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The 81 was historically intended for use with the Type 50 tube. The 81 is a classical mono diode for relatively high voltage, such as found with large directly heated triodes. Normally two 81 tubes are used for full wave rectification. The heaters of two tubes should be put in parallel, thus the equivalent of a normal dual diode will result. In this configuration It can produce conveniently an output voltage of 500....700 Volt, at maximum 170mA. Other than dual diodes such at 5U4G, a pair of 81 can be used at maximum voltage and maximum current at the same time.
Attention: This rectifier has a 7Volts heater
For ultra low ripple, it is recommended to use the Lundahl LL1673 dual coil choke in low CMR configuration. In this configuration, there is virtually no field radiation from the choke. (See link to circuit diagram, at the bottom of this page - though for 5U4G here, a pair of mono diodes would give the same configuration.
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