Another LM338 related question

[the_eagle]

I am adventuring myself in the design of a 28VDC linear power supply (for the other specs of the power supply I am using as reference MIL-STD-704F).
AC mains (UK) is rectified from a 100VA transformer which gives an unregulated voltage of about 52VDC.

Because 52-28=24V, the continuous output current of the LM338 is limited to only 2A. Therefore I am thinking to shift the unregulated DC output from 0/52VDC to -17/35VDC such that Vout-Vin of the LM338 is less than 10V.

I have drawn a circuit in LTSpice which seems to give 28VDC output voltage, but when I add a load of 10 Ohm, the output voltage drops to 300mV. Why is this happening?

I am more than happy to do a complete change of the initial design

EDIT: confused Peak-to-Peak with Peak voltage. Removed transformer secondary voltage

[Circlotron]

As soon as you apply your load the voltage drop across R3 and R4 causes the input voltage to the regulator to drop to a very low value. Also, an 18VAC transformer should give you about 25.4V, varying somewhat with load.

[the_eagle]

I was doing automatic conversions from rms to peak voltage but I didn't notice the result was peak-to-peak instead of Peak voltage.

Thank you for the explanation. What can I use as an alternative to the resistors?

[Circlotron]

I’d say use a 25vac transformer and no resistors.