I have a whole series of fly-killers, that use a Cockroft-Walton voltage-multiplier system, I have the knowledge, to fix these,
except they use Germanium Diodes.
Please Help,if you can.

I'm not familiar with the brand, but germanium diodes are still obtainable. Do you have a type number?

In an application like this, where the low forward voltage drop isn't critical, silicon diodes could probably be substituted. The standard 1N4007 is rated at 1 kV PIV @ 1A , and might be a good starting point.

Paul,
This is not a Brand of Voltage multiplier,
but a name given to a certain circuit to provide the multiplication.
It uses a series of Diodes and Capacitors, to effect a change in voltage.
Believe me, I've fixed thousands of these units,over a course of 10 years, with working as a Site Electrician in a Freezing
Works, and other shops where food is served.

Oh well, you learn something new every day!

I'm familiar with the common diode/capacitor voltage-multiplier chains, but I've never seen them go by that name before.

As NJ said, in this non-critical application I'm sure you could substitute silicon for germanium diodes. A forward-biased silicon PN junction usually drops about 0.6V instead of the 0.2 to 0.3V of the germanium type.

Just a thought: Could you substitute some of the "stick" rectifiers that are commonly used in the EHT multiplier chain of a TV set? They're readily available at reasonable cost.

Paul,
Just to pick your brains, while we are talking about Germanium Diodes,I recently had a problem with a Stereo Amplifier(old NZ-made), that had Germanium Transistors in the Final Power Output section,(AC188, AC187), would it be possible to replace these with say a, BD139 or BD682,as they have similar bias currents and voltages.
The original devices were damaged when the speaker wires were shorted.
This type of amp has adjustment for the Collector current,although the Emitter resistors are blackened(excessive current?)
These resistors are wire-wound, almost like an inductor, and are air-cored,made of ceramic coated copper wire.
Helllllllppppp!!!!.

It's certainly possible to substitute silicon for germanium transistors in some designs, but not always a simple swap. You may need to change quite a few other components to set the biasing correctly.

Many older power-output stages using Ge transistors used a couple of diodes to set the base bias. These usually need to be changed to account for the 0.6V forward bias of a Si junction.

From the shorted speaker connections burning out the transistors, I'm assuming the amp has a DC-coupled speaker output -- Quite unusual as most amps of the germanium period would be capacitor coupled (or in earlier models, have an output transformer).

You'd probably find it easier to obtain Ge replacements. The AC187 and AC188 were quite rare (at least on British equipment), but their AC127 and AC128 cousins were very common, and still obtainable, although getting scarcer.