On 04/15/2011 01:55 PM, Edward Knobloch wrote:
On 4/14/2011 8:40 PM, Kenneth Scharf wrote:
I would test any wire would resistor with a grid dip meter to make sure
it ISN'T self resonant on any ham band, otherwise it will fail again.
OR You can prevent the self resonant problem by putting a .01 uf 1000v
ceramic capacitor in parallel with it.

Hi, Ken

I have to disagree with you about the added capacitor you suggest.

The added resistor is intended to act as a fuse,
on the output side of the HV supply. If it opens (by exploding),
the parallel capacitor will have 3 KV across it, assuming
the short condition is still present downstream, so a 1KV rated
capacitor is inadequate.
Plus, we want all of the voltage spike to be across the resistor,
so it blows up "real good". A capacitor across the resistor
would have the effect of slowing the resistor failure time.

There should be little or no rf current in the resistor,
so no need to test it with a grid dipper.
The resistor is isolated from the 3-500Z by the plate choke,
and bypass capacitors shunt the rf to ground in the HV line.

73,
Ed Knobloch
If there IS an rf bypass capacitor between the 'cold' end of the plate
choke and ground then my suggestion isn't needed. If the only bypass at
the cold end of the choke is though the power supply, then one should be
added. In some amplifiers a second rf choke is placed in series with
the main plate choke and a bypass capacitor to ground is placed between
them. Such a bypass capacitor should be one of those 'doorknob' types.