context, drum staff

[Stefan Thomas]

Dear Lilypond users,
in the below quoted example I get two staves, although I would like to have one!
What could be wrong?

\version "2.11.49"
global = { \time 4/4 s1 }
drumsA = {
    \context Voice <<  { \global }  { \drummode {  tamb4 tamb tamb tamb } } >> }
   
\new DrumStaff
{ \drumsA }