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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.

half-wave rectifier

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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 Photon
    Dec 24 '16 at 21:48
  • \$\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_1818839
    Dec 24 '16 at 22:00
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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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    \$\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\$
    – Jack Creasey
    Dec 24 '16 at 22:58
  • \$\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\$
    – Spehro Pefhany
    Dec 25 '16 at 11:35
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According to the RCA Receiving Tube Manual (30th Edition):

enter image description here

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.

enter image description here enter image description here

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.

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