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'69 Twin with sore throat

1/1/2001 10:33 PM
'69 Twin with sore throat
A young guy in this area recently bought a non-mv, early silverface Twin Reverb. He wanted a master volume and more distortion in it. I told him I thought the tone was pretty sweet without the mv, but he really wanted it. I changed the pi / driver section to blackface specs, and a friend of mine changed the filters. Then, we put in a pre-pi master volume in place of the tremolo intensity pot, following the mv in the 70s silverfaces.  
It's doing two weird things now. First, the amp is still fairly loud with the mv turned all the way down, although it gets a hell of a lot louder with the mv turned up. First the bottom of the pot was connected to ground. Then I connected it to the point where the negative feedback ties into the pi, which is 100 ohms above ground. That's how it's wired in the mv silverfaces. No difference.  
That's not my main concern. The irritating part is this harsh, raspy tone that the amp gets in the top end when it distorts. My buddy had put in a silver mica, 500p treble cap. I changed it back to the ceramic 250p. Then I put a 120p cap across the two sides of the bottom of the driver section. That tamed some of the raspiness, but it's still there.  
My friend and I both have master volume silverfaces, and they have never sounded like that. Does anyone have any ideas how to cure the sore throat in the kid's 69 Twin? Don't bother saying not to put in a mv - his mind is still in JCM800 mode, and we won't be cutting the chassis.  

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