Bassman 5F6-A Guitar Preamp Schematic
The Bassman 5F6-A preamp contains two voltage amplifiers, one for the bright inputs and one for the normal inputs. The preamp is designed to boost the relatively weak signals from the guitar pickup and to suppress radio frequency interference. The boost comes from a medium-mu 12AY7 triode with a fully bypassed cathode. (In designs based on the 5F6-A this tube is often replaced by a high-mu 12AX7 triode.) The RF suppression results from 68k grid stoppers in combination with the Miller Capacitance of the tube. There are four inputs: fully amplified normal and bright channels (the #1 inputs) and attenuated normal and bright inputs (the #2 inputs).
Except for the highest audio frequencies, where Miller Capacitance becomes a factor, the input impedance is 1 megohm at the #1 inputs and 136k at the #2 inputs. Based on a graphical analysis of the tube's AC characteristics at the DC operating point, the 12AY7 amplification factor is estimated to be 49.1 and the plate resistance is equal to 29.9k. The voltage gains are thus -32.2 and -16.1, respectively, depending on which input is used. These are not the gains that are achieved when connected to the next stage, however, because the output impedance is significant: 23k.
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