Re: I have heard it speculated that the extra controll grid in the 4-500z is causing the problem and that what I really need is a 3-500z triode. David: Power is power. Apparently you aren't getting it with the 4-500z. Shouldn't matter whether it is triode, pentode, tetrode, or whatever. (INCLUDING TRANSISTORS, SCR'S, MOSFETS,IGBT'S, ETC) Ed Phillips