The top of the federal pay pyramid once the preserve of a few hundred very senior civil servants, will get 10,000 new tenants next month when most of Uncle Sam's million-plus white collar workers get their 4 percent pay raise.

But for the 18,000 people already at the magic $57,500 level, the raise most likely will be something they only hear about.

The House Appropriations Committee voted last week to prevent senior federal officials from getting catchup raises of from $6,000 to $8,000 due them next month, and from getting the 4 percent that most other U.S. workers will receive. The committee's action almost certainly ensures that the executives will not get a pay raise, because it will force Congress to do what it never wanted to do -- vote on the issue.

The news is bad, but not unexpected, for senior federal officials. They have grown accustomed to freezes imposed because members of Congress (who get just over $60,000 a year) don't like the idea of unelected colleagues downtown making as much as they do and because members of Congress are loath to take pay raises themselves in an election year.

According to the government's own pay data, top civil servants should be earning more than $78,000 a year to keep pace with pay for people of similar (sometimes lesser) responsibility in the private sector.

Congress hoped to give itself a modest raise this year, and allow federal executives to catch up for three years of pay freezes, by changing executive-legislative pay rules this time around.

Idea was to let Congress get whatever rank-and-file civil servants receive (4 percent) and -- by doing nothing -- allow executive federal pay to rise when the current cap expires on Sept. 30.

Too many members of Congress are worried that any kind of pay raise for themselves could lead to unemployment this November, and if they can't have a raise, they are determined not to let government executives have one, either.

Thousands of employes in the middle of Grade 15 will move up to the $57,500 limit in October. Nearly everybody in Grade 16, and everyone in Grades 17 and 18 will remain capped at $57,500. Members of the Senior Executive Service will stay at the $58,500 level.

Failure to get a raise means most executiVes Will take home less money in 1983 than they did this year.

That is because the executives -- along with other federal workers -- will begin paying a Medicare tax of 1.3 percent in 1983.

In addition to less 1983 income for executives, federal employes covered by the government's health program will be hit by a big rise -- some are predicting a 25 percent increase -- in health insurance premiums next January.