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Typically 5 watt Zeners are what you want in an 18 watter.
The position of the Zeners effects their efficiency and how many volts will be dropped. If you place a Zener before the first filter cap, you can expect it to drop much less than the rated voltage (usually about 50% in my experience).
The ideal placement for the Zeners is between the 1st and 2nd filter cap. Since the DC voltage is filtered in this position, the Zeners work efficiently and will drop the rated voltage.
I recommend using more than 1 Zener if dropping any significant amount of voltage. For example, I needed to drop 40VDC on my recent Tremolo TMB build. I used two 5W 20V Zener diodes in series. Take a look:
Here's the layout drawing:
As you can see, the Zener diode stripes must always point towards the voltage source. I marked the heat shrink tubing so I know which direction the stripes are going.
Another thing to note is that by connecting it in this position, you can also connect the output transformer after the diodes. This will help ensure that your output tube plate voltages aren't too high.
If you're only dropping 10V, then just use a 5W 10V Zener. 25V or more, and I would use 2 or 3 5W Zeners, or switch to a higher wattage.
Here's some examples of what I use (but use whatever suits your needs safely):