Hi Mark, I built one amplifier with 3x QB5/1750 in 1977 . All coils are copper-tubing 9-12 mm, being dull brown already for a long time. So far I never measured any change in efficieny from the beginning. Probably coating is only cometic value, but who cares ? 73 de Peter, DF3KV --- StripMime Report -- processed MIME parts --- multipart/alternative text/plain (text body -- kept) text/html ---