Insider Case ECG cable Faces U

A Federal prosecutor said yesterday that he is conducting a preliminary investigation of an insider trading case involving Thomas C. Reed, who is now a national security consultant on the White House staff.

John S. Martin, the United States Attorney in Manhattan, said in response to a reporter's inquiry that the insider trading case against Mr. Reed had been brought to the attention of his office. He said it was now in ''a preliminary stage of investigation'' to determine if any further action should be taken.

The Securities and Exchange Commission had filed a civil complaint that was ECG cable terminated when Mr. Reed, while denying any wrongdoing, agreed to set up an escrow account of more than $400,000 to pay off any judgments against him resulting from private litigation. An investigation by the United States Attorney's office can result in criminal charges.

Mr. Reed is the son of Gordon W. Reed of New York, a member of the board of directors at AMAX. complaint charged that, at the Feb. 5, 1981, meeting of the AMAX board, AMAX's chairman discussed conversations he had had with Socal, which Gordon Reed subsequently mentioned to his son. Thomas Reed was accused of placing an order for 500 AMAX call options on March 4, one day before an AMAX board meeting was scheduled. On March 5, the AMAX board announced the Socal offer. Although the AMAX board rejected the offer, its stock continued to rise when trading resumed March 6.