I am having trouble wrapping my brain around this power transformer circuit. I must be missing something very fundamental, so I apologize in advance for that.

enter image description here

Can someone explain to me why the center tap on this schematic does not have to be rectified? Also, if the center tap is 0v, how is it supplying power? It appears to be powering all of the tubes except the final 807s. The transformer is a 300v. I'm not sure if that means 150-0-150, or 300-0-300.

I must be missing something simple.

  • 1
    \$\begingroup\$ Your error is in thinking the centre-tap is at zero volts. It is at half voltage. The winding ends alternately are clamped to zero volts. The centre-tap is full-wave half voltage (+) \$\endgroup\$
    – glen_geek
    Dec 18, 2018 at 23:56
  • \$\begingroup\$ The EL84 datasheet I just looked at suggests a max of about \$250\:\text{V}\$. So I'm pretty sure the transformer is a center-tapped \$300\:\text{V}_\text{RMS}\$ (or, as you write, \$150-0-150\$.) The ground symbol on the schematic is the negative rail. The center-tap is the mid-point rail. \$\endgroup\$
    – jonk
    Dec 18, 2018 at 23:56
  • \$\begingroup\$ Excellent. Thank you both for your help. This clarifies a lot. Any idea why the center tap does not need to be rectified? Or is it being rectified in some way that I am not seeing? Thanks again. \$\endgroup\$
    – McMurdo
    Dec 19, 2018 at 0:30
  • \$\begingroup\$ It is being rectified in a way that you are not seeing -- basically, the way that @glen_geek explained. It may help to grab a pencil and paper and draw out the rectifier and transformer, then redraw it with the transformer replaced with a pair of 300V voltage sources in series, minus to plus, then redraw it again with the voltage polarities reversed. That's how I have to do it every time, although with time I've learned to do it in my head. \$\endgroup\$
    – TimWescott
    Dec 19, 2018 at 0:41
  • \$\begingroup\$ Thank Tim Wescott. I drew it out three times like you suggested. Something just isn't clicking in my head. I'm not seeing how that is DC coming out of the center tap without going through a rectifier. Seems like it would be 150VAC. Is the center tap being rectified because the entire secondary (300v) is going through the bridge rectifier? \$\endgroup\$
    – McMurdo
    Dec 19, 2018 at 2:09

1 Answer 1


While one half of AC period, two bridge rectifier diodes are open (see effective chematic below). So, one end of secondary is connected to ground and another - to 300 V output capacitor. At the same time, center tap produces half of full secondary voltage.

On another half of AC period, picture is almost the same, just anoter two diodes of bridge rectifier are opened.

So, center tap produces fully rectidied waveform with half of full secondary amplitude.


simulate this circuit – Schematic created using CircuitLab


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