Purpose of Center Tap on Full Wave Bridge?

[phydauex]

This is a question related to the UL 135 watt Twin in previous post. The power supply uses a Full Wave Bridge with the center tap connected to the mid point of the two series caps in the first stage. Why? Everything I read says don't connect the CT if your using a FWB. But this is how Fender layed it out on the schematic, I can't seem to find any examples of this on electric theory web sites. So I'll ask the genius's here.

This point is sitting at @247v, does connecting the CT here lift the ground somehow? It seems to me that it could be a source for noise. I don't know, scratching my head on this one.

Matt

[greekie]

It is used to balance the voltage between the two caps. Just like two 220K resistors might.

[Rhodesplyr]

Yes, but this amp already has a pair of 100k divider resistors on the power supply capacitors. I'm not 100% sure of the purpose, but what it does is to put the secondary winding at a DC voltage that's 1/2 the B+ voltage. I'd suspect it has something to do with protecting the bridge diodes and/or the snubber capacitors attached to them.

[52 Bill]

I'd have to look up the schematic for that model, but some Fender amps of that era had a voltage doubler to get the high B+ supply.

[phydauex]

I did some digging last night and found it used by Marshall on their 200 watt Major amps. The marshall's also used 2-FWB in series, which I believe bumps up the voltage even more.

"but what it does is to put the secondary winding at a DC voltage that's 1/2 the B+ voltage. I'd suspect it has something to do with protecting the bridge diodes and/or the snubber capacitors attached to them. "
This I guess would make sense to keep the voltage at the diodes lower relative to the center tap, Like not exceeding the cathode to voltage spec on some preamp tubes, by raising the heater relative to ground.

I don't know enough about electronic theory & formulas, I just figure it out as I go and ask lots of ???. Why? Why?

Thanks for the replies, Matt

[Rhodesplyr]

Duh. I missed that one. Yes, it is a full-wave bridge voltage doubler. I'm more used to seeing voltage doubler rectifier circuits used with non center-tapped transformer secondaries as in the Marantz 8 power amp.

Full Wave Bridge Voltage Doubler - MetroAmp Wiki