The 6K7 DX Receiver
Since the announcement of the first one tube dx contest, I have been wondering what I was going to do for an entry. None of my previous sets had been built with dx as a prime consideration. I had done well with some of my high performance crystal sets, such as my #35 in the last crystal set contest. Then all of a sudden the bright light came on over my head. "Why not build a dx tube radio the same way as a dx crystal set?"
So here it is folks. The pictures show the regenerative detector board. The antenna is connected to a separate board using a dual section variable capacitor and a 660/46 litz wire coil. This circuit is on the left side of the schematic at the bottom of this page. My #35 and 50 set pages show the input network. So I won't discuss that much here.
The regenerative detector centers around the use of a 6K7 tube that I had. It is a very nice looking glass version with the grid cap on the top. You frequent visitors probably remember that I build stuff for the looks too.
The circuit starts with a large litz spider coil. I used 165/46 litz instead of larger litz because regenerative radios tend to be "Q multipliers". I thought that the 660/46 wire would make the radio too sharp and perhaps unstable.
The tuning capacitor is an old timer made by Pilot. The plates are shaped so that the frequencies at the high end aren't so crowded together. This little band spread at the top end of the dial can be helpful. A vernier drive is an essential item here as the tuning is very sharp. I made the dial from white styrene. Use your imagination to make a dial of your own. The frequencies are marked with labels made from a Brother P-Touch labeler. The label edges hardly show up, so the look is good.
The octal tube socket is surface mounted to the breadboard. If you can't find such a socket, you can substitute an 8 pin plug in relay socket made by NTE and others. There are plenty of bypass capacitors used to keep this radio stable. Without a chassis shield, bad oscillations can occur and bypassing helps keep those away.
I used a Bogen T725 transformer as a way to match the plate of the tube to my headphones. I took a guess at which tap to use. I went for the top tap. I have good volume with my sound powered headphones. I wanted to keep the B+ off the headphone wires, so there is a .47 uf, 250 volt capacitor from the red tap on the Bogen to the headphones. Connecting the phones directly to the plate and B+ is ok in a battery set, but I don't like doing that in higher voltage sets.
I did cheat a little and used a 24 volt zener diode across the
regeneration pot. I wanted a quick way to reduce the voltage at the pot
and the regulation does help make the radio easier to adjust.
The tickler coil is on a wooden coil form and is not too critical. Make sure the coil is facing the right direction for regeneration, rather than degeneration.
The power supply is the same one that I used for my
battery charger project. I used
the power supply with my Pentaflex too.
Another bell and whistle is a volume control. I usually use the regen control as a volume control. But with this set, I want to crank up the regeneration control, just below the squeal. At that point there is maximum selectivity.
The 4 silicon diodes next to the volume control is my way of cutting down on noise spikes. When my circulator pump turns on or off, there is a loud crack in the headphones. This helps quiet the set down. The diodes make a quick and dirty limiter. Again, not very 1930's.
A major innovation is the inclusion of the "Hobbydyne" circuit in this radio. Since this circuit was brought to my attention, I have been using it quite a bit. This allows the tank circuit to be as sharp as it can be. There is some sacrifice of volume, but it is well worth it. The 7-45 pf trimmer is set to around 20-30 pf. This was the best for my situation, but other settings might suit you. The 27 mh choke is one that I have used in several recent crystal sets. The Hobbydyne is the best dx circuit of this millenium. Thanks Jim.
This radio operates much as my high performance crystal sets mentioned earlier. This is a two hand tuning system and both tuned circuits must be properly adjusted to receive signals. If you need more selectivity, crank up the regeneration pot to the point before oscillation. Don't let the radio oscillate as you might bother reception of others. This radio can act as a fairly efficient transmitter. You wouldn't went to get the FCC all Riley'd up.
Due to the higher voltages required to run this radio, this project is only recommended for experienced builders and users. If you are unfamiliar with higher voltage circuit construction, I would recommend making a radio with a transistor or field effect transistor. Do not use this radio in the bathtub! :)
Good dx de N2DS