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Detailed Circuit Description

The signal enters the first 12AX7 triode, center-biased and partially bypassed for maximum mid to high frequency gain, and is boosted well above the noise floor.  From there the signal enters a Fender style tone stack with BASS, MIDDLE, and TREBLE controls.  The tone stack is followed by the GAIN control to adjust the level of distortion generated in the following stages.

The signal then enters the second 12AX7 triode, center-biased and unbypassed for moderate gain.  The signal then enters a filter network and is split and sent to both the third 12AX7 triode and to the LEVEL control.

The third 12AX7 triode is center-biased and fully bypassed for maximum gain across all audio frequencies.  The signal is attenuated a bit and sent along to the fourth 12AX7 triode.

The fourth 12AX7 is cold-biased and partially bypassed to distort the higher frequencies.  The signal is then attenuated and mixed with the signal from the second 12AX7 triode at the LEVEL control.  The LEVEL knob controls the signal level that reaches the fifth12AX7 triode, the output stage.

The output stage is configured as an AC-coupled cathode follower, but with a large anode load to restrict headroom to safe line levels; this prevents large signals from escaping the preamplifier and damaging line level devices, but maintains the cathode follower’s low (as far as tube devices are concerned) output impedance. The tone is crafted before this stage, so don’t crank the LEVEL for tone’s sake (use the GAIN for that); cranking the LEVEL beyond what is needed will only add noise and possibly unpleasant clipping.

The filaments of all three tubes are powered by a 6.3 VAC supply, fitted with a humdinger trimpot with grounded center tap.  This trimpot is adjusted at the factory to remove nearly all heater noise. DO NOT OPEN THE CHASSIS OR ATTEMPT TO ADJUST THIS TRIMPOT YOURSELF; DANGEROUS HIGH VOLTAGES ARE PRESENT VERY NEAR THE TRIMPOT.