Households with income under $9,000 a year will be exempt from the City of Alhambra's 5% tax on utility bills, starting July 1, the City Council has decided. Dennis Danner, finance director, estimated that 5,000 of the city's 26,000 households will qualify for the exemption. The tax, which was imposed in 1983, raises about $2.5 million a year for the city. The exclusion of low-income households will reduce city revenue by $84,000, Danner estimated. The tax applies to bills for electric, gas, telephone, water and cable television service.