CTLR CATCHES OWN ERROR IN CLRING PA28 TO ALT OF B747 IN IAD CLASS B AIRSPACE.

Narrative

ON DEP FROM JYO WITH RADAR VECTOR TOWARDS BWI APCH CTL AIRSPACE. WHILE LEVEL AT 4000 FT MSL; I WAS GIVEN A CLB TO 6000 FT ON A 060 DEG VECTOR. POS WAS APPROX 14 NM N OF IAD ARPT. WE ENTERED A SNOWSHOWER IN THE CLB; AND AT 4700 FT MSL THE CTLR YELLED FOR US TO DSND IMMEDIATELY TO 4000 FT. I DID SO AND ACKNOWLEDGED THE REQUEST AS I DSNDED. BREAKING THROUGH THE SNOWSHOWER; I SAW A B747 AT 12 O'CLOCK POS ABOUT 1000 FT HIGH AND 2 MI AWAY. THE B747 WAS TURNING L UNDER INSTRUCTION OF CTLR. THE CTLR APOLOGIZED FOR FRIGHTENING US WITH THE 'EMER DSCNT' INSTRUCTIONS. HE SAID HE HAD CANCELED THE ORIGINAL CLB CLRNC FROM 4000 FT TO 6000 FT; HOWEVER; THAT XMISSION WAS NOT HEARD. HE LATER SAID MAYBE IT DID NOT GET THROUGH AND APOLOGIZED AGAIN AS HE TURNED US OVER TO BWI APCH. HE WAS EXTREMELY BUSY AND VERY HARRIED WHILE WE WERE ON HIS FREQ. OTHER XMISSION MISTAKES WERE MADE AND CORRECTED AS WE TRANSITED THE AREA. WX WAS VMC WITH OCCASIONAL CLOUDS AND SNOW SHOWERS AT 4000 FT TO 7000 FT. THE B747 WAS PROBABLY ON A VECTOR FOR THE R DOWNWIND FOR RWY 1R AT DULLES. MORE THAN 1 XMISSION WAS 'STEPPED ON' BEFORE WE LEFT THAT FREQ.