Metal detector circuit to grid-dip oscillator?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jolyon Vater Cox by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>

Having seen a schematic for a super-simple metal detector  based on a 
Colpitts oscillator and diode-transistor detector in the "Ingenuity 
Unlimited" section of the  UK magazine "Everyday & Practical Electronics" 
March 2003,
I have been wondering if anyone on the List has read the article or knows 
whether or not  the circuit could be converted to a grid dip oscillator for 
TC testing
-by changing the fixed capacitors in the Colpitts tank for a dual-ganged 
variable capacitor so as to allow the oscillator frequency to be made 
adjustable.

Incidentally, there is no meter as such in the published circuit -only an 
LED which goes out when metal is detected (by a fall in voltage at the 
detector,caused by a decrease in the Q of the oscillator coil)
But then, is a meter really be necessary in a GDO
(despite the name!) when really all would is needed is an indication of 
resonance at a particular frequency-
and if the "depth" of the dip were an issue
couldn't that be handled by tailoring the sensitivity of the detector to 
the need of the user?

Also, with an inverting transistor on the output
would it not also possible to have an LED which turned on when the dip was 
detected or a combination of two LEDs, possibly in different colours, to 
indicate the untuned and the tuned conditions respectively?

Jolyon