Power Supply Question

[dorrisant]

Can someone explain why the PT center tap is connected to the reservoir where it is... Also, why is there a note to not install the balancing resistors?

Line 6 Spider PS.png

[sluckey]

It's a FWB rectifier. Connecting the CT to the junction of the series connected caps forces the voltage to divide equally across the caps. This is also why you don't need the two balancing resistors.

[dorrisant]

I figured it was simple... just didn't recall ever seeing that before. Thanks slukey!!

[roberto]

The Major was like that too:
http://drtube.com/schematics/marshall/200w.gif

Other Marshalls too:
http://drtube.com/schematics/marshall/100w-67.gif

[R.G.]

This is also why you don't need the two balancing resistors.

Yep. Works well that way.

However, it's still smart to put in resistance to run the caps down in some reasonable time for safety reasons.

[tictac]

Never noticed this before! Most commonly a power supply with a CT Secondary uses a FW rectifier with CT to ground and non-CT secondaries with a FWB.....

So what advantage would there be in using a FWB with a CT secondary in this configuration as opposed to using a FW rectifier and grounding the CT?

A higher B+ voltage? Better filtering?

TT

[sluckey]

When using a Full Wave Bridge, the secondary only needs to produce half as much AC voltage to produce the same B+ as a conventional full wave rectifier. Having a center tap connected as shown above forces the B+ to divide equally across the series connected reservoir caps. For example, let's say the B+ produced by the FWB is 600V. You could use that 600v for the plates and operate the screens at 300v. And you don't have to use any big dropping resistors or fancy circuit to drop the screens.

[tictac]

Interesting..... thanks for that clear explanation!

TT

[dorrisant]

Thanks again sluckey... Good to know!