According to the Japanese JIS nomenclature for valves (tubes), the code 6D-H3 indicates that this device was the third, miniature, high-mu triode that Nippon Electric (NEC) produced.
This miniature tube was designed specifically to act as the impedance converter in capacitor (condenser) microphones with particular design consideration given to the reduction of all kinds of noise (microphony, hum, flicker & shot noise) as well as to the minimisation of initial-velocity current.
The 6D-H3 was widely adopted especially by SONY who used this device in many classic microphones which are today compromised by the total inability to find the original NEC 6D-H3 tubes or reasonable substitutes. These include the: SONY C-220A; C-107; C-57; and the C-17B microphones.
To rectify this situation, Phædrus Audio have developed the 6D-H3 Electronic TubeTM.
The Phaedrus Audio 6D-H3 Electronic TubeTM is suitable as a direct replacement for the NEC 6D-H3 tube in microphones using the original NEC device as a cathode-follower (the normal configuration for SONY). A typical circuit is given (left).
The Phædrus Audio 6D-H3 is also termed the AC701 Green Dot because it may be used to substitute the AC701 Telefunken tube in cathode-follower circuits as in the classic SONY CU-2 microphone developed for NHK (right) and in the AKG C60 microphone.
Click here for the datasheet for the Phaedrus Audio 6D-H3 Electronic TubeTM.
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