On Sun, 29 Nov 2015, OldGuy wrote:

Looking for recommendations for a Grid Dip Meter built or a kit.

You can build simple or you can build complicated. With semiconductors,
the cost of extra transistors and the space they take up doesn't amount to
much, but you can end up with a better device, and one that can be used
for more things.

The biggest problem seems to be false dips, coming from an rf choke
feeding the oscillator that resonates in the wrong place, or just the
layout of the circuit. I suppose it's made worse if the oscillator
doesn't oscillate very well as you move up in frequency.

Calibration was always kind of vague, but nowadays with so many having
frequency counters, one can build something that has a buffer to feed a
frequency counter, so you can either have vague calibration on the dial,
or none at all.

It's probably also worth adding a buffer and output jack so you can use it
as a rudimentary signal generator.

SOme circuits are better than others. Something that uses a two terminal
coil is simpler (Heathkit used phono plugs in at least one of their GDOs)
but I suppose you get more control with more contacts. If nothing else,
you can pre-adjust the level of the oscillator from band to band. Of
course a circuit that uses only two terminals can then be useful in other
ways, put a crystal where the coil goes and you can test that crystal,
connect a shielded coil where the coil goes and with the variable
capacitor at minimum, you get the general frequency of that shielded coil
if you listen for it in a receiver, or have the frequency counter hooked
up. Since coupling to shielded coils can be a problem, this works out
well.

Like regen receivers, for a long time a GDO was a minimal circuit, yet
nowadays you can complicate it. Why not have a nice 3terminal voltage
regulator? Why not use an exotic oscillator circuit that uses more than
one transistor?

For a long time, most GDOs were about the same, the difference being that
anytime someone wrote about their GDO, they'd built it just like someone
else's, except changed the coils a bit or used a tube socket instead of a
phono jack for the coils, or vice versa. Even the Measurements
Corporation or the Millen GDO were fairly simple circuits, though I gather
Millen put a lot of effort into a nice flat output. It was the move to
solid state that shook things up, technically those were no longer GDOs
but everyone knew what was meant. So Heathkit used a tunnel diode in an
early sixties one, not sure if it gave any advantage. In some circuits
there wasn't anything that really dipped, so they'd measure the signal
strenght of the oscillator and see where it dipped. Bipolar transistors
often didn't work so well, at least not if someone was basically
transcribing a tube GDO to a bipolar oscillator. Later FETs and mosfets
came along, more like tube GDOs. IN the seventies, Heathkit and some
others used mosfets, and more peripheral devices, still fairly simple.
The December 1971 issue of QST (I'm pretty sure it was that issue) had an
article by someone from Millen talking about their effort to come out with
a solid state GDO. I think they set out to use the same variable
capacitor and coil set used by their tube one, but I remember one thing
was they had to use ferrite beads and RF chokes in series to get rid of
false dips.

GDOs can still be found at hamfests on the used market. I think someone
is still making new ones, but I'm not sure who. There was period when
people were taking tube GDOs and solid stating them, the magazines had
articles about that.

Michael