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Alternate Preamp/Reverb Circuit For Blackface Fenders

 

If you are familiar with the Fender blackface with reverb circuit , you are probably also familiar with the ever present background hiss of this circuit and have possibly been annoyed by it, especially in recording situations. This page will discuss an alternative design that could be implemented on a DIY amp in place of the Fender design. I am not advocating modding a vintage amp, although this mod could certainly be implemented on a blackface in a reversible manner. I should also add that this mod is applicable only if you use the amp for clean or a slight blues distortion, as it involves some changing of the gain structure of the amp. The discussion below assumes familiarity with the blackface circuit.

 

Background

The BF design, with its large padding of the dry signal and subsequent re-amplification, creates additional noise that can be reduced by some circuit redesign. This padding is done to provide correct mixing with the reverb circuit, but the reverb return circuit is not arranged to maximize the reverb signal available. If the reverb side of the mixer is arranged to provide more signal, then logically less padding of the dry is required. This is the basic concept of this mod.

The circuit presented here makes maximum use of the gain already available in the preamp and changes the gain structure of the reverb section. If you will use your blackface style amp for clean sounds, the volume control probably doesn't get set past seven. This mod trades off the gain not used in the preamp to better the signal to noise ratio.

 

Circuit Description

The two major elements that are different from the blackface are as follows:

  1. Change the gain stage that follows the mixer in the blackface to a cathode follower.
  2. Change the mixer to make best use of the reverb signal generated and minimize the padding of the dry signal.

 

Schematic

Here is a SCHEMATIC of the preamp. I built this design into a push pull 6V6GT amp of my own design built with major components (cabinet, chassis, transformers, circuit board) purchased from Allen Amplification. These major components are normally part of the Accomplice kit, and since I was looking to build a small combo similar in size to the Accomplice, why not? This approach made sense as it saved me the hassle of getting a custom cabinet built, the chassis was set up for the correct number of tubes, it was available immediately, etc.

 

 Sound

 So how does it sound?

There is definitely a reduction in noise/hiss. It still has the classic BF sound, just a little cleaner with more clarity; It makes my PP 6V6GT Deluxe type amp sound kind of like a lower volume Twin Reverb.

If your use of the BF circuit will be for clean sounds, give it a try, the same number and types of tubes required, just a little rewiring.

 

 

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