Power supply question: Center tap -> capacitor reference

[jmaf]

Hi, can any of the more experienced members please explain technically why this is done?

This is a Marshall Major 200 W example, but I've seen this on other amps as well. The center-tap of the high voltage secondary is not grounded, but is connected at the intersection of two series reservoir caps. I guess this is a pretty basic power supply question, but I've been asked about it more than once and I'm not 100% sure why exactly it's like that.

[Wilder Amplification]

Hi, can any of the more experienced members please explain technically why this is done?

This is a Marshall Major 200 W example, but I've seen this on other amps as well. The center-tap of the high voltage secondary is not grounded, but is connected at the intersection of two series reservoir caps. I guess this is a pretty basic power supply question, but I've been asked about it more than once and I'm not 100% sure why exactly it's like that.

It's what they call a full wave bridge voltage doubler, which isn't really a "doubler" at all unless you look at it from the angle that it's doubling the CT voltage, which running the BR across the full winding does that all on its own.

However, they use it to balance the voltage across the filter caps without having to use bleeder resistors on them. This works because one half of the secondary charges up one cap while the other half charges up the other, while the BR switches which 1/2 of the secondary charges which cap. It's really just a full wave center tap dual polarity power supply with the ground reference relocated to the negative supply output rather than having the CT and the junction point of the two caps referenced to ground.

[jmaf]

It's what they call a full wave bridge voltage doubler, which isn't really a "doubler" at all unless you look at it from the angle that it's doubling the CT voltage, which running the BR across the full winding does that all on its own.

However, they use it to balance the voltage across the filter caps without having to use bleeder resistors on them. This works because one half of the secondary charges up one cap while the other half charges up the other, while the BR switches which 1/2 of the secondary charges which cap. It's really just a full wave center tap dual polarity power supply with the ground reference relocated to the negative supply output rather than having the CT and the junction point of the two caps referenced to ground.

Jon, the explanation was clear as water, especially comparing it to a dual polarity supply. Thanks.

I have a 1970's Brazilian made amp which still uses the bleeder resistors across that capacitor pole, two 100k resistors. Would you say that's a redundant feature?

[Enzo]

No. Bleeder resistors discharge the filter caps when power is removed. It is a desirable safety feature.

[jmaf]

No. Bleeder resistors discharge the filter caps when power is removed. It is a desirable safety feature.

Thanks, Enzo. In that case, will leave them in.