Another, rarer, alternate used with directly heated cathodes is to have two cathodes heated by a Scott wound transformer that gives two phase AC current with a 90 degree phase difference. This reduced heater generated noise because the noise is a function of the square of the heater voltage and sin^2 + cos^2 = 1, effectively a DC component. I have only ever seen this arrangement used on broadcast equipment. Lee. . ___________________________________________________________ To help you stay safe and secure online, we've developed the all new Yahoo! Security Centre. http://uk.security.yahoo.com