Uncle Peter wrote:

It is kind of hard to get the proper coupling on PCB style coils.

Bingo.

Those were intended for larger tube circuit designs, not PCB
based gear.

Daily Double!

Dana

On Sat, 22 Nov 2003 22:02:03 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:

remove ,xnd to reply (Spam precaution!)
The negative resistance fet circuit I mentioned used a pair of jfets,
one P channel and one N channel with the sources tied together and the
gates of each transistor tied to the drain of the other. In this way
each transistor acts as the self bias resistor of the other. The
resulting voltage/current curve of the combo exhibits a region where the
current decreases as the voltage increases, looking much like the 'kink'
in the plate circuit curves of an old time tetrode tube (such as the
24A). The major disadvantage of this circuit for a gdo is that it is
such a strong oscillator that you don't get much of a dip (you need to
sample the output and feed it to a rectifier and meter) unless you load
the coil down with a shunt resistor....and you need a different value
for each range. Also P channel fets with good vhf performance are as
rare as hen's teeth.

Interesting. The circuit I rebuilt my old Tradiper with used two
dual-gate MOSFETs., 3SK88s I believe they are. It gives a nice fragile
ouput which is easily pulled down by an external tuned circuit,
thereby giving rise to a good, deep dip. The only problem is the size
of the blasted snifffer coils! Whilst this is fine for big old valve
type constructions, it *is* totally impractical for today's ever
smaller PCB jobs. I guess the time is coming to abandon it in favour
of some other method of measurement, probably involving a scope and
sig-gen.

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Sat, 22 Nov 2003 22:02:03 -0500, Kenneth
%wa2mze%@%bellsouth%.%net% wrote:

remove ,xnd to reply (Spam precaution!)
The negative resistance fet circuit I mentioned used a pair of jfets,
one P channel and one N channel with the sources tied together and the
gates of each transistor tied to the drain of the other. In this way
each transistor acts as the self bias resistor of the other. The
resulting voltage/current curve of the combo exhibits a region where the
current decreases as the voltage increases, looking much like the 'kink'
in the plate circuit curves of an old time tetrode tube (such as the
24A). The major disadvantage of this circuit for a gdo is that it is
such a strong oscillator that you don't get much of a dip (you need to
sample the output and feed it to a rectifier and meter) unless you load
the coil down with a shunt resistor....and you need a different value
for each range. Also P channel fets with good vhf performance are as
rare as hen's teeth.

Interesting. The circuit I rebuilt my old Tradiper with used two
dual-gate MOSFETs., 3SK88s I believe they are. It gives a nice fragile
ouput which is easily pulled down by an external tuned circuit,
thereby giving rise to a good, deep dip. The only problem is the size
of the blasted snifffer coils! Whilst this is fine for big old valve
type constructions, it *is* totally impractical for today's ever
smaller PCB jobs. I guess the time is coming to abandon it in favour
of some other method of measurement, probably involving a scope and
sig-gen.

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill