This year, I have two different active device receiver entries. One is a direct conversion
receiver using a sheet beam tube, that I built two years ago, and used in the Radioboard
summer DX contest in 2008, but never in this contest. Shortly after I built it, I built a sheet
beam superhet receiver and the direct conversion receiver was mostly forgotten. I wanted
to dust it off, make a few improvements and enter it in another contest. It took a while to
get to it. So, I began the contest using the same superhet that I entered last year. That
receiver is completely unchanged, and you can find a description of it under the 2009
entries, or see the writeup on my webpage.

The direct conversion receiver has a tuned front end to help reduce interference from
strong local stations and out of band signals. The signal enters the control grid and is
mixed with the oscillator signal which appears on the deflectors. The result is an audio
signal which appears as a differential signal on the plates. This audio signal drives the
push pull primary of the interstage audio transformer. The secondary is connected back
to the control grid in series with the incoming RF so that it is amplified reflex style. The
resulting amplified audio is now a common mode (in-phase signal) at the plates, and so
has no effect on the interstage transformer, and passes through the primary winding
and on to the output transformer which is in series. The secondary of the output trans-
former drives the headphones.

Feedback for the local oscillator is taken differentially from the plates, through an audio
blocking capacitor to the primary of the oscillator coil. One half of the secondary of the
oscillator coil forms a tuned circuit with one gang of the variable capacitor. This side of
the secondary drives one deflector, and the other half of the secondary produces an
opposite phase signal to drive the other deflector. This oscillator is very stable and is
almost completely immune to pulling towards the incoming RF. In fact this is a bit of a
disadvantage, because it makes it very touchy tuning in AM signals without getting a
beat frequency.

In the first version of this receiver, I had included feedback from the cathode to a one
turn tickler on the RF coil, to provide a small amount of regeneration. However, after
making a number of modifications and tests, I finally concluded that there was little to
no benefit. So, I finally removed it. A description of the receiver is here.

There is a DPDT switch which reverses the phase of the interstage transformer
secondary. This is to correct for slight unbalances in the tube which could cause
oscillation when the volume control is turned all the way up. There is a bit of phase
shift that varies across the band, and so the best position of the switch changes at
different frequencies. I had tried various other plate balancing techniques (such as in
my superhet receiver) but found that nothing worked as well as the reversing switch.

The receiver worked reasonably well. It was built as an experiment to see what could
be done with a sheet beam tube. If you read the write up on my website, I sounded
much more enthusiastic right after I built it. However, nearly two years later, and having
built a better receiver in the interim, I find that listening to AM for any length of time on
this set can be tiresome with the touchy tuning and almost constant beat tone. So, after
using it for a couple of days and logging a respectable number of stations, I retired it
and switched back to the superhet which outperforms it in almost every respect.

It seems that each year, I manage to learn a new trick or two. This year, I connected a
set of wavetraps between my loop antenna pickup coil and the receiver. I wasn't sure
how wall wavetraps would work with the loop. But, using the antenna orientation to
null out a local station on 860kHz, and then using two of the wavetraps tuned to the
same frequency to further null it out, I was able to log my first station ever on 870 kHz:
WWL New Orleans, which by the distance/power formula was my second best catch of
the contest. My best catch was KVNS 880, Brownsville TX. Attempts to log a MW station
outside of North America have failed so far. Hopefully, one of these days it will happen.

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