[HBR] A Homebrew design that is rarely mentioned

[Walt Hutchens]

Dave said:

> I came across some more good info on beam  deflection mixers at this link:

http://www.mail-archive.com/amradio@mailman.qth.net/msg03105.html
 
> According  to this article the 7360 won't have much (if any) gain as a mixer
> working into a  9 MHz IF.  He says it approaches 22 dB loss at 9 MHz, but
> works OK at 1  MHz.  He does say it makes a good product detector.

My gut feeling is that there's something wrong with the 22 dB loss number.
Maybe (as Ian says) the deflection voltage was low.
 
> I personally  built a product detector using the 7360 in a BC-348.  At
> first I had lots  of hum from the power transformer which was about 3 inches
> away.  No amount  of shielding had much effect.  I tried tube shields,
> mu-metal, and even a  piece of iron pipe!  I found that rotating the power
> transformer about 90  degrees would null out the hum.  I wonder what these
> stray magnetic fields  would do when the tube is used as an RF mixer.

Same thing as if the deflecting signal -- generally the LO -- was hum
modulated.   There's an electron beam in there, and although the tube is
structured for electrostatic deflection, magnetic deflection will certainly
work.  (The laws of physics are not optional ...)  The tube characteristics
bulletin warns against locating the tube in magnetic fields.   An iron pipe
shield will help -- I've done that for WEAK fields -- but start with plenty
of spacing.   

It's tough to retrofit these tubes: In addition to the magnetic field issue
the supply voltages are odd and since you're dealing with a beam (an
electrostatic lens) it's probably a good idea to stick to the values in the
characteristics info.   One really wants to design the receiver around the
beam tube, right from the start.

You can see where G2DAF was coming from when he set out to design his
receiver to see how much could be done with conventional tubes:  Beam tubes
aren't terribly 'friendly' to the designer.

Ah ... another thought about the 22 dB loss:   If the electrode voltages
were significantly wrong the beam focus could go to pot and that would
certainly reduce the gain.

The next major project is likely to be a beam tube receiver along the W5OMX
lines.   

Walt
KJ4KV