▁of ▁the ▁pre amp ▁circuit . ▁The ▁effect ▁was ▁achieved ▁through ▁" b ias ▁w ig gle ", ▁in ▁which ▁the ▁bias ▁of ▁a ▁t ube , ▁in ▁the ▁pre amp ▁or ▁output ▁stage , ▁was ▁mod ulated ▁( turn ed ▁off ▁and ▁on , ▁or ▁partly ▁off ▁and ▁on ) ▁in ▁a ▁pure ▁s ine ▁wave . ▁Such ▁circ uits ▁typically ▁had ▁controls ▁for ▁speed ▁and ▁depth , ▁and ▁produced ▁an ▁effect ▁described ▁as ▁" l ush , ▁warm , ▁and ▁round ly ▁pul sing ". ▁ ▁Later ▁ampl ifiers , ▁and ▁particularly ▁the ▁F ender ▁Black face ▁am ps ▁of ▁the ▁mid ▁ 1 9 6 0 s ▁and ▁the ▁later ▁Silver face ▁am ps , ▁used ▁a ▁much ▁more ▁complex ▁circuit , ▁producing ▁the ▁kind ▁of ▁effect ▁that ▁was ▁especially ▁popular ▁with ▁sur f ▁mus icians . ▁Mod ulation ▁was ▁produced ▁using ▁an ▁opt oc ou pler , ▁a ▁light - dependent ▁res istor ▁whose ▁p uls ating ▁signal ▁( produ cing ▁a ▁l ops ided ▁wave ) ▁affect s ▁the ▁pre amp ▁circuit . ▁ ▁References ▁ ▁Category : Effect s ▁units <0x0A> </s> ▁Or nitz ▁is ▁a ▁surname . ▁Notable ▁people ▁with ▁the ▁surname ▁include : ▁ ▁Arthur ▁Or nitz ▁( 1 9 1 6 – 1 9 8 5 ), ▁American ▁cinemat ograph er ▁Don ▁Or nitz ▁( 1 9 2 0 – 1 9 7 2 ), ▁American ▁magazine ▁and ▁celebr ity ▁photograph er ▁Samuel ▁Or nitz ▁( 1 8 9 0 – 1 9 5