[AMRadio] RE: new home brew receiver progress

[Brett Gazdzinski]

I am making good progress on the new receiver.
I have tried lots of circuits for the local oscillator,
and have come up with a good circuit.
Its very hard to build a good stable oscillator without
complex coils with feedback windings.

I found a circuit in the Bill Orr handbook, out of the C-W man's receiver
that uses one untapped coil, one side goes to ground, a variable cap across
the coil to ground, an RF choke in the cathode, and a pair of caps
in series from the grid to ground, the center (between the caps)
goes to the cathode for feedback.

I tried various coil types, various size ceramic slug tuned forms
with various gauge wire on them, pre made phenolic slug tuned forms
that I unwound some wire from till I got to the correct frequency,
large ceramic coil forms, and B+W coil stock.

The best results are from the B+W coil stock, a much higher
Q coil, gives less harmonics out.

I changed the design some, instead of using the 6U8, I used a 6C4,
since the ARRL handbook says a separate oscillator tube results
in a more stable receiver.
2nd harmonics are down more than 45 DB with the B+W coil, much less with 
some of the others, some generate harmonics out to 60 Mhz
or so!
The oscillator keeps working down to as little as 10 volts on the plate,
and I may run it at 75 volts.
I just tack parts together on the workbench, hooked up to
the old heathkit variable high voltage power supply, 
so the finished product should actually work better, with
short wires and a sensible layout.

To switch between 80 and 40 meters, all I need to do is
short part of the coil to ground.
One end is always connected to ground, and a tap to
ground will give 40 meters, I tested it, and it works well,
no drop in output, no increase in harmonics.
I plan on putting the coil and switch in a small box to isolate
it.

I think the mixer tube will be a 6AH6, and I will try to inject
the osc into the cathode.
If that does not work well, I can inject it into the grid.
The ARRL 1967 handbook has lots of good info about
this sort of mixer stuff on page 99.

I think the IF amps will be 2 or 3 6BZ6 tubes.
The same handbook gives circuits and values for various tubes.

I have a circuit for the low distortion detector, and the S meter
amp, but have to change from octal tubes to 7 or 9 pin
tubes, from a 6SN7 for the S meter circuit, and the detector needs
a diode, and a triode, like the 6SN7.

I may be able to drive the S meter directly with an IF
amp plate current, like the Gonset G76 does.
I found a nice S meter from an old heathkit receiver 
on E bay a while ago. It can be lit up from the back, a nice touch,
and its only an S meter, not out of a transceiver with plate current
and so on..

I have the chassis, the side supports and front panel,
the IF transformers, the digital frequency display, the tuning cap
and various reduction drives, nice ceramic tube sockets, the antenna
tuning coil forms, loads of VR tubes, and all the other tubes.

The Kiwi filters have not arrived yet, and I may need to buy a
power transformer, to give 200 volts out with choke input, 
and filament windings.

Its really getting to be ham radio season, with building and 
operating, and I am having FUN!


Brett
N2DTS