Maker's Name: 
QMax (Electronics) London
Where made: 
7.7 × 9 × 25 cm

A Grid Dip Oscillator (GDO) is used to check the resonant frequencies of RF filters, amplifiers, and antennas or more generally as a convenient RF source.

They were very popular with amateur radio operators. This GDO contains a valve in a Colpitts oscillator circuit (split capacitance, two lead inductances, no tap). One of the inductance coils A - H is plugged into the socket on the top to address the frequency range of interest. The capacitance is then varied to provide a variable oscillation frequency. The coil is linked to the circuit to be tested via a pickup loop, or by inserting it into an inductance in the cicuit under test. When the resonant frequencies of the GDO and test circuit coincide, the absorption of energy by the test circuit causes a 'dip' in the DC grid current of the GDO as indicated on the meter. The frequency can then be read off the scale on the dial corresonding to the coil chosen.

Coil Ranges: A 1.5 - 2.9 Mcs; B 2.9 - 5.6 Mcs; C 5.6 -10.8 Mcs; D 10.8 - 20.5 Mcs; E 20.5 - 39.0 Mcs; F 39.0 - 75 Mcs; G 75.0 - 150.0 Mcs; H 150.0 - 300.0 Mcs.