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An integrated A/B switch + attenuator + tube reverb designed specifically for vintage Fender 2-channel amplifiers


I have a beloved Fender AB763 amp whose circuitry I am completely unwilling to modify. I've always loved these Fenders, but always considered the 'Normal' channel to be a rather useless appendage, at least for my own requirements, as I rarely share an amp with an accordion player, or what have you. Obviously, many others had this same idea. Randall Smith started Mesa Boogie around the idea of modifying these venerable old amps to configure that extra channel as a switchable alternative, and/or extra gain stages. Many others had similar ideas, but the common factor was that they all involved either modifying an existing amplifier or building an entirely new one.

I set out to solve a similar problem, but in an entirely different, non-destructive manner.

To my ear, the vibrato channel of any of the AB763 variations is the perfect rhythm-guitar tone. Sure, you can crank it up to where it really sings, but typically that results in some rather apocalyptic volume levels. Moreover, when switching from rhythm to lead, one a) needs to accomplish the change instantaneously, and b) wants any volume boost to be controlled, not necessarily a jump of mega-decibels. Also, many players, if they use reverb on their leads at all, want it less intense than in their rhythm mix. Finally, one often wants some other effects specific to one scenario or the other. The alternatives are either a lot of 'tap-dancing', or an elaborate switching system.

Although no attenuation system will allow you to play a dimed 100W stack at bedroom volumes, while maintaining tonal integrity, a reactive attenuator (more on this distinction later) will allow an amp to produce its cranked tones, uncompromised, at a normal club performance level, which is exactly what I was looking for.


Rather than glaze you over with internal details, a quick run-through of the setup will better convey the essentials:

1. The unit, which, cosmetically, resembles a vintage Fender stand-alone reverb unit, sits on top of the amp.
2. It has a remote footswitch.
3. The speaker out of the amplifier plugs into the unit.
4. A jack on the unit connects to the amp's speakers.
5. The guitar is plugged into the unit.
6. Two jacks on the unit plug into the 'Normal' and the 'Vibrato' channels of the amp.
7. Using the footswitch, select the 'Vibrato' channel.
8. Adjust it as usual to a level appropriate to the venue.
9. IMPORTANT: Turn the attenuation level control on the effects unit to maximum (zero volume).
10. Turn the 'Normal' channel volume on the amp up to a level where you usually get the drive you want; typically 6-8 or so.
11. Gradually adjust the attenuation control on the effects unit until the perceived volume is about the same as your rhythm ('Vibrato' channel) level, or perhaps a bit louder.
12. Adjust the amp's 'Normal' channel tone as desired.
13. Since the 'normal' channel doesn't have reverb, if reverb is desired then adjust the reverb control on the effects unit, which contains its own, tube-driven, spring reverb, very similar to the stock unit in the Fender amp.

That's it.

When the remote switch is toggled, two things happen simultaneously:
1. The input signal is routed to either the 'Normal' or the 'Vibrato' input of the amp.
2. The output from the power transformer is routed either directly to the speaker, or through the reactive attenuator, and thence top the speaker, introducing reverb into the signal if so-configured.

Your stock Fender can now sing or roar at the tap of a single switch.

For further information about attenuators in general, see {coming soon}|.