© ESO 2018. We present an analysis of 507 spectra of 173 stripped-envelope (SE) supernovae (SNe) discovered by the untargeted Palomar Transient Factory (PTF) and intermediate PTF (iPTF) surveys. Our sample contains 55 Type IIb SNe (SNe IIb), 45 Type Ib SNe (SNe Ib), 56 Type Ic SNe (SNe Ic), and 17 Type Ib/c SNe (SNe Ib/c). We have compared the SE SN subtypes via measurements of the pseudo-equivalent widths (pEWs) and velocities of the HeI λλ5876, 7065 and OI λ7774 absorption lines. Consistent with previous work, we find that SNe Ic show higher pEWs and velocities in OI λ7774 compared to SNe IIb and Ib. The pEWs of the HeI λλ5876, 7065 lines are similar in SNe Ib and IIb after maximum light. The HeI λλ5876, 7065 velocities at maximum light are higher in SNe Ib compared to SNe IIb. We identify an anticorrelation between the HeI λ7065 pEW and OI λ7774 velocity among SNe IIb and Ib. This can be interpreted as a continuum in the amount of He present at the time of explosion. It has been suggested that SNe Ib and Ic have similar amounts of He, and that lower mixing could be responsible for hiding He in SNe Ic. However, our data contradict this mixing hypothesis. The observed difference in the expansion rate of the ejecta around maximum light of SNe Ic (Vm = √2Ek/Mej ≈ 15 000 km s-1) and SNe Ib (Vm ≈ 9000 km s-1) would imply an average He mass difference of ∼1.4 M⊙, if the other explosion parameters are assumed to be unchanged between the SE SN subtypes. We conclude that SNe Ic do not hide He but lose He due to envelope stripping.