Do not put more than one tube in device. The meter is a very delicate and extremely sensitive instrument. When Tubes Need Charging. The No. 290 Jefferson Tube checker is to your tubes what the hydrometer is to your storage battery. It will test all tubes, large and small, except the UV 199 type, which can also be tested by using an adapter.
Test shows good, fair or poor tubes.
Poor and fair need charging. Full instructions with each instrument. Case finished in crystallized green, trimmed in nickel, and equipped with cord and plug. These are twisted together and exit the transformer case through a porcelain grommet that says Jefferson 100 1. The other end has an old style plastic two-prong plug.