CT plus FWBR?

[xtian]

Check out the power supply from this THD Univalve schematic:

Screen Shot 2017-11-01 at 7.36.06 PM.png

It shows the heater winding with a grounded center tap, AND a full-wave bridge rectifier with negative end ALSO grounded. Isn't that a no-no?

[sluckey]

That has to be a drawing error. It will work like this...

[xtian]

Ooh. So the reservoir cap negative side does not connect to chassis ground?

[xtian]

Boneheaded basic theory question: why cant both CT and FWBR be grounded together? Why can’t they be at the same potential?

[martin manning]

Notice in the original schematic there is a diode connected directly across each half of the filament winding, which will short half of the cycle.

[pdf64]

Most gear using +ve and -ve rails uses a centre tapped winding and a FWB rectifier, with the CT connecting to circuit 0V, and the rectifier outputs forming the raw +ve and -ve supplies.
ie the CT will provide a low impedance dc source half way between the + and - rectifier outputs, see http://schems.com/schematicheaven.net/m ... _112se.pdf
Some tube amps, including this VHT, use the CT as an option for the HT, to provide a half HT (=1/4 power) arrangement.

With the VHT heaters, the schematic must be wrong, either the CT or the rectifer -ve output isn't connected to 0V.

[RJ Guitars]

Been there, done that, I have smelled the incense and seen the smoke signals indicating that center taps do not play well with FWBR power supplies. I did not have any additional diodes to create the 1/4 voltage option so I took half of my secondary straight to ground. ... in this case I can testify that the practical experiment and the theory align. YMWNV (Your Mileage Will Not Vary - in this case)

[xtian]

Thx, guys, but let me redirect your attention to the heater circuit, not the HT circuit.

First, the PT secondary coil with CT: if you connect the CT to chassis ground, the CT is at 0v AC and DC, and the other wires are 3.15vAC each.

Next, the secondaries go to the FWBR recto, output of which (right side of FWBR in schematic) gives you DC relative to the opposite end of the FWBR (left side). Right? Now, why can’t you connect that left side of the bridge to chassis ground? So that the DC is relative to chassis ground?

[Ten Over]

They are talking about the heater circuit (most of them). Re-read what martin said about the diodes shorting half the winding.

If the AC side is referenced to ground, the DC side will also be referenced to ground so that you don't need to ground the DC side.

[sluckey]

Now, why can’t you connect that left side of the bridge to chassis ground? So that the DC is relative to chassis ground?

Martin answered that. Look at the filament winding and the bridge rectifier. Nothing else matters. Look closely at the two diodes in the left side of the bridge. The top diode is directly across the top half of the filament winding. That places 3.15VAC directly across that diode. For one half cycle the diode is reverse biased and nothing happens. But the next half cycle the diode becomes forward biased and becomes a short circuit because there is no series resistance to limit current flow. So the diode will self destruct because it's current limit will be exceeded. Hopefully the diode will fail open but if it should fail shorted, then the top half of the heater winding will begin to smoke.

Now look closely at the bottom diode. It is connected directly across the bottom half of the filament winding. The exact same thing will happen to that diode that happened to the top diode, except it will happen during opposite half cycles. So, after one complete cycle of the AC voltage (16.7mS), both of the left side diodes will be destroyed. Hopefully both diodes will burn open and not take out the heater winding.

Heater windings are pretty robust and can take a direct short for a very short time. But if you make this kind of mistake in the HT winding, it will probably burn open too before the diodes actually burn open.

[xtian]

OK, thanks for the slap upside the head. It's funny how you can miss the obvious.