WAD Pre II conversion

[Cyprus Martin]

In the FAQ Upgrades & Tweaks section regarding the above it recommends replacing ZD1 with a 200V version and bypassing it with a .1uF 400v cap. What sort of capacitor is it best to use and does it replace C3 or supplement it?

[John Caswell]

Hi Martin,
This change ONLY applies if you convert the PreII to a cathode follower not if you keep it as the transformer coupled version.
If converted then the Zener needs to be a 200V/5 Watt version or a combination of them in series to = 200V eg 4 x 50 V and the 0.1µF is in parallel with C3 to improve its RF decoupling performance any good quality polypropylene is fine.
Don't get paranoid about the volts 190V or 210V will be just as good and you will need to allow about 10mA through the zener to keep it comfortably "turned on" ie conducting so R4 - 22K will probably need to be reduced to about 10K.

Regards

John aka Dr John

PS Just looking at the FAQ see that NealG recommends R4 to be 18K fine! John

[Greg.]

Hi Martin,

For the 0.1uF cap I used an Arcotronics polypropelene RS114-604 and for the 18K resistor, Welwyn 6W RS199-5505. After the changes, my voltages were spot on.

Best wishes,

Greg