Mike Peace

ERP Systems Architect / Expert Witness

4cx250b HF Amplifier Schematics

The tube requires no more than 2000 volts on the plate so you need a transformer that has an AC secondary of 1350-1400 when using a capacitor type filter. The plate current would be around 500 mA for a pair of tubes when properly run but for AM service I suggest a current rating of 1 amp for the transformer.The screen supply should be around 350 volts so you need a transformer with an AC secondary of around 250 volts.I suggest you do not tap off the plate supply with dropping resistors for the screen supply. It can be done but is not a good idea for AM or SSB service where the plate current is rather dynamic as it will cause fluctions in the screen voltage. I suggest you start looking at places like RFparts or Surpluss Sales of Nebraska unless you want to pay top dollar from Peter Dahl for the iron you need. Do you really need dual voltage primary BTW? And what about the 5 and 10% taps?? Are they really necessary as well? Maybe a bit nice to have if being picky about setting voltages but the amp won’t mind a bit of leeway.Generally big amateur amps and comercial stuff have a series of taps to compensate for local line voltage variations but they are far from necessary for homebrew projects.