I found the image below in an old book about vacuum tube rectifiers. Could someone explain why the negative terminal of the load is connected to the 6.3V transformer? I understand why it would be connected to the 350V transformer but not the other one too.
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\$\begingroup\$ Think about it this way: Why would you connect the 6.3 V transformer to the common terminal of the 350 V terminal? \$\endgroup\$– The PhotonDec 24, 2016 at 21:48
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\$\begingroup\$ Good question, because connecting it to ground certainly stresses the thin heater-cathode insulation. I suspect the reason is that the 6.3V winding can be shared with the heaters of all the other tubes itn the design, saving a winding, and reducing the cost of the transformer. \$\endgroup\$– user_1818839Dec 24, 2016 at 22:00
2 Answers
The 6.3V supply is for the tube heater. The heater and cathode get surrounded by a cloud of free electrons. By putting the heater at the most negative point in the circuit you ensure that these electrons are not attracted back to the heater.
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3\$\begingroup\$ Great answer from someone who obviously remembers working with valves. The center tapped heater drive here was a way to ensure symmetrical distribution of voltage over the heater element wrt the cathode. \$\endgroup\$ Dec 24, 2016 at 22:58
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\$\begingroup\$ Though if it's hum you are concerned about (eg. other tubes with the same filament supply), it's better to have the heater somewhat (tens of volts) positive wrt the cathode. \$\endgroup\$ Dec 25, 2016 at 11:35
According to the RCA Receiving Tube Manual (30th Edition):
As others have said, this would apply to other tubes in this device, not to the power supply rectifier.
Note the the heater-cathode voltage is rated at -500VDC to +100VDC (DC limitations shown in notes g, h). In this case, it will be running right at the negative voltage rating (-500VDC) since 350V RMS will produce about 500VDC, so the filament needs to be somewhere between the negative rail (call it ground) and +600V.
Also note that this is not two different transformers. As typical for the era, the plate and filament voltages come from two secondary windings on a single transformer. You can tell this by the fact the parallel lines (representing the magnetic laminations) span both secondary windings.