GDOs are sort of dated by now. i build one last year when i was just
getting back into hamming. i found very little use for it over the
last 3 months that i have actively been homebrewing. let me explain
why ...

a gdo is primarily used to check for resonance of a tuned circuit. if
you knew the inductance and the capacitance, you could easily compute
the resonanating frequency youself.

as another poster mentioned, getting a dip is a fight. so, what i do
use is a combination of three things: an rf probe with a high
impedance voltmeter, a test oscillator and a frequency counter. all
these things are in themselves pretty useful. but i seldom go wrong in
getting properly tuned circuits.

i have a test oscillator (see the schematic
http://farhan.net.co.nr/testosc.gif). i plug in a coil with a 330pf
capcitance in series between the base of the vfo transistor and the
ground. and measure the frequency on the counter. that gives me a
pretty accurate (within 1%) measure of the coil's inductance. it
involves a bit of calculating, but once i have cast the values, there
is seldom need to change them. i tend to do simple maths in my head
using 10MHz as starting value for resonance (100pf with 2.5uH). I can
now scale up or down without resorting to a calculator.

As for peaking a circuit, it is best done by the ear or using an
oscilloscope. peaking by the ear is probably the best, if u can manage
it. now, i dont mean to be rude, but frankly very few people have the
ear to be able to tune for best fidelity rather than loudness. it
takes patience and care (i have very little of either). so, i depend
upon a scope. it is a little like knowing morse. it is the best mode
of communicating, but not all want to use it.

a poor man's alternative is using the RF probe. the RF probe will
never show distortions. But it can show clear peaks while tuning up a
circuit. be sure that you also terminate the output of the tuned stage
properly! otherwise you maybe be tuning away from the sweet point.

i would rather that you invested into building a simple PIC based
counter. It is pretty accurate, you will never need to caliberate it.
(I never got around to caliberating mine, it is off by 1.5KHz at
10MHz). That with the test oscillator, you would be completely
informed about your coils. An RF probe is a 10 minute project and if
you already have a good VOM, you might not need a High impedance
voltmeter. I brewed my own voltmeter to keep things completely
homebrewed.

the counter can always be used with all your projects as a standard
read out. The rf probe will the most useful tool in tuning up any
transmitter. The voltmeter is indispensable.

- farhan