Chris at Parts Connexion stated to me that upon request he will test each 6C33C and offer a 90-day warranty.
After many retubings of 6C33Cs in the VK75SE without Pearl Tube coolers, I noted (subjectively) about a 50% extension in tube life with the Coolers. I now use them everywhere, particularly with hot-running & valuable 6H30-DR input tubes. However I don't think the Pearl Coolers extend tube life 2x-5x as claimed.
After many retubings of 6C33Cs in the VK75SE without Pearl Tube coolers, I noted (subjectively) about a 50% extension in tube life with the Coolers. I now use them everywhere, particularly with hot-running & valuable 6H30-DR input tubes. However I don't think the Pearl Coolers extend tube life 2x-5x as claimed.