The AM Forum

I've been picking through the jonque boxe gathering parts for an exciter.  I'm going with the proven "Longfeller" design with 6AG7 and 6V6 which should do nicely for driving the 813.  I noticed there is a .005 capacitor in the output of the oscillator.  I have a .002 and .003 micas I can parallel, but while scrounging I came across several tubular plastic jobs with a stripe on one end and am curious.  I considered using this Cornell Dubilier PM6D47 in place of the micas.  It's .0047 at 600 volts.  Close enough.  Question is which would make the best coupler?  I'm guessing the micas.  But my real question is what is the significance of that stripe?  If it was used in any circuit, especially RF, I'm guessing the striped-end would be output or does it make a difference?

I have an alternate design for the oscillator but it's more complex.  The original (schematic follows) appears to be a proven design.  I have enough parts to build either one.

The stripe or banded end of a capacitor indicates the lead that is connected to the outer layer of foil.

If the capacitor is used for RC coupling in low-level circuits, the banded end should normally be connected to the lower impedance, or source, side of the circuit, that is the plate of the driving stage.  This outer foil then effectively shields the higher impedance grid circuit, thus minimizing hum and noise pickup. 

When using the capacitor as a bypass, the banded end should be connected to the grounded side of the circuit.  This reduces stray coupling to other nearby components.

By golly, that's what I thought...outer layer of foil...just wan't sure especially which lead would go where because technically it doesn't designate polarization.  Thanks!

I wouldn't use the .005 paper cap in the oscillator.  Use one of the micas you have if they are any good.  You don't need 5000pfd as a coupling cap in a RF circuit.  1000pfd is more than enough (.001).

Fred

My original oscillator design called for a .001 while the "Longfeller" shows .005. The advantage of the former is a VFO can be plugged into the XTL socket and no switching is necessary.  I have all the parts necessary to build the exciter using either oscillator.