There are several types of tube-testers for the serviceman. Simpson manufactures all different accepted types. Among these, the Simpson Models 305 and 305RC are the most modest in price. But this does not mean that they are limited in performance of necessary functions. Such instruments are simply not produced by Simpson, because Simpson does not manufacture to sell at a price. First consideration is the design of an instrument that will do a thorough job. Simpson Models 305 and 305RC are designed to provide accurate testing for all tubes with filament voltages from .5 volts to and including 120 volts. It tests loctals, single ended tubes, bantams, midgets, miniatures, ballast tubes, gaseous rectifiers, acorn tubes, Christmas tree bulbs, and all popular radio receiver tubes. Like other Simpson tube-testers, the 305's incorporate 3-way switching which makes it possible to test any tube regardless of its base connections or the internal connections of its elements. This method, the result of exhaustive research and expensive construction, protects the Model 305's against obsolesence. No adapters or special sockets are required. In addition to having a complete set of sockets for every tube now on the market, this tester has a spare socket, to provide for future tube developments. On the Model 305RC, the "No-Backlash" Roll Chart prevents the chart from being torn or from getting out of alignment by automatically keeping chart in constant tension. This gives precision selection at all times. If you will remove the Roll Chart unit you will see the very superior Simpson construction throughout. It is, beyond question, the finest roll chart mechanism, in design and in quality manufacture, on any tube tester today.