The 1967 Shure schematic shows that B+ is an unregulated 100 V. This volume knob could occupy the spot originally filled by the Shure amplifier’s channel-select switch. This initial gain reduction would be accomplished by a potentiometer at the input. I wanted very little gain, so an obvious architecture for the new preamp was a common-cathode amplifier (for gain) followed by a cathode-follower stage (to lower the output impedance).įigure 1: I located the common-cathode amplifier nominal operating point on the 12AX7 datasheet’s “average plate characteristics” graph.īecause common-cathode amplifiers produce much more gain than this application needed, ironically, it was necessary to reduce the level of the input signal before sending it into the preamp. In Shure’s original preamp, each audio channel was a series pair of common-cathode amplifiers (because the design needed lots of gain). The schematic for the original Model 65 preamp and datasheets for the 12AX7 tubes are easy to find online. The old Shure preamp used two 12AX7 dual-triode tubes (one for each stereo channel). The new preamp design would be relatively simple because it was receiving a low-impedance signal and driving a high-impedance load. It would be a perfect mechanical platform for the preamp I wanted. However, the little preamp had a chromed case with shielded tubes, audio connectors as well as a small B+ and heater transformer built in. Mechanically, it was beautiful, but the channel gain was extremely high. It was a small, 1960s Shure Brothers stereo preamplifier (Model 65) that had been in the closet for decades (see Photo 1). The effort involved in constructing all the mechanicals for a stereo tube preamp, combined with the fact that this was not a pressing problem, kept the project off my priority list for years.Įventually, I realized that I had a flea market find that could supply the mechanicals. Placing a solid-state preamp between the switched line-level audio inputs and the old tuner and power amplifier would be acoustically clean, but building a tube preamp always seemed more appealing. It would be replaced by a volume control. The switch on the left selected various amplifier paths (tuner, mic, tape). Note the two, shielded 12AX7 tubes, the old capacitor can (which I disconnected but retained for aesthetics) and the small power transformer with B+ and heater output windings. Photo 1: The Shure Brothers Model 65 stereo conversion preamplifier.
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