[Amps] 15M OSCILLATION? AL-1500

[ROY JR BONVILLIAN]

Yesterday made a quick qso on 15m all ok , with the amp in operate heard a buzzing sound in receiver and looked at amp plate current about 500 ma and grid about 60 ma. Went to stand by reading returned to normal and no buzzing.Check ground on 3cpx1500 look ok. Turned radio off grounded antenna and with amp in stby ok as soon as I go to operate the above situation happens.The amp works on other bands worked 40m and 20m last for a few hours. HELP and ideas?ThanksWD5DBVROY Roy & Peggy Bonvillian