I am building a crystal tester and the circuit calls for two 220pf
caps in the oscillator. I didnt have any of these caps in the junque
box so I thought about making a couple gimmick caps. I understand this
is done by twisting some insulated wires together. Is there some guide
to use that will give you an approximate value? In other words, if I
have twisted some #26 enameled wire together does 1" equal some pf
value? Any info you may have on this will be appreciated.
w5kcm

w5kcm wrote:
I am building a crystal tester and the circuit calls for two 220pf
caps in the oscillator. I didnt have any of these caps in the junque
box so I thought about making a couple gimmick caps. I understand this
is done by twisting some insulated wires together. Is there some guide
to use that will give you an approximate value? In other words, if I
have twisted some #26 enameled wire together does 1" equal some pf
value? Any info you may have on this will be appreciated.
w5kcm
"Gimmick" capacitors are only good for a few pf, and are
used to do light coupling or introduce
feedback/self-oscillation. even if you could build one, it
would be pretty unstable for test use. 220 pf is
substantial (an air capacitor would have many plates). In
the day, we'd use silver-mica type capacitors for stability.
If other friends' junque boxes are available, I'd try
those. Thank goodness for modern meters--at my age, reading
the color codes is hard!
--W9NPI

"Gimmick" capacitors are only good for a few pf, and are
used to do light coupling or introduce
feedback/self-oscillation. even if you could build one, it
would be pretty unstable for test use. *220 pf is
substantial (an air capacitor would have many plates). *In
the day, we'd use silver-mica type capacitors for stability.
* If other friends' junque boxes are available, I'd try
those. *Thank goodness for modern meters--at my age, reading
the color codes is hard!
--W9NPI

Yes, the more I have lookad at it 220 pf would be too large for a
gimmick. I will keep digging in the junque box, maybe find a few other
values to series or parallel to get close to 220 pf.
thanks, w5kcm

afcsman wrote:
w5kcm wrote:
I am building a crystal tester and the circuit calls for two 220pf
caps in the oscillator. I didnt have any of these caps in the junque
box so I thought about making a couple gimmick caps. I understand this
is done by twisting some insulated wires together. Is there some guide
to use that will give you an approximate value? In other words, if I
have twisted some #26 enameled wire together does 1" equal some pf
value? Any info you may have on this will be appreciated.
w5kcm
"Gimmick" capacitors are only good for a few pf, and are used to do
light coupling or introduce feedback/self-oscillation. even if you could
build one, it would be pretty unstable for test use. 220 pf is
substantial (an air capacitor would have many plates). In the day, we'd
use silver-mica type capacitors for stability. If other friends' junque
boxes are available, I'd try those. Thank goodness for modern
meters--at my age, reading the color codes is hard!
--W9NPI
http://web.telia.com/~u85920178/begin/gimmik-0.htm

w5kcm ) writes:
I am building a crystal tester and the circuit calls for two 220pf
caps in the oscillator. I didnt have any of these caps in the junque
box so I thought about making a couple gimmick caps. I understand this
is done by twisting some insulated wires together. Is there some guide
to use that will give you an approximate value? In other words, if I
have twisted some #26 enameled wire together does 1" equal some pf
value? Any info you may have on this will be appreciated.
w5kcm

"Gimmick capacitors" are when you need only the tiniest amount of
coupling. They are just a step up from placing a wire near another one
(such as when there is enough stray coupling from a bfo to a diode
detector for a sufficient beat). They are likely sub-pF, and if
they are any higher then a couple of picofarad. In other words, the
value needed is so low that you can get by with a pair of twisted wires.
If you need something substantial, then you use a real capacitor.

Another advantage of a "gimmick capacitor" is that it's somewhat variable,
sicne you can adjust the twist and even trim the wires to get a different
capacitance.

Michael VE2BVW

In article ,
w5kcm wrote:

"Gimmick" capacitors are only good for a few pf, and are
used to do light coupling or introduce
feedback/self-oscillation. even if you could build one, it
would be pretty unstable for test use. =A0220 pf is
substantial (an air capacitor would have many plates). =A0In
the day, we'd use silver-mica type capacitors for stability.
=A0 If other friends' junque boxes are available, I'd try
those. =A0Thank goodness for modern meters--at my age, reading
the color codes is hard!

Yes, the more I have lookad at it 220 pf would be too large for a
gimmick. I will keep digging in the junque box, maybe find a few other
values to series or parallel to get close to 220 pf.

Gimmick caps in that range are probably made from sheet metal cut out of
tin cans. I find Altoids tins work well.

Use a cheap DMM with a capacitance function to measure them... then cut
them down until the value is correct.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

................................................. ........................................if I
have twisted some #26 enameled wire together does 1" equal some pf
value?


I didn't know Jack Benny was still alive!..

On Feb 14, 10:39*am, w5kcm wrote:
"Gimmick" capacitors are only good for a few pf, and are
used to do light coupling or introduce
feedback/self-oscillation. even if you could build one, it
would be pretty unstable for test use. *220 pf is
substantial (an air capacitor would have many plates). *In
the day, we'd use silver-mica type capacitors for stability.
* If other friends' junque boxes are available, I'd try
those. *Thank goodness for modern meters--at my age, reading
the color codes is hard!
--W9NPI

Yes, the more I have lookad at it 220 pf would be too large for a
gimmick. I will keep digging in the junque box, maybe find a few other
values to series or parallel to get close to 220 pf.
thanks, w5kcm

OK, last night I found another stash with some capacitors and found a
couple small 200 pf silver micas in there. That should work. Thanks
for all the input. Not sure about the Jack Benny, that one sailed
right over.
73, Randy