248 Valve Amplifiers The wirewound resistor to be avoided at all costs is the old-fashioned wirewound rheostat occasionally found lurking at the back of a cupboard in an old physics laboratory. With a power rating of 200 W or more, these beasts seem ideal as power amplifier dummy loads, but their typical % 1v (25 mm) core diameter and length of % 4v (100 mm) mean that their induc- tance becomes significant at 1 kHz, and if their DC resistance is used in a V 2 /R power calculation, the assumed output power of an amplifier will be wrong. As an example, the author used one of these devices while measuring the output power of his `Scrapbox Challenge' single-ended amplifier for the third edition, and measured 6.8 W when 6 W was expected. The reason for the error was that the very slightly higher impedance load allowed the ampli- fier to swing a higher voltage than expected, and this error was magnified by the V 2 term in the power calculation. Non-Inductive Thick Film Power Resistors The hybrid technology that was initially used to make wide bandwidth amplifiers by printing resistors and tracks directly to a ceramic substrate and then adding surface mount transistors has been borrowed to make non- inductive power metal film resistors. At the lower end (5 W) of the range,