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I'm currently disassembling an old (1960's) tube radio and trying to reproduce parts of the circuit. When i come to the power supply part, i found this:

PSU

I wonder about some things:

  • Is it a 230V/460V transformer or a 230V/230V transformer?
  • What is the purpose of this "center tap"?
  • Is there an alternative to achieve the exact same thing with "modern parts"?
  • Isn't this exact the same thing like using P1 and P2 as P3 and P4? If no, why?
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  • \$\begingroup\$ Even some modern parts still use center tap transformer. \$\endgroup\$
    – Marko Buršič
    Mar 25, 2019 at 13:40
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    \$\begingroup\$ A vital safety reason for the transformer is isolation as mentioned in the answer below. Do not remove the transformer unless you know why the isolation is no longer needed. \$\endgroup\$
    – KalleMP
    Mar 25, 2019 at 17:29

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Is it a 230V/460V transformer or a 230V/230V transformer?

It's for the radio's HV DC supply.

Full-wave rectifiers were not easily created using valve technology. The centre-tapped transformer allows for easy full-wave rectification using two valves (tubes) or two rectifiers combined into one tube.

enter image description here

Figure 1. A full-wave tube rectifier. Each anode (on the left) will conduct when its supply goes positive. Image source: Electronics Notes.

See the linked article for much more on the topic.

What is the purpose of this "center tap"?

That should be clear by now.

Is there an alternative to achieve the exact same thing with "modern parts"?

Yes. A single secondary and a semiconductor bridge rectifier.

Isn't this exact the same thing like using P1 and P2 as P3 and P4? If no, why?

No. One of the mains wires will be neutral and so will have zero volts with respect to earth. Doing as you suggest would result in half-wave rectification.

In addition the transformer provides isolation between the mains and the radio. This may offer some protection in the event of a high voltage fault or problem with neutral.

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    \$\begingroup\$ Can you really call 230V HVDC? \$\endgroup\$
    – user16307
    Mar 25, 2019 at 14:05
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    \$\begingroup\$ @user16307 - while that may be accepted terminology in the utility power transmission business, it's a more-or-less arbitrary number made up by someone in that business. In the electronics world, for all practical purposes 230Vdc is high voltage. \$\endgroup\$
    – brhans
    Mar 25, 2019 at 14:16
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    \$\begingroup\$ @user16307: "Can you really call 230V HVDC?" Only after you've touched it. Note that 230 V is the RMS value. The DC value will be about \$ 230 \sqrt 2 \ \text V \$. \$\endgroup\$
    – Transistor
    Mar 25, 2019 at 14:26
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    \$\begingroup\$ @Transistor I know that and still it doesnt make your terminology correct:) HVDC starts at 100kV \$\endgroup\$
    – user16307
    Mar 25, 2019 at 14:27
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    \$\begingroup\$ It is all context related. In typical electronics circuits 300 V would be considered high voltage. In power transmission and other contexts it might not. What do you recommend that I call it? Note that in the context of this question there is a heater supply which is typically 6.3 V AC. See also Wikipedia's High tension (vacuum tube) which supports my use of the term. And we all know Wikipedia is never wrong! ;^) \$\endgroup\$
    – Transistor
    Mar 25, 2019 at 14:47

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