TUT5 Swede power supply arrangement

[capacitivewaves]

Hi all

I tried building the Swede power supply to power my next build but I haven't been very lucky


As far as I understand the schematic in fig 18-2, it's a full wave rectifier bridge, one side provides the B+ supply to the amp and the other side provides the negative voltage for the bias supply, that's why 2 diodes are reversed, and the CT is connected to ground. However, in a 4 diode bridge configuration the CT isn't normally used as far as I know (I did blow a few fuses discovering that) So i'm not sure I understand the schematic properly. Maybe someone can enlighten me? 
So far my build is only blowing fuses and would love to understand why

Thanks
Sarah

[K O'Connor]

Hi Guys

Maybe you do not know the difference between "full-wave rectification" and a "full-wave bridge"? TUTs 1&2 explain this.

If you have a transformer winding with one end grounded and a single diode as a rectifier, that diode blocks half the wave and conducts for half the wave. During conduction, the diode is 'on' and has practically zero resistance, so can pass current from the PT to the load. This is obviously called "half-wave rectification".

If we configure four diodes into the standard "bridge" arrangement, the winding can no longer be tied to ground. The AC terminals of the bridge tie to the ends of the winding; the DC terminals of the bridge provide current to the load for both halves of the AC wave, so we have "full-wave rectification" AND we are using a "full bridge".

We can have a winding with a centretap and use a single diode connected to each end with their outputs joined and the CT grounded. For one half of the AC cycle, one diode conducts current to the load. For the other half cycle the second diode conducts current to the load. The load receives current over the full AC cycle, so we have full-wave rectification again BUT we have a "half-bridge rectifier".

In the case of a tube amp that requires a high-voltage positive plate supply and a moderate-voltage negative bias supply, we can add a half-wave rectifier to one end of the CTed winding with its diode reversed. This produces a negative DC voltage output BUT uses only half the AC wave. TUT readers know that this is far from ideal, and we can add another half-wave reversed-diode from the other end of the winding and join the two negative outputs to achieve a bias supply derived from full-wave AC. The bias supply is now more reliable and easier to filter.

We now have a CTed winding with a centretap and a full-bridge rectifier producing plus and minus DC rails. This is the most common type of supply for solid-state amplifiers.

With any of these connections, the DC output is either negative nor positive until we decide to ground one end or the other, as RSG explains.

[capacitivewaves]

Thank you Kevin, that makes more sense to me. Will try wiring the supply again with the new insight and hopefully it will work!