Wednesday, November 23, 2016

Disabling the Master Volume and Pull Boost On A Twin Reverb

There's a generation of hand wired Fender amps starting from about 1974 or 1975 that had a master volume and a pull boost added. These are generally despised as tone sucking turds, and although I wouldn't go that far, they can be easily done away with while you're reverting your Fender amp to an earlier and tastier configuration such as the AB763.

Enter the Quad Reverb. This was a 1975 issue Quad Reverb which is nothing more than the venerable Twin Reverb circuit in a 4x12" combo cabinet. In terms of wretched excess it is exceeded only by the Super Six and the early Peavey Vintage 6x10"-which I've only seen in pictures.

Well, nevermind. I acquired this Quad Reverb in a trade for some amp work I did for a guy, he got it from the widow of a friend in exchange for some house painting so we all made out good. The widow got her painting, my bass player pal got his Ampeg spruced up and I got the Quad Reverb.

And that's where things stood for about five years until last weekend when I asked myself what in heck I was doing letting this amp sit.

I'd recapped it way back when, so that was OK. The capacitor board was dangling by the wires and the cake pan was long gone. I decided it needed to be reverted to the original AB763 configuration and that involved changing it from bias balance to bias level, removing the master volume and boost switch, working my way down the circuit board and changing out everything that was non conforming and finally replacing most of the blue Paktron capacitors with Mallory 150s and Mojo Dijons. I finished it off with an earlier Twin Reverb faceplate to cover the hole and polished it up.

There are plenty of resources on the internet to guide you through this process. Suffice it to say, here are the steps and the links to the steps.

1) convert bias balance to bias level.

You may need to tailor the 15k or 27k resistor a bit to give you the range you need to bias your power tubes correctly. In my case an 8.2k did the trick.

2) remove and discard master volume and pull boost. Props to Mike Pascale for this.

3) Retrogress to AB763 configuration. You can either use this link

or do as I did and take the board layout and work your way from right to left and conforming your circuit board to what's on the original schematic. There will be a couple of orphan parts left over from removing the pull boost which takes its drive off the reverb circuit but if you leave them I won't tell.

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