Have you ever wanted a tube phono pre-amp? Or the ability to have inputs for magnetic and ceramic pick-ups, for hi-fi sound, and the convenience of an automatic changer? And used a simple circuit and minimum of tubes? Well, I was, until I found this circuit. Based off an old Stromberg-Carlson design from the golden age of hi-fi, this circuit has been an interesting one to tinker with.
It is presented here without a power or filament supply circuit schematic. My tinkering has been to incorporate this into an already existing amp, and use its power supply for the B+, and some sort of DC current supply for the filaments. You may want to do likewise, so the slate is clean. The schematic shown is for one channel. Thus, you will need two 7025 tubes for stereo. The circuit features RIAA equalization, and uses an inverse feedback loop between the two stages.
Click on the schematic for a larger view.
|Resistor ID||Value||Function, Notes|
|R1||100K 1/2W 5%||Ceramic load|
|R2||2.2M 1/2W 5%||Signal coupling|
|R3||100K 1/2W 5%||Grid leak|
|R4||6800 1/2W 5%||Cathode|
|R5||120K 1/2W 5%||Feedback coupling|
|R6||140K 1/2W 5%||Tone comp|
|R7||100K 1/2W 5%||Plate load|
|R8||354K 1/2W 5%||Tone comp|
|R9||140K 1/2W 5%||Tone comp|
|R10||1M 1/2W 5%||Grid leak|
|R11||3300 1/2W 5%||Cathode|
|R12||100K 1/2W 5%||Plate load|
|R13||100K 1/2W 5%||Output|
|Capacitor ID||Value||Function, Notes|
|C1||10mmF 500V||Tone comp, silvered mica|
|C2||50mF 50V||Cathode, electrolytic|
|C3||.0047mF 630V||Coupling, metalized polyester|
|C4||10000mmF 500V||Tone comp, silvered mica|
|C5||470mmF 500V||Feedback, silvered mica|
|C6||10000mmF 500V||Tone comp, silvered mica|
|C7||10000mmF 500V||Tone comp, silvered mica|
|C8||3300mmF 500V||Feedback, silvered mica|
|C9||.047mF 630V||Output coupling, metalized polyester|
Shielding of audio leads and dress of all component leads is very important to prevent hum in this circuit. If using AC for tube heaters, use twisted pair for heater wiring and keep wires as close to chassis as possible. Do not run AC heater supply with one wire grounded to chassis.
A single, common ground point is recommended.
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