[HBR] Mixer questions and other tech questions..

[Tim Shoppa]

K1JZH writes:

> One thing I'd like to try is using balanced mixers for the first and
> second mixer, and for the PD  using 6JH8 tubes.  I'm wondering how
> these tubes will perform on the upper bands, where Ted used the 2nd
> harmonic of the LO to drive the mixer? I have a feeling that these
> tubes aren't suited for this application, and that may limited my RX
> to 30 meters and below. I'm also wondering which injection scheme
> would work best with these tubes using 2nd harmonic LO injection--LO
> injection to the control grid, or balanced LO injection to the
> defector electrodes?

The best way by far is balanced LO injection to the deflector electrodes.
e.g. the Squires-Sanders articles from the early 60's. There were other
articles showing injection to the control grid but you lose all the advantages
of using a beam deflection tube if you do that.

The gotcha is that for a balanced mixer like this, the 2nd harmonic is a very
very poor way to do it. With balanced mixers you want to inject a square
wave for best performance, or in other words a wave with lots of odd harmonics.
The 2nd harmonic, unless you've done a very good job of removing all traces
of the fundamental and have a really strong 2nd harmonic that drives the
beam deflection all the way is a very very poor way to inject to a balanced mixer.

The ridiculously-weak gimmick coupling of the LO to the mixer in the HBR's is
also not at all the way you want to drive beam deflection mixers. You
want tens of volts of LO on the deflectors. You really have to rethink
the whole LO drive issue from the get-go and not just change the mixer.

Beam deflection will work great at higher frequencies but the most likely
route will involve a plug-in tuned LO output coil that has a balanced winding for driving
the deflectors.

Tim N3QE