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The third version is in serial numbers 3460 and up. | The third version is in serial numbers 3460 and up. | ||
All three versions use a 50mA 6pF germanium tunnel diode | All three versions use a 50mA 6pF germanium tunnel diode | ||
to generate the actual output pulse. | to generate the actual output pulse. | ||
The circuit versions differ in how they | In early 661 production, a [[1N3130]] tunnel diode was used. | ||
Then it was replaced by a [[TD1081]]. | |||
The circuit versions also differ in how they | |||
bias and trip the output tunnel diode. | bias and trip the output tunnel diode. | ||