2203 reservoir cap and PT CT

[xtian]

Can someone explain what's going on with the reservoir cap in the 2203? There are two 100u caps in series, with the PT's center tap connected to the junction of the two caps. Won't that raise the circuit ground to 200+ Vdc?

[martin manning]

The CT is always at half of B+, but that's where you want the center of the caps to be. Notice there are no balance resistors.

[xtian]

The FWB is referenced to chassis ground, and the CT is referenced to 200v. How can the HT coil oscillate around both 0Vdc AND 200Vdc?

[romberg]

The FWB is referenced to chassis ground, and the CT is referenced to 200v. How can the HT coil oscillate around both 0Vdc AND 200Vdc?

If it is this circuit:

http://en.wikipedia.org/wiki/File:Full- ... oubler.svg

Then the center tap just balances the voltage across each cap. There is no ground reference until one is made at the bottom of the stack of caps.

I think (and could be wrong) you could place the ground at the center tap and have a power supply that has +1/2 and -1/2 the full PT voltage. Or ground it at the negative end for full B+ and finally ground it at the top for full negative voltage.

Mike

[rp]

What's the advantage of a voltage doubler? i.e. why do some designers choose it assuming transformer availability isn't an issue.

[cbass]

A FWB isnt a voltage doubler .I think I read you can get the same current from a smaller trannie if its wound properly

[schaublin65]

Hi

the Marshall voltage doubler puts the caps under a voltage a lot lower than their rating. The B+ would leave little or no margin on a 500 volt cap if the doubler wasn't used.

take care..

John

[romberg]

Yea. Even though you see this circuit described as a voltage doubler I think it is only so in that it is double the voltage of the center tap to either end. I'd guess the only advantage of using this (for a big company that could spec any transformer they wanted) is you'd save the cost of resistors for a voltage divider across the caps. Every penny counts!

Mike

[schaublin65]

Hi,

this voltage doubler was used on Superleads with dual 50uF stacked.

There is some discussion on the metroamp forum about that amp needing a certain amount of voltage sag to avoid redplating. I don't know if this is relevant to a 2203 but a doubler may sag more under load than a straight FWB.

Take care..

John

[martin manning]

This Marshall thing is not a voltage doubler. It's a FWB with a CT added to balance the stacked reservoir. The DC voltage is approximately equal to the scondary's peak AC voltage. A voltage doubler only has two diodes and produces DC voltage approximately equal to two times the secondary's peak AC voltage.

[schaublin65]

Hi

it seems that calling it a voltage doubler is not correct

Full wave doublers are possible but this isn't one.

The reason for this circuit is the voltage rating of the filter caps.

take care..

John