A tunable millimeter-wave (mmw) DNP probe head is disclosed that is compatible with efficient H/X/Y/e− DNP in samples that may have volume large compared to λ03, where λ0 is the free-space wavelength at the frequency fe of the electron paramagnetic resonance (EPR) when placed in an external polarizing field B0, where B0 is typically in the range of 6.5 T to 35 T, corresponding to fe in the range of 180-1000 GHz, and corresponding to proton resonance frequency fH in the range of 280 MHz to 1500 MHz. The probe head comprises a tune cavity of adjustable volume, a sample cavity that is large compared to the sample wavelength, a tapered conical feed transition filled with a high dielectric material at the small end, and a selectively reflective wall that is substantially reflective of electromagnetic radiation at fe but substantially transparent to electromagnetic radiation at fH.