Normster wrote:Is it this one? I've got one of these on the shelf that I was going to use in my next build. Looks like you need a voltage doubler to get high enough HT voltage.
you don't have to voltage double it----just ignore the ct and use a bridge---it works out the same and (depending on how high the n.l. dc voltage) can save you a cap over the marshall style of doing things. rh
I think Normster is using the term voltage doubler to mean bridge rectifier, which lots of people seem to do.
I don't consider it a voltage doubler at all, just a different way of wiring your rectifier (which happens to make more efficient use of your transformer secondary as well). A voltage doubler to me would be something like a charge pump.
Marshall don't use a cap either; they just use a bridge rectifier. The centre-tap exists only to balance the voltage between the first pair of
filter caps which are placed in series to get a high enough voltage rating. They could have omitted it though, and used balancing resistors across the caps instead.
yes very true all you said, i agree with all---more complete than my post. sometimes i don't like to type so much (finger, wrist, etc pain ) and tend to leave out some of extras like you included. rh
Many thanks to those of you who posted on my topic. I apologize for not being able to engage this past week, but I was out of town on business and had no internet connection.
I finally got some tech info out of the supplier, and it is as y'all suggest: a Marshall-style PT intended to be wired without the CT. I wish they'd sent this info when I first had this problem, not 3 weeks later, after the PT and a replacement were both sent back.
If they send it back to me, I'll see how it goes. I'm not convinced the HT will be quite as high as I want it, but we'll see.
I've just finished a 100 watter using a 370-0 Marshall-style transformer and see 515V on the plates. Scaling that down to the 350-0 you've got you can expect something like 490V, so I think you should be about spot on with this one.
If you're using a totem-pole arrangement of first filter caps then you can always use the centre-tap to balance the voltage across them since it'll be at 1/2 B+. Personally I don't use this method because it means that you require two secondary fuses in order to protect against all modes of failure and use 220K balancing resistors on the filter caps, but it's an option nonetheless.
) and tend to leave out some of extras like you included. rh