the Breezeshooters Hamfest, I decided to dig into the homebrew 1934 superhet to try and resolve the issues as well as try some improvements. The receiver was knocked far enough out of alignment during the trip to the hamfest to cause one of the IF tubes to break into oscillation when the gain control was advanced to a certain point. That was easy enough to fix with a complete re-alignment.
Another annoying issue was an intermittent “raspiness” that would develop on CW notes. Perhaps a bad solder joint, dirty tube socket wipers, etc. Ultimately, this particular problem was resolved while troubleshooting another issue that I hadn’t noticed before. When I powered up the receiver on the bench after returning home, I noticed a very high pitched note (perhaps 10-12 kHz) in the audio. In trying to identify the source, I touched the grid cap of each tube. The signal stopped when I got to the BFO tube. After swapping in a different 57, the high pitched note was gone. Subsequently, so was any further intermittent raspiness.
There were a number of improvements I decided to implement while I had the rig on the bench. Continue reading 1934 Superhet Improvements
The high voltage (approx. 1150 VDC) B+ power supply has taken on various configurations over the last couple of years:
But in both of the above renditions, a full wave rectifier with modern diodes was used. To bring the power supply more in line with period practices, it was completely re-built and the modern diodes replaced with a pair of 866A Mercury Vapor tube rectifiers. The output filter consists of a 20-4 H swinging choke, 5 uF oil cap (two 10uF / 1kV in series), a 8.5H choke, and 6 uF / 1.5 kV oil cap. A 25K ohm / 100W bleeder is used and a 50mA ammeter connected to the cold end of the bleeder and ground. Under normal conditions with no load, the current is 46mA, translating into about 1150 VDC.