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  • Guitar Amp Effect Loop Tube Buffer

  • Created: Oct 24, 2015

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Summary

This is a Guitar Amp Effects Loop Buffer based off the Vox V941. While the basic functionality is the same as the original Vox model, several improvements are offered in this design. First, this plan uses a greatly simplified and improved rectification circuit. Also, this plan utilizes a center-tapped 6.3V secondary for the tube heaters. Additionally, there is a plate capacitor added to tame the extreme high end frequencies on the second gain stage. (Please note, a dedicated single-tube transformer is recommended for this project. These are readily available from specialty guitar amp part suppliers)

This unit is ideal for connecting low-headroom guitar pedals and/or low impedance studio effects into a guitar' amplifier's signal path without loss of tone or unwanted distortion.

The unit functions by taking the signal from a guitar amp's Effect Loop send into the unit's Input. The audio is fed via potentiometer to the first half of a dual triode tube (12AX7 recommended) configured as a cathode follower. This reduces the volume of the signal by at least 6dB and lowers output impedance to 600Ω. This buffered signal is then sent out the unit's Send jack to feed external effects, which might otherwise distort from high signal levels straight from an amplifier effect loop, or reduce sound quality due to impedance mismatching. From the external effects, the signal is fed back to the Unit's Return jack, where it is routed via potentiometer into the second half of a 12AX7 with a potential gain of about 30dB. Finally, the audio signal is fed back into the amplifier's Effect Return with an output impedance of about 38kΩ.

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