  We construct an $\epsilon$-deformation of W algebras, corresponding to the
additive version of quiver $\text{W}_{q,t^{-1}}$ algebras which feature
prominently in the 5d version of the BPS/CFT correspondence and refined
topological strings on toric Calabi-Yau's. This new type of algebras fill in
the missing intermediate level between $q$-deformed and ordinary W algebras. We
show that $\epsilon$-deformed W algebras are spectral duals of conventional W
algebras, in particular the $\epsilon$-deformed conformal blocks manifestly
reproduce instanton partition functions of 4d $\mathcal{N}=2$ quiver gauge
theories in the full $\Omega$-background and give dual integral representations
of ordinary W conformal blocks.
