Altec 1566 vacuum tube mic preamp and direct box

V1 and V2 are 12AX7 dual triodes. Each half of V1 is wired as a Class A voltage amplifier in a configuration known as Common Cathode. By comparison, both halves of V2 are wired together in the Cathode Follower configuration for current gain. Don ’t freak dudes and dudettes! Tubes are real high impedance (Hi-Z) devices — just like a passive guitar or bass
Altec 1566 vacuum tube mic preamp and direct box - schematic

— and, in addition to making things louder (voltage gain), they can also convert impedance — in this case — from high to medium (current gain). Unlike transformers, tubes and transistors can manipulate impedance without a level loss. Remember, the outside world is a nasty place. Have you ever had an instrument cable that crackled when stepped on That phenomenon is not an example of a bad cable so much as it ’s the wrong cable for the application. (It ’s capacitance is too high and the insulation between conductors is inadequate. ) The reason there are transformers at both the input and the output is to match impedance with the outside world. Low-impedance (Lo-Z) balanced sources, such as microphones outfitted with XLR connectors, can drive long lengths of cable. The input transformer converts Lo-Z to Hi-Z and in doing so takes a small signal and steps it up to a higher albeit more vulnerable signal. (The power supply transformer manipulates voltage and current in the same way. ) After the tubes do their thang, the output transformer brings the impedance down to a level that ’s semi-impervious to electrical interference. While on the subject, the filament in a vacuum tube heats the cathode so that it will emit electrons, which are negatively charged. The high voltage at the plate accelerates and attracts the electrons while the grid controls electron flow. Each 12AX7 filament requires 150 milliamps (mA) so that the two in...

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