 Some also produce significant 3rd harmonic distortion, which is less pleasant to the ear.Discussion is often heard about the distortion of triode versus pentode valves, and single ended versus push pull, which affect the types of distortion produced, but these issues seem to be secondary in practice to the ones discussed in this article, and are already well covered in other articles.Homemade pre-war sets usually used some form of moving iron speaker, usually horn or cone loaded, and occasionally disc loaded.