RO 4-Stage RCA Tube Preamp PUBLIC

RO 4-Stage Tube Preamp Using RCA Circuit Topologies

Richter Original 4-stage tube preamp using RCA circuit topologies, for electric guitar. Controls Include: Input Level, Drive, Bass, Mid-Center, Treble, and Volume. Design based on two circuits from the 1965 RCA Receiving Tube Manual # RC-24 (p. 544, 545); "Two-Stage Input Amplifier" and "Bass and Treble Tone-Control Amplifier". Frequency Range 30 Hz to 16 kHz. Suitable for 6/7/8-string guitar or 4/5/6-string bass. Overall Gain +20 dBV (Vo/Vi = 10). Experimental circuitry. No simulation. Untested design. Breadboard test before final soldering. Power-supply not shown. Requires a regulated single-polarity DC supply providing +12VDC @ >= 750 mA. Rackmount implementation is recommended, with vibration damping using a grounded metal enclosure, and pads on the tubes to reduce vibration noise. Example Power-Supply: 12V DC 1000 mA Regulated Switching Power Adapter, Product ID: 798. $8.95 each. https://www.adafruit.com . Notes On The 12U7: The 12U7 is a space-charge type of twin-triode preamp-tube designed to be operated using a +12V supply-voltage on heaters AND plates. It is not a "starved" 12AU7, but was engineered to work and sound like a 12AU7 that is operated from a high-voltage supply, and provides the same gain (mu = 20). Thus, its distortion is reported to be the same as that of a 12AU7, and the only difference in tone is when it is compared to a starved 12AU7, operated at +12V supply-voltage, which sounds weak and thin when starved. Otherwise, the 12U7 at 12V sounds very close to a 12AU7 at high-voltage, and with full gain, which was after all part of the original design intent. Here, distortion can be obtained by overdriving the input and/or setting the Drive control to get V2 to overdrive V3. Also, the tone-controls have an adjustment range of +-12 dB for Treble and Bass each, while the Center control will move the mid-center between 1 kHz and 6 kHz, though its effect is not discernable when the Treble and Bass controls are both at 50%.